39.1C: Amphibian and Bird Respiratory Systems - Biology

39.1C: Amphibian and Bird Respiratory Systems - Biology

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Birds and amphibians have different oxygen requirements than mammals, and as a result, different respiratory systems.

Learning Objectives

  • Differentiate among the types of breathing in amphibians and birds

Key Points

  • Amphibians utilize gills for breathing early in life, and develop primitive lungs in their adult life; additionally, they are able to breathe through their skin.
  • Birds have evolved a directional respiratory system that allows them to obtain oxygen at high altitudes: air flows in one direction while blood flows in another, allowing efficient gas exchange.

Key Terms

  • gills: A breathing organ of fish, amphibians, and other aquatic animals.

Amphibian Respiration

Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they do not leave the water. As the tadpole grows, the gills disappear and lungs grow (though some amphibians retain gills for life). These lungs are primitive and are not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing through the lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist. It has vascular tissues to make this gaseous exchange possible. This moist skin interface can be a detriment on land, but works well under water.

Avian Respiration

Birds are different from other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli.

Flight poses a unique challenge with respect to breathing. Flying consumes a great amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to sustain flight. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs, and is then expelled during exhalation. The details of breathing between birds and mammals differ substantially.

In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.

30.1 Systems of Gas Exchange

In this section, you will explore the following questions:

  • How does air pass from the outside environment to the lungs?
  • How are lungs protected from particulate matter?

Connection for AP ® Courses

Much of the information in this chapter is not within the scope for AP ® . However, studying the respiratory system provides an opportunity to apply concepts we have previously explored, including chemistry and pH, cell membrane structure, and diffusion of molecules across membranes. The respiratory systems of both invertebrate and vertebrate animals—from the tracheal system of insects and gills of fish to lungs in reptiles, birds, and mammals—reflect a distinct pattern of evolution as animals transitioned from aquatic to terrestrial environments. The function of all respiratory systems is to facilitate the exchange of oxygen (O2) and carbon dioxide (CO2) with the environment. Energy production in cellular respiration requires O2, and CO2 is produced as a toxic by-product. Some animals, such as worms and amphibians, use their entire body surface for respiration. Terrestrial animals, including humans, needed a more efficient system for gas exchange. So, take a deep breath (pun intended!) as we take a dive into the activities and functions of the respiratory system.

Do not confuse the respiratory system with cellular respiration although both involve the movement of O2 and CO2 (gas exchange) between organism and environment.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 2.7 The student is able to explain how cell size and shape effect the overall rate of nutrient intake and the rate of waste elimination.
Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.6 Interactions among living systems and with their environment result in the movement of matter and energy.
Science Practice 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 4.15 The student is able to use visual representations to analyze situation or solve problems quantitatively to illustrate how interactions among living systems and within their environment result in the movement of matter and energy.

The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide, a cell waste product. The main structures of the human respiratory system are the nasal cavity, the trachea, and lungs.

All aerobic organisms require oxygen to carry out their metabolic functions. Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system is correlated with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell (// This email address is being protected from spambots. You need JavaScript enabled to view it. :[email protected]/30-1-Systems-of-Gas-Exchange#fig-ch39_01_01">Figure 30.2). Diffusion is a slow, passive transport process. In order for diffusion to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as many flatworms (Platyhelminthes). Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by complex circulatory systems, to transport oxygen throughout their entire body.

Direct Diffusion

For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist and gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane (Figure 30.3). The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen.

Skin and Gills

Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin and facilitates gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.

Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the oxygen concentration is much smaller than that. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water (Figure 30.4). Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.

The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration), as shown in Figure 30.5. Similarly, carbon dioxide molecules in the blood diffuse from the blood (high concentration) to water (low concentration).

Tracheal Systems

Insect respiration is independent of its circulatory system therefore, the blood does not play a direct role in oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system is the most direct and efficient respiratory system in active animals. The tubes in the tracheal system are made of a polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body (Figure 30.6) and regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.

Mammalian Systems

In mammals, pulmonary ventilation occurs via inhalation (breathing). During inhalation, air enters the body through the nasal cavitylocated just inside the nose (Figure 30.7). As air passes through the nasal cavity, the air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particles are important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation serves several purposes in addition to bringing oxygen into the respiratory system.


  1. pharynx → trachea → larynx → bronchi→ bronchioles.
  2. pharynx → larynx → trachea → bronchi→ bronchioles
  3. pharynx → larynx → trachea → bronchioles → bronchi
  4. pharynx → trachea → larynx → bronchioles → bronchi

From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes its way to the trachea (Figure 30.7). The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long and 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavity where it divides into the two primary bronchi at the midthorax. It is made of incomplete rings of hyaline cartilage and smooth muscle (Figure 30.8). The trachea is lined with mucus-producing goblet cells and ciliated epithelia. The cilia propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The smooth muscle can contract, decreasing the trachea’s diameter, which causes expired air to rush upwards from the lungs at a great force. The forced exhalation helps expel mucus when we cough. Smooth muscle can contract or relax, depending on stimuli from the external environment or the body’s nervous system.

Lungs: Bronchi and Alveoli

The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. The right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 30.9). The muscular diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of the thoracic cavity.

In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs through the two primary (main) bronchi(singular: bronchus). Each bronchus divides into secondary bronchi, then into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split and spread through the lung. Like the trachea, the bronchi are made of cartilage and smooth muscle. At the bronchioles, the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. In humans, bronchioles with a diameter smaller than 0.5 mm are the respiratory bronchioles. They lack cartilage and therefore rely on inhaled air to support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases.

The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 30.10). In the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are approximately 100 alveolar sacs, each containing 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Because there are so many alveoli (

300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m 2 . This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells.

Amphibian Respiration

Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they do not leave the water. As the tadpole grows, the gills disappear and lungs grow (though some amphibians retain gills for life). These lungs are primitive and are not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing through the lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist. It has vascular tissues to make this gaseous exchange possible. This moist skin interface can be a detriment on land, but works well under water.

39.1C: Amphibian and Bird Respiratory Systems - Biology

This page has been translated into Belorussian by Paul Bukhovko and is available at

This page has been translated into Swedish by Johanne Teerink and is available at

The avian respiratory system delivers oxygen from the air to the tissues and also removes carbon dioxide. In addition, the respiratory system plays an important role in thermoregulation (maintaining normal body temperature). The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs plus nine air sacs that play an important role in respiration (but are not directly involved in the exchange of gases).

( A). Dorsal view of the trachea (circled) and the lung of the Ostrich (Struthio camelus). The lungs are deeply entrenched into the ribs on the dorsolateral aspects (arrowhead). Filled circle is on the right primary bronchus. Note that the right primary bronchus is relatively longer, rather horizontal and relatively narrower than the left primary bronchus. Scale bar, 1 cm. (B) Close up of the dorsal aspect of the lung showing the deep costal sulci (s). Trachea indicated by an open circle filled circle = right primary bronchus. Scale bar, 2 cm (Maina and Nathaniel 2001).

Avian respiratory system
(hd = humeral diverticulum of the clavicular air sac adapted from Sereno et al. 2008)

The air sacs permit a unidirectional flow of air through the lungs. Unidirectional flow means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content. In contrast, air flow is 'bidirectional' in mammals, moving back and forth into and out of the lungs. As a result, air coming into a mammal's lungs is mixed with 'old' air (air that has been in the lungs for a while) & this 'mixed air' has less oxygen. So, in bird lungs, more oxygen is available to diffuse into the blood (avian respiratory system).

Pulmonary air-sac system of a Common Teal (Anas crecca). a. Latex injection (blue) highlighting the location of air sacs.
b, Main components of the avian flow-through system. Abd, abdominal aire sac Cdth, caudal thoracic air sac Cl, clavicular
air sac Crth, cranial thoracic air sac Cv, cervical air sac Fu, furcula Hu, humerus Lu, lung Lvd, lateral vertebral diverticula
Pv, pelvis and Tr, trachea (From: O'Connor and Claessens 2005).

The alveolar lungs of mammals (Rhesus monkey A) and parabronchial lungs of birds (pigeon B) are subdivided into large
numbers of extremely small alveoli (A, inset) or air capillaries (radiating from the parabronchi B, inset). The mammalian respiratory
system is partitioned homogeneously, so the functions of ventilation and gas exchange are shared by alveoli and much of the lung volume.
The avian respiratory system is partitioned heterogeneously, so the functions of ventilation and gas exchange are separate in the air sacs
(shaded in gray) and the parabronchial lung, respectively. Air sacs act as bellows to ventilate the tube-like parabronchi (Powell and Hopkins 2004).

Comparison of the avian 'unidirectional' respiratory system (a) where gases are exchanged between the lungs and the blood in the parabronchi, and the bidirectional respiratory system of mammals (b) where gas exchange occurs in small dead-end sacs called alveoli (From: West et al. 2007).

Credit: Zina Deretsky, National Science Foundation

Bird-like respiratory systems in dinosaurs -- A recent analysis showing the presence of a very bird-like pulmonary, or lung, system in predatory dinosaurs provides more evidence of an evolutionary link between dinosaurs and birds. First proposed in the late 19 th century, theories about the animals' relatedness enjoyed brief support but soon fell out of favor. Evidence gathered over the past 30 years has breathed new life into the hypothesis. O'Connor and Claessens (2005) make clear the unique pulmonary system of birds, which has fixed lungs and air sacs that penetrate the skeleton, has an older history than previously realized. It also dispels the theory that predatory dinosaurs had lungs similar to living reptiles, like crocodiles.

The avian pulmonary system uses "flow-through ventilation," relying on a set of nine flexible air sacs that act like bellows to move air through the almost completely rigid lungs. Air sacs do not take part in the actual oxygen exchange, but do greatly enhance its efficiency and allow for the high metabolic rates found in birds. This system also keeps the volume of air in the lung nearly constant. O'Connor says the presence of an extensive pulmonary air sac system with flow-through ventilation of the lung suggests this group of dinosaurs could have maintained a stable and high metabolism, putting them much closer to a warm-blooded existence. "More and more characteristics that once defined birds--feathers, for example--are now known to have been present in dinosaurs, so, many avian features may really be dinosaurian," said O'Connor. A portion of the air sac actually integrates with the skeleton, forming air pockets in otherwise dense bone. The exact function of this skeletal modification is not completely understood, but one explanation theorizes the skeletal air pockets evolved to lighten the bone structure, allowing dinosaurs to walk upright and birds to fly.

Some hollow bones are providing solid new evidence of how birds evolved from dinosaurs.

  • one interclavicular sac
  • two cervical sacs
  • two anterior thoracic sacs
  • two posterior thoracic sacs
  • two abdominal sacs

Air sacs and axial pneumatization in an extant avian. The body of bird in left lateral view, showing the cervical (C), interclavicular (I), anterior thoracic (AT), posterior thoracic (PT), and abdominal (AB) air sacs. The hatched area shows the volume change during exhalation. The cervical and anterior thoracic vertebrae are pneumatized by diverticula of the cervical air sacs. The posterior thoracic vertebrae and synsacrum are pneumatized by the abdominal air sacs in most taxa. Diverticula of the abdominal air sacs usually invade the vertebral column at several points. Diverticula often unite when they come into contact, producing a system of continuous vertebral airways extending from the third cervical vertebra to the end of the synsacrum. Modified from Duncker 1971 (Wedel 2003).

Computerized axial tomogram of an awake, spontaneously breathing goose air is darkest. A large percentage of the bird's body is filled with the several air sacs. Upper left: At the level of the shoulder joints (hh, humeral head) is the intraclavicular air sac (ICAS), which extends from the heart cranially to the clavicles (i.e., furcula or wishbone). S, sternum FM, large flight muscles with enclosed air sac diverticula, arrowheads t, trachea. Upper right: At the level of the caudal heart (H) is the paired cranial thoracic air sacs (TAS). Arrowhead points to the medial wall of the air sac (contrast enhanced with aerosolized tantalum powder). The dorsal body cavity is filled with the lungs, which are tightly attached to the dorsal and lateral body wall. V, thoracic vertebrae. Lower left: At the level of the knees (K) is the paired caudal thoracic air sacs (PTAS) and paired abdominal air sacs, with the abdominal viscera (AV) filling the ventral body cavity. The membrane separating the abdominal air sacs from one another (arrowhead) and from the caudal thoracic air sacs (arrows) can be seen. Lower right: At the level of the caudal pelvis, the abdominal air sacs, which extend to the bird's tail, can be seen. Arrow, membrane separating abdominal air sacs (Brown et al. 1997).

Birds can breathe through the mouth or the nostrils (nares). Air entering these openings (during inspiration) passes through the pharynx & then into the trachea (or windpipe). The trachea is generally as long as the neck. However, some birds, such as cranes, have an exceptionally long (up to 1.5 m) trachea that is coiled within the hollowed keel of the breastbone (shown below). This arrangement may give additional resonance to their loud calls (check this short video of calling Sandhill Cranes).

Sandhill Cranes calling in flight

The typical bird trachea is 2.7 times longer and 1.29 times wider than that of similarly-sized mammals. The net effect is that tracheal resistance to air flow is similar to that in mammals, but the tracheal dead space volume is about 4.5 times larger. Birds compensate for the larger tracheal dead space by having a relatively larger tidal volume and a lower respiratory frequency, approximately one-third that of mammals. These two factors lessen the impact of the larger tracheal dead space volume on ventilation. Thus, minute tracheal ventilation is only about 1.5 to 1.9 times that of mammals (Ludders 2001).

Examples of tracheal loops found in Black Swans (Cygnus atratus), Whooper
Swans (Cygnus cygnus), White Spoonbills (Platalea leucorodia), Helmeted Curassow (Crax pauxi),
and Whooping Cranes (Grus americana).

The trachea bifurcates (or splits) into two primary bronchi at the syrinx. The syrinx is unique to birds & is their 'voicebox' (in mammals, sounds are produced in the larynx). The primary bronchi enter the lungs & are then called mesobronchi. Branching off from the mesobronchi are smaller tubes called dorsobronchi. The dorsobronchi, in turn, lead into the still smaller parabronchi. Parabronchi can be several millimeters long and 0.5 - 2.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, & anastomosing 'air capillaries' surrounded by a profuse network of blood capillaries (Welty and Baptista 1988). It is within these 'air capillaries' that the exchange of gases (oxygen and carbon dioxide) between the lungs and the blood occurs. After passing through the parabronchi, air moves into the ventrobronchi.

Semi-schematic drawing of the lung-air sac system in situ. The cranial half of the dorsobronchi (4) and the parabronchi (6) has been removed. 1 = trachea, 2 = primary bronchus, 3 = ventrobronchi with the connections into (A) cervical, (B) interclavicular and (C) cranial thoracic air sacs, 5 = laterobronchi and the caudal primary bronchus open into the (D) posterior thoracic and (E) abdominal air sacs (From: Duncker 2004).

Avian respiratory system showing the bronchi located inside the lungs. Dorsobronchi and ventrobronchi branch off of the primary bronchus parabronchi extend from the dorsobronchi to the ventrobronchi. Light blue arrows indicate the direction of air flow through the parabronchi. The primary bronchus continues through the lung and opens into the abdominal air sac. (Source:

Birds exhibit some variation in lung structure and, specifically, in the arrangement of parabronchi. Most birds have two sets of parabronchi, the paleopulmonic (&lsquoancient lung&rsquo) and neopulmonic (&lsquonew lung&rsquo) parabronchi. However, the neopulmonic region is absent in some birds (e.g., penguins) and poorly developed in others (e.g., storks [Ciconiidae] and ducks [Anatidae]). In songbirds (Passeriformes), pigeons (Columbiformes), and gallinaceous birds (Galliformes), the neopulmonic region of the lung is well-developed (Maina 2008). In these latter groups, the neopulmonic parabronchi contain about 15 to 20% of the gas exchange surface of the lungs (Fedde 1998). Whereas airflow through the paleopulmonic parabronchi is unidirectional, airflow through the neopulmonic parabronchi is bidirectional. Parabronchi can be several millimeters long and 0.5 - 2.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, and anastomosing air capillaries surrounded by a profuse network of blood capillaries.

Differences among different birds in the development of the neopulmonic region of the lung. (a) Penguin lungs are entirely paleopulmonic.
(b) Some birds, such as ducks, have a relatively small neopulmonic region. (c) Songbirds have a well-developed neopulmonic region.
1, trachea, 2, primary bronchus, 3, ventrobronchus, 4, dorsobronchus, 5, lateral bronchus, 6, paleopulmonic parabronchi,
7, neopulmonic parabronchi A, cervical air sac, B, interclavicular air sac, C, cranial thoracic air sac, D, caudal thoracic air sac,
E, abdominal air sac. The white arrows indicate changes in volume of the air sacs during the respiratory cycle (From: McLelland 1989).

So, how does air flow through the avian lungs & air sacs during respiration?

Air flow through the avian respiratory system during inspiration (a) and expiration (b).
1 - interclavicular air sac, 2 - cranial thoracic air sac, 3 - caudal thoracic air sac, 4 - abdominal air sac
(From: Reese et al. 2006).

A schematic of the avian respiratory system, illustrating the major air sacs and their connections to the lung. (A) The lateral and dorsal direction of motion of the rib cage during exhalation is indicated by arrows. (B) The direction of airflow during inspiration. (C) The direction of flow during expiration (From: Plummer and Goller 2008).

Avian respiratory cycle
This Flash diagram shows the paths that air takes through the respiratory system when a bird breathes.

        • Use the toolbar to step through the five pages of the diagram.
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        During inhalation, air moves into the posterior air sacs and, simultaneously,
        into the lungs and through the parabronchi and into the anterior air sacs.

        During exhalation, air moves out of the posterior air sacs into and through the parabronchi and, simultaneously,
        out of the anterior air sacs and out of the body via the trachea.

        During inhalation, all air sacs expand as inhaled air enters the posterior air sacs and lungs and, simultaneously, air moves out of the lungs
        and into the anterior air sacs. During exhalation, the air sacs diminish in volume as air moves (1) from the posterior air sacs through the lungs and
        (2) from the anterior air sacs and out of the body via the trachea.

        The above Shockwave Flash and Adobe Flash animations were created by John McAuley (Thanks John!).
        (To install Adobe Shockwave Player, go to
        To install Adobe Flash: and, for 64 bit,

        Respiratory airflow in avian lungs. Filled and open arrows denote direction of air flow during inspiration (filled arrows) and expiration (open arrows), respectively. Relative thickness of the arrows indicates the proportion of air streaming through the different areas of the respiratory system during the respiratory cycle. Dotted arrows indicate the volume changes of air sacs. In bird lungs (A), most air directly enters the caudal air sacs during inspiration (thick black arrow), whereas a lesser part flows through the parabronchi/air capillaries into cranial air sacs (thin black arrows). During expiration the major part of inspired air streams from the reservoirs (caudal air sacs, thick open arrows) through the parabronchi/air capillaries into major distal airways, where it mixes with the deoxygenated respiratory gas stored in cranial air sacs during the inspiratory phase. Consequently, respiratory gas flow through the parabronchi, atria, and the gas-exchanging air capillaries is unidirectional and continuous during both inspiration and expiration. This principle is achieved by cranio-caudal pressure gradients in the respiratory system changing between inspiration and expiration and the consecutive opening and closing of valve systems between mesobronchi/air sacs and the parabronchi (not indicated in the figure). Hence, airflow is constant and high in the parabronchi, atria, and the gas-exchanging air capillaries (From: Bernhard et al. 2004).

        Surfactant SP-B (in the figure above) is mixture of phospholipids and specific proteins that functions to maintain airflow through the 'tubes' of the avian respiratory system. Surfactant SP-A has only been detected in the mesobronchi of birds. SP-A plays an important role in innate host defense and regulation of inflammatory processes and may be important in the mesobronchi because air flow is slower and small particles could tend to accumulate there (see figure below). Surfactant SP-C is not found in the avian respiratory system (or, if so, in very small quantities), but is found in the alveoli of mammals along with SP-A and SP-B. Because the mammalian respiratory system (below) includes structures that are collapsible (alveoli) and areas with low airflow, all three surfactants are important for reducing surface tension and innate host defense (Bernhard et al. 2004).

        Airflow in mammalian lungs is bidirectional during the respiratory cycle, with highly reduced airflow
        in peripheral structures, i.e., bronchioles and, particularly, the gas-exchanging alveoli. Consequently, small particles (< 1 µm)
        that enter the alveoli may sediment, making a system of first line of defense necessary, comprising alveolar macrophages
        (white blood cells), SP-A, and (phospholipid) regulators of inflammatory processes (From: Bernhard et al. 2004).

        A: A high-power view of a foreign particle (p) being engulfed by an epithelial cell (e) in an avian lung.
        Arrows, elongated microvilli. B: Surface of an atrium of the lung of the domestic fowl showing red blood
        cells with one of them (r) being engulfed by the underlying epithelial cell (arrow): e, epithelial surface m, a free
        (surface) macrophage. Scale bars: A = 0.5 µm B = 10 µm (From: Nganpiep and Maina 2002).

        Air flow is driven by changes in pressure within the respiratory system:

        • During inspiration:
          • the sternum moves forward and downward while the vertebral ribs move cranially to expand the sternal ribs and the thoracoabdominal cavity (see diagrams below). This expands the posterior and anterior air sacs and lowers the pressure, causing air to move into those air sacs.
            • Air from the trachea and bronchi moves into the posterior air sacs and, simultaneously,
            • air from the lungs moves into the anterior air sacs.

            Changes in the position of the thoracic skeleton during breathing in a bird. The solid lines represent
            thoracic position at the end of expiration while the dotted lines show the thoracic position
            at the end of inspiration (Source:

            Drawing of a bird coelom in transverse section during expiration (gray bones) and inspiration (white bones). Dashed lines illustrate the
            horizontal septum that separates the pleural cavity (PC) where the lungs are located from the subpulmonary cavity (SP) where most
            of the air sacs are located (except the abdominals that are in the peritoneal cavity), and the oblique septum that separates the air sacs from
            the abdominal cavity (AC) and digestive viscera. Both septa insert on the ventral keel of vertebrae. The volume of the pleural cavity changes
            very little with respiratory rib movements, but the volume of the subpulmonary cavity (and the air sacs) is greatly increased when the oblique
            septum is stretched during inspiration (Adapted from: Klein and Owerkowicz 2006). The increase in volume lowers air pressure and draws air
            into the air sacs.

            Schematic representation of the lungs and air sacs of a bird and the pathway of
            gas flow through the pulmonary system during inspiration and expiration. For purposes of clarity, the neopulmonic lung
            is not shown. The intrapulmonary bronchus is also known as the mesobronchus. A - Inspiration. B - Expiration

            • During expiration:
              • the sternum moves backward and upward & the vertebral ribs move caudally to retract the sternal ribs and reduce the volume of the thoracoabdominal cavity. The reduces the volume of the anterior & posterior air sacs, causing air to move out of those sacs.
                • Air from the posterior sacs moves into the lungs &, simultaneously,
                • air from the anterior sacs moves into the trachea & out of the body.

                So, air always moves unidirectionally through the lungs and, as a result, is higher in oxygen content than, for example, air in the alveoli of humans and other mammals.

                Variation in length of uncinate processes -- Birds with different forms of locomotion exhibit morphological differences in their rib cages: (A) terrestrial (walking) species, Cassowary (Casuaris casuaris) (B) a typical flying bird, Eagle Owl (Bubo bubo) and (C) an aquatic, diving species, Razorbill (Alca torda). Uncinate processes are shorter in walking species, of intermediate length in typical birds, and relatively long in diving species (scale bar, 5 cm). Muscles attached to uncinate processes (appendicocostales muscles) help rotate the ribs forwards, pushing the sternum down and inflating the air sacs during inspiration. Another muscle (external oblique) attached to uncinate processes pulls the ribs backward, moving the sternum upward during expiration. The longer uncinate processes of diving birds are probably related to the greater length of the sternum and the lower angle of the ribs to the backbone and sternum. The insertion of the appendicocostales muscles near the end of the uncinate processes may provide a mechanical advantage for moving the elongated ribs during breathing (Tickle et al. 2007).

                Ward presented his ideas at the 2003 annual meeting of the American Geological Society in Seattle. See:

                In the avian lung, oxygen diffuses (by simple diffusion) from the air capillaries into the blood & carbon dioxide from the blood into the air capillaries (shown in this figure and in figures below ). This exchange is very efficient in birds for a number of reasons. First, the complex arrangement of blood and air capillaries in the avian lung creates a substantial surface area through which gases can diffuse. The surface area available for exchange (SAE) varies with bird size. For example, the ASE is about 0.17 m 2 for House Sparrows (about 30 gms Passer domesticus), 0.9 m 2 for Rock Pigeons (about 350 gms Columba livia), 3.0 m 2 for a Mallard (about 1150 gms Anas platyrhynchos), and 8.9 m 2 for a male Graylag Goose (about 3.7 kg Anser anser) (Maina 2008). However, smaller birds have a greater SAE per unit mass than do larger birds. For example, the SAE is about 90 cm 2/gm for Violet-eared Hummingbirds (Colibri coruscans Dubach 1981), about 26 cm 2/gm for Mallards, and about 5.4 cm 2/gm for Emus (Dromaius novaehollandiae Maina and King 1989). Among mammals, there is also a negative relationship between SAE and body size, with smaller mammals like shrews having a greater SAE per unit mass than larger mammals. However, for birds and mammals of similar size, the SAE of birds is generally about 15% greater (Maina et al. 1989).

                A second reason why gas exchange in avian lungs is so efficient is that the blood-gas barrier through which gases diffuse is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness. Among terrestrial vertebrates, the blood-gas barrier is thinnest in birds. Natural selection has favored thinner blood-gas barriers in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles. Among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds. For example, the blood-gas barrier is 0.099 &mum thick in Violet-eared Hummingbirds and 0.56 &mum thick in Ostriches (West 2009).

                Comparison of the mean thickness of the blood-gas barrier of 34 species of birds, 37 species of mammals,
                16 species of reptiles, and 10 species of amphibians revealed that birds had significantly thinner blood-gas
                barriers than the other taxa (West 2009).

                Also contributing to the efficiency of gas exchange in avian lungs is a process called cross-current exchange. Air passing through air capillaries and blood moving through blood capillaries generally travel at right angles to each other in what is called cross-current flow (Figure below Makanya and Djonov 2009). As a result, oxygen diffuses from the air capillaries into the blood at many points along the length of the parabronchi, resulting in a greater concentration of oxygen (i.e., higher partial pressures) in the blood leaving the lungs than is possible in the alveolar lungs of mammals (Figures below).

                (A) Micrograph of lung tissue from a Brown Honeyeater (Lichmera indistincta) showing (a) parabronchi, (b) blood vessel, and (c) exchange tissue (bar, 200 micrometers). (B) Electron micrograph from the lung of a Welcome Swallow (Hirundo neoxena) showing (a) blood-air barrier, (b) air capillary, (c) blood capillary, and (d) red blood cell in the blood capillary (bar, 2 micrometers). (From: Vitali and Richardson 1998).

                A) Medial view of the lung of a domestic chicken (Gallus gallus domesticus). p, primary bronchus v, ventrobronchus d, dorsobronchus r, parabronchi. Scale bar, 1 cm. (B) An intraparabronchial artery (i) giving rise to blood capillaries (c) in the lung of an Emu (Dromiceus novaehollandiae). a, air capillaries. Scale bar, 15 &mum. (C) Air capillaries closely associated with blood capillaries (arrows) in a chicken lung. Scale bar, 10 &mum. (D) Blood capillaries (c) closely associated with air capillaries (spaces) in a chicken lung. Scale bar, 12 &mum. (From: Maina 2002).

                An individual air capillary (AC) surrounded by a dense network of blood
                capillaries (asterisk) in a chicken lung. The blood capillaries drain into a
                larger vein (V6) adjacent to an infundibulum (IF). Note that the general direction
                of blood flow through the blood capillaries is perpendicular to the flow of air through
                the air capillaries, i.e., cross-current flow (From: Makanya and Djonov 2009).

                In birds, the thickness of the blood-gas barrier in the 7.3-g Violet-eared Hummingbird ( Colibri coruscans ) is 0.099 µm, whereas that of an immature 40-kg Ostrich ( Struthio camelus ) is 0.56 µm (Maina and West 2005).

                Relationship between the harmonic mean thickness of the blood-gas barrier (the thickness of the barrier that affects the diffusion of oxygen from air capillaries into blood capillaries) against body mass in the lungs of bats, birds, and non-flying mammals. Birds have particularly thinner barriers than bats and non-flying mammals
                (Maina 2000).

                Light micrographs of a portion of the lung of a chicken (A) and rabbit (B).
                Note the small diameter of the air capillaries in the chicken lung vs. that of the rabbit alveoli (same magnification).
                (A) In the chicken lung, pulmonary capillaries are supported by 'struts' of epithelium (arrows). (B) In the rabbit lung,
                pulmonary capillaries are suspended in the large spaces between alveoli (Watson et al. 2007).

                Cross-current exchange:

                Top: Air flow (large arrows) and blood flow (small arrows) illustrating the cross-current gas-exchange mechanism operating
                in the avian lung (between the blood capillaries and air capillaries). Note the serial arrangement of blood capillaries running from the periphery to the lumen of the parabronchus and the air capillaries radially extending from the parabronchial lumen. The exchange of gases (simple diffusion of O2 and CO2) occurs only between blood capillaries and air capillaries. As air moves through a parabronchus and each successive air capillary, the partial pressure of oxygen (PO2) declines (as indicated by the decreased density of the stippling) because oxygen is diffusing into the blood capillaries associated with each air capillary. As a result of this diffusion, the partial pressure of oxygen in the blood leaving the lungs (pulmonary vein) is higher than that in blood entering the lungs (pulmonary artery) (as indicated by the increased density of the stippling).

                Bottom: Relative partial pressures of O2 and CO2 (1) for air entering a parabronchus (initial-parabronchial, PI) and air leaving a parabronchus (end-parabronchial, PE), and (2) for blood before entering blood capillaries in the lungs (pulmonary artery, PA) and for blood after leaving the blood capillaries in the lungs (pulmonary vein, PV). The partial pressure of oxygen (PO2) of venous blood (PV) is derived from a mixture of all serial air capillary-blood capillary units. Because of this cross-current exchange the partial pressure of oxygen in avian pulmonary veins (PV) is greater than that of the air leaving the parabronchus (PE) air that will be exhaled. In mammals, the partial pressure of oxygen in veins leaving the lungs cannot exceed that of exhaled air (end-expiratory gas, or PE) (Figure adapted from Scheid and Piiper 1987). Importantly, the partial pressure of oxygen in blood leaving the avian lung is the result of 'mixing' blood from a series of capillaries associated with successive air capillaries along the length of a parabronchus is mixed as the blood leaves the capillaries and enters small veins. As a result, the direction of air flow through a parabronchus does not effect the efficiency of the cross-current exchange (because gases are only exchanged between blood capillaries and air capillaries, not between the parabronchus and the blood). So, in above diagram, reversing the direction of air flow would obviously mean that the air capillary on the far left would have the highest partial pressure of oxygen rather than the air capillary on the far right (so the stippling pattern that indicates the amount of oxygen in each air capillary would be reversed). However, because of the 'mixing' of blood just mentioned, this reversal would have little effect on the PV, the partial pressure of oxygen in blood leaving via pulmonary veins (the PO2 would likely be a bit lower because some oxygen would have been lost the first time air passed through the neopulmonic parabronchi). This is important because most birds have neopulmonic parabronchi as well as paleopulmonic parabronchi and, although air flow through paleopulmonic parabronchi is unidirectional, air flow through neopulmonic parabronchi is bidirectional.

                Diagram showing the flow of air from the parabronchial lumen (PL) into the air capillaries (not shown) and arterial blood from the periphery of the
                parabronchus into the area of gas exchange (exchange tissue, ET). The orientation between the flow of air along the parabronchus and that of blood into
                the exchange tissue (ET) from the periphery is perpendicular or cross-current (dashed arrows). The exchange tissue is supplied with arterial blood
                by interparabronchial arteries (IPA) that give rise to arterioles (stars) that terminate in blood capillaries. After passing through the capillaries, blood flows
                into the intraparabronchial venules (asterisks) that drain into interparabronchial veins (IPV). These in turn empty into the pulmonary vein which returns the
                blood to the heart. (From: Maina and Woodward 2009).

                Control of Ventilation:

                Ventilation and respiratory rate are regulated to meet the demands imposed by changes in metabolic activity (e.g., rest and flight) as well as other sensory inputs (e.g., heat and cold). There is likely a central respiratory control center in the avian brain, but this has not been unequivocally demonstrated. As in mammals, the central control area appears to be located in the pons and medulla oblongata with facilitation and inhibition coming from higher regions of the brain. It also appears that the chemical drive on respiratory frequency and inspiratory and expiratory duration depend on feedback from receptors in the lung as well as on extrapulmonary chemoreceptors, mechanoreceptors, and thermoreceptors (Ludders 2001).

                Central chemoreceptors affect ventilation in response to changes in arterial P CO 2 and hydrogen ion concentration. Peripheral extrapulmonary chemoreceptors, specifically the carotid bodies (located in the carotid arteries), are influenced by P O 2 and increase their discharge rate as P O 2 decreases, thus increasing ventilation they decrease their rate of discharge as P O 2 increases or P CO 2 decreases. These responses are the same as those observed in mammals. Unlike mammals, birds have a unique group of peripheral receptors located in the lung called intrapulmonary chemoreceptors (IPC) that are acutely sensitive to carbon dioxide and insensitive to hypoxia. The IPC affect rate and volume of breathing on a breath-to-breath basis by acting as the afferent limb of an inspiratory-inhibitory reflex that is sensitive to the timing, rate, and extent of CO 2 washout from the lung during inspiration (Ludders 2001).

                Respiration by Avian Embryos

                Bernhard, W., A. Gebert, G. Vieten, G. A. Rau1, J. M. Hohlfeld, A. D. Postle, and J. Freihorst. 2001. Pulmonary surfactant in birds: coping with surface tension in a tubular lung. American Journal of Physiology - Regulatory Integrative and Comparative Physiology 281: R327-R337.

                Bernhard, W., P. L. Haslam, and J. Floros. 2004. From birds to humans: new concepts on airways relative to alveolar surfactant. American Journal of Respiratory Cell and Molecular Biology 30: 6-11.

                Dubach, M. 1981. Quantitative analysis of the respiratory system of the House Sparrow, Budgerigar, and Violet-eared Hummingbird. Respiration Physiology 46: 43-60.

                Duncker, H.-R. 1971. The lung air sac system of birds. Advances in Anatomy, Embryology, and Cell Biology 45: 1�.

                Klein, W., and T. Owerkowicz. 2006. Function of intracoelomic septa in lung ventilation of amniotes: lessons from lizards. Physiological and Biochemical Zoology 79: 1019-1032.

                Ludders, J.W. 2001. Inhaled anesthesia for birds. In: Recent advances in veterinary anesthesia and analgesia: companion animals (R. D. Gleed and J. W. Ludders, eds.). International Veterinary Information Service, Ithaca, NY. (

                Maina, J.N. 1989. The morphometry of the avian lung. Pp. 307-368 in Form and function in birds (A.S. King and J. McLelland, eds.). Academic Press, London.

                Maina, J. N. 2002. Structure, function and evolution of the gas exchangers: comparative perspectives. Journal of Anatomy 201: 281-304.

                Maina, J. N. 2008. Functional morphology of the avian respiratory system, the lung-air system: efficiency built on complexity. Ostrich 79: 117-132.

                Maina, J. N., and A. S. King. 1989. The lung of the Emu, Dromaius novaehollandiae: a microscopic and morphometric study. Journal of Anatomy 163: 67-74.

                Maina, J. N., A. S. King, and G. Settle. 1989. An allometric study of the pulmonary morphometric parameters in birds, with mammalian comparison. Philosophical Transactions of the Royal Society of London B 326: 1-57.

                Maina, J. N., and C. Nathaniel. 2001. A qualitative and quantitative study of the lung of an Ostrich, Struthio camelus. Journal of Experimental Biology 204: 2313-2330.

                Maina, J. N., and J. D. Woodward. 2009. Three-dimensional serial section computer reconstruction of the arrangement of the structural components of the parabronchus of the Ostrich, Struthio camelus lung. Anatomical Record 292: 1685-1698.

                Makanya, A. N., and V. Djonov. 2009. Parabronchial angioarchitecture in developing and adult chickens. Journal of Applied Physiology 106: 1959-1969, 2009.

                McLelland, J. 1989. Anatomy of the lungs and air sacs. In: Form and function in birds, vol. 4 (A. S. King and J. McLelland, eds.), pp. 221-279. Academic Press, San Diego, CA.

                Powell, F.L. 2000. Respiration. Pp. 233-264 in Avian physiology, fifth edition (G. Causey Whittow, ed.). Academic Press, New York, NY.

                Powell, F. L. and S. R. Hopkins. 2004. Comparative physiology of lung complexity: implications for gas exchange. News in Physiological Science 19:55-60.

                Reese, S., G. Dalamani, and B. Kaspers. 2006. The avian lung-associated immune system: a review. Vet. Res. 37: 311-324.

                Scheid, P., and J. Piiper. 1987. Gas exchange and transport. In: Bird respiration, volume 1 (T. J. Seller, ed.), pp. 97-129. CRC Press, Inc., Boca Raton, FL.

                Sereno, P. C., R. N. Martinez, J. A. Wilson, D. J. Varricchio, O. A. Alcober, and H. C. E. Larsson. 2008. Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina. PLoS ONE 3(9): e3303.

                Tazawa, H. 1987. Embryonic respiration. Pp. 3 - 24 in Bird respiration, vol. 2 (T. J. Seller, ed.). CRC Press, Boca Raton, FL.

                Tickle, P. G., A. R. Ennos, L. E. Lennox, S. F. Perry, and J. R. Codd. 2007. Functional significance of the uncinate processes in birds. Journal of Experimental Biology 210: 3955-3961.

                Watson, R. R., Z. Fu, and J. B. West. 2007. Morphometry of the extremely thin pulmonary blood-gas barrier in the chicken lung. American Journal of Physiology. Lung Cellular and Molecular Physiology 36: L769-L777.

                Wedel, M.J. 2003. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29: 243�.

                Welty, J.C. and L. Baptista. 1988. The life of birds, fourth edition. Saunders College Publishing, New York, NY.

                West, J. B. 2009. Comparative physiology of the pulmonary blood-gas barrier: the unique avian solution. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 297: R1625-R1634.

                West, J. B., R. R. Watson, and Z. Fu. 2007. The human lung: did evolution get it wrong? European Respiratory Journal 29: 11-17.

                BIOEE 2740

                Course information provided by the Courses of Study 2020-2021.

                Introductory course in vertebrate organismal biology that explores the anatomy and function of vertebrates with an emphasis on vertebrate evolution. Lectures cover topics such as the origin, anatomy, physiology, paleontology, and evolution of various vertebrate groups, with a focus on organ systems (such as the nervous, circulatory, and respiratory systems), life history, locomotion, behavior, and conservation. This course prepares students for advanced courses on the biology of fishes, amphibians and reptiles, birds, and mammals pre-vet and pre-med students benefit from its comparative anatomical approach to understanding the organization of the vertebrate body.

                Fees Course fee: $35.
                Prerequisites/Corequisites Prerequisite: two majors-level biology courses.

                Distribution Category (OPHLS-AG, PBS-AS, BIO-AS)

                Comments Laboratories include dissections of fresh and preserved vertebrate animals and noninvasive live animal demonstrations.

                • Describe how the ten major organ systems interact in a vertebrate's body.
                • Identify major anatomical structures in diverse vertebrate species, including cartilaginous and bony fishes, amphibians, mammals, reptiles and birds.
                • Understand major events and comparative anatomical transitions in the evolutionary history of vertebrates, such as the origin of land vertebrates.
                • Classify vertebrate species to the appropriate major group using correct scientific names, e.g., sharks and other cartilaginous fishes belong to Chondrichthyes.
                • Look at the anatomy of a living or fossil vertebrate and be able to make logical predictions about its way of life.
                • Understand that different groups of vertebrates have different life histories that impose different conservation concerns and outcomes.

                Regular Academic Session. Choose one lecture and one laboratory.

                Credits and Grading Basis

                4 Credits Stdnt Opt (Letter or S/U grades)

                Class Number & Section Details

                1511 BIOEE 2740 LEC 001

                Meeting Pattern
                Additional Information

                Instruction Mode: Online
                Prerequisite: two majors-level biology courses. Course Fee: $25.

                Class Number & Section Details

                1512 BIOEE 2740 LAB 401

                Meeting Pattern
                Additional Information

                Instruction Mode: Online
                At least 2 asynchronous lab hours required per week.

                Class Number & Section Details

                1513 BIOEE 2740 LAB 402

                Meeting Pattern
                Additional Information

                Instruction Mode: Online
                At least 2 asynchronous lab hours required per week.

                Class Number & Section Details

                2257 BIOEE 2740 LAB 403

                Meeting Pattern
                Additional Information

                Instruction Mode: Online
                At least 2 asynchronous lab hours required per week.

                Watch the video: GSC101 Complete grand quiz fall 2021 (July 2022).


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