16.3: Circulatory System - Biology

The circulatory system is extremely important in sustaining life. More accurately the vascular network directly connected to the heart is just blood and blood vessels, but do not be surprised to see the terms circulatory and cardiovascular used interchangeably. Here we are focusing on the heart and vascular network that transports blood.

The Heart

The heart is the life-giving, ever-beating muscle in your chest. From inside the womb until death, the thump goes on. The heart for the average human will contract about 3 billion times; never resting, never stopping to take a break except for a fraction of a second between beats. At 80 years of age, a person’s heart will continue to beat an average of 100,000 times a day. Many believe that the heart is the first organ to become functional. Within weeks of conception the heart starts its mission of supplying the body with nutrients even though the embryo is no bigger than a capital letter on this page. The primary function of the heart is to pump blood through the arteries, capillaries, and veins. There are an estimated 60,000 miles of vessels throughout an adult body. Blood transports oxygen, nutrients, disease causing viruses, bacteria, hormones and has other important functions as well. The heart is the pump that keeps blood circulating properly. Americans today have many options to take care of their heart and circulatory system. Expanding medical technology has made it much easier to do so.

The heart is a hollow, muscular organ about the size of a fist. It is responsible for pumping blood through the blood vessels by repeated, rhythmic contractions. The heart is composed of cardiac muscle, an involuntary muscle tissue that is found only within this organ. The term “cardiac” (as in cardiology) means “related to the heart” and comes from the Greek word kardia, for “heart.” It has a four-chambered, double pump and is located in the thoracic cavity between the lungs. The cardiac muscle is self-exciting, meaning it has its own conduction system. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart’s rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influence such as exercise or the perception of danger.

The Cardiovascular System


Arteries are muscular blood vessels that carry blood away from the heart, oxygenated and deoxygenated blood. The pulmonary arteries will carry deoxygenated blood to the lungs and the systemic arteries will carry oxygenated blood to the rest of the body. Arteries have a thick wall that consists of three layers. The inside layer is called the endothelium, the middle layer is mostly smooth muscle and the outside layer is connective tissue. The artery walls are thick so that when blood enters under pressure the walls can expand.


An arteriole is a small artery that extends and leads to capillaries. Arterioles have thick smooth muscular walls. These smooth muscles are able to contract (causing vessel constriction) and relax (causing vessel dilation). This contracting and relaxing affects blood pressure; the higher number of vessels dilated, the lower blood pressure will be. Arterioles are just visible to the naked eye.


Capillaries are the smallest of a body’s vessels; they connect arteries and veins, and most closely interact with tissues. They are very prevalent in the body; total surface area is about 6,300 square meters. Because of this, no cell is very far from a capillary, no more than 50 micrometers away. The walls of capillaries are composed of a single layer of cells, the endothelium, which is the inner lining of all the vessels. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body.

The “capillary bed” is the network of capillaries present throughout the body. These beds are able to be “opened” and “closed” at any given time, according to need. This process is called autoregulation and capillary beds usually carry no more than 25 percent of the amount of blood it could hold at any time. The more metabolically active the cells, the more capillaries it will require to supply nutrients.


Veins carry blood to the heart. The pulmonary veins will carry oxygenated blood to the heart awhile the systemic veins will carry deoxygenated to the heart. Most of the blood volume is found in the venous system; about 70% at any given time. The veins outer walls have the same three layers as the arteries, differing only because there is a lack of smooth muscle in the inner layer and less connective tissue on the outer layer. Veins have low blood pressure compared to arteries and need the help of skeletal muscles to bring blood back to the heart. Most veins have one-way valves called venous valves to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. If a person is standing still for long periods or is bedridden, blood can accumulate in veins and can cause varicose veins. The hollow internal cavity in which the blood flows is called the lumen. A muscular layer allows veins to contract, which puts more blood into circulation. Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes, nutrition, and medications (intravenous delivery).


A venule is a small vein that allows deoxygenated blood to return from the capillary beds to the larger blood veins, except in the pulmonary circuit were the blood is oxygenated. Venules have three layers; they have the same makeup as arteries with less smooth muscle, making them thinner.

Learning Objectives

Watch this video for a quick survey of the human circulatory system:

A YouTube element has been excluded from this version of the text. You can view it online here:

11.3 Circulatory and Respiratory Systems

Animals are complex multicellular organisms that require a mechanism for transporting nutrients throughout their bodies and removing wastes. The human circulatory system has a complex network of blood vessels that reach all parts of the body. This extensive network supplies the cells, tissues, and organs with oxygen and nutrients, and removes carbon dioxide and waste compounds.

The medium for transport of gases and other molecules is the blood, which continually circulates through the system. Pressure differences within the system cause the movement of the blood and are created by the pumping of the heart.

Gas exchange between tissues and the blood is an essential function of the circulatory system. In humans, other mammals, and birds, blood absorbs oxygen and releases carbon dioxide in the lungs. Thus the circulatory and respiratory system, whose function is to obtain oxygen and discharge carbon dioxide, work in tandem.


The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue, and it pumps blood through blood vessels by repeated, rhythmic contractions. As shown in Figure 14.2.3, the heart has four inner chambers: a right atrium and ventricle, and a left atrium and ventricle. On each side of the heart, blood is pumped from the atrium to the ventricle below it, and from the ventricle out of the heart. The heart also contains several valves that allow blood to flow only in the proper direction through the heart.

Figure 14.2.3 The right side of the heart includes the right atrium and right ventricle. The left side includes the left atrium and left ventricle.

As you may have noticed, the Figure 14.2.3 diagram labels the right side of the heart on the left side of the diagram, and vice versa. This is because it is assumed that in this diagram, the heart appears as if the patient was facing us – the patient’s left side is on our right side!

Unlike skeletal muscle, cardiac muscle routinely contracts without stimulation by the nervous system. Specialized cardiac muscle cells send out electrical impulses that stimulate the contractions. As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart.

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Introduction to the Human Body 10 th Edition Binder Ready Version offers a balanced introduction to the human body, especially developed to meet the needs of the one-semester A&P course. It provides an effective blend of stunning art and clearly written text to illuminate the complexities of the human body. Class-tested pedagogy is woven into the narrative and illustrations to ensure that students gain a solid understanding of the material. This text is an unbound, binder-ready version.

Gerard J. Tortora is Professor of Biology at Bergen Community College in Paramus, New Jersey. He teaches human anatomy and physiology as well as microbiology. He received his bachelor's degree in biology from Fairleigh Dickinson University and his master's degree in science education from Montclair State College. He is active in professional organizations such as the Human Anatomy and Physiology Society, the American Society of Microbiology, American Association for the Advancement of Science, National Education Association, and the Metropolitan Association of College and University Biologists.

Ms. Lima Biology Teacher

You need to know the factors that affect blood flow through the cardiovascular system:

blood pressure, blood volume, resistance, disease and exercise.

You need to know how these factors affect blood flow (increases or decreases blood flow)

The cardiovascular system is a transport system that carries nutrients and oxygen to the cells of the body and carries waste products such as carbon dioxide away from the cells. Also, transports chemical messages between cells and plays a role in fighting disease

The cardiovascular system is made up of the heart, blood vessels and blood

2. Blood vessels are hollow tubes. The three main types are:

Arteries : carry oxygen-rich blood away from the heart

Veins: carry oxygen-poor blood to the heart

Capillaries: Tiny blood vessels that connect arteries to veins. The walls of capillaries are very thin. Gases and nutrients can pass through them and be exchanged between the blood and the cells of the body.

3. Blood is a tissue made up of plasma, blood cells and platelets

Plasma the liquid part of blood made up mostly of water. Plasma contains dissolved nutrients, minerals, salts, and gases.

Red blood cells carry oxygen to the cells of the body and carry carbon dioxide from the cells to the lungs.

White blood cells: There are several types they fight disease either by engulfing microorganisms or by producing chemical called antibodies to fight infection.

Platelets: Platelets are irregularly shaped cell fragments that clump together into clots that help prevent blood loss from injury. Form a blood clot.

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Arteries have strong walls

Veins are less muscular than arteries. There are valves in most veins to prevent backflow. If veins did not have valves blood would have difficulty returning to the heart.

YOU NEED TO KNOW Factors affecting blood flow through the cardiovascular system (blood pressure, blood volume, resistance, disease and exercise)

When your heart beats, it pumps blood through your body to give it the energy and oxygen it needs. As the blood moves, it pushes against the sides of the blood vessels. The strength of this pushing is your blood pressure.

mv2.jpg/v1/fill/w_160,h_128,al_c,q_80,usm_0.66_1.00_0.01,blur_2/Blood%20Pressure.jpg" />

Blood Pressure: pressure that is exerted by the blood upon the walls of the blood vessels and especially arteries as the heart pumps blood through the body

Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow.

Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow.

Hypertension (High Blood Pressure)

A condition present when blood flows through the blood vessels with a force greater than normal. Abnormal high blood pressure increases the resistance to the flow of blood and reduces blood flow. Hypertension can strain the heart, damage blood vessels, and increase the risk of heart attack, stroke, kidney problems, and death.

Blood volume is the volume of blood (both red blood cells and plasma) in the circulatory system of any individual.

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Low blood volume, called hypovolemia , may be caused by bleeding, dehydration, vomiting, severe burns and some medications for hypertension.

Example: If an individual experience a severe injury leading to a drop in blood volume, a decrease in blood pressure will occur, as a result the flow of blood will decrease.

&ldquoAs blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase&rdquo

Resistance to blood flow within a vascular network is determined by the size of individual vessels (length and diameter) and the physical characteristics of the blood (viscosity)

A substance called PLAQUE sometimes builds up inside blood vessels. CHOLESTEROL is a waxy substance that is a major component of plaque. PLAQUE MAKES BLOOD VESSELS NARROWER and SLOWS the flow of blood.

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mv2.png/v1/fill/w_119,h_128,al_c,usm_0.66_1.00_0.01,blur_2/Stages%20of%20Atherosclerosis.png" />

Atherosclerosis is a condition in which fatty deposits called plaque build up in artery walls and eventually causes arteries to stiffen. Decreasing the radius of an artery would increase the resistance to blood flow and reduces the flow of blood. A narrowed artery sometimes become blocked by a blood clot.

Hypertension causes small tears in blood vessels, which sets the stage for ARTERIOSCLEROSIS buildup of plaque in arteries. Decreasing the radius of an artery would increase the resistance to blood flow.

Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured&mdashwhich means that hypertension can promote arteriosclerosis, as well as result from it.

Viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud.

Disease like Diabetes affects the flow of blood. Patients with diabetes have a high level of sugar in blood which increases the viscosity of blood. A high viscosity of blood decreases blood flow

In a patient experiencing hypothermia, blood viscosity may increase. The resistance to blood flow would increase, and the flow of blood will decrease.

As the viscosity of the blood increases, resistance increases and the flow of blood decreases

As the viscosity of the blood decreases, resistance decreases and the flow of blood incre ases

5. Diseases that affect blood flow

Patients with diabetes have a high level of sugar in blood which increases the viscosity of blood. A high viscosity of blood decreases blood flow

Hypertension causes small tears in blood vessels, which sets the stage for ARTERIOSCLEROSIS buildup of plaque in arteries. Decreasing the radius of an artery would increase the resistance to blood flow.

Heart attack or Stroke due to blood clots that block arteries which reduces blood flow

Exercise increases blood flow.

When a person exercises heart rate increases and blood vessels in the muscles dilate to increase blood flow.

Athletes&rsquo heart rate is lower than normal. Exercise strengthens the athletes heart muscle so more blood is pumped with each beat.

16.3: Circulatory System - Biology

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The human circulatory system, a complex network of cylindrical vessels and a central pump, transports nutrients, gases, and wastes throughout the body via three divisions. Coronary, providing blood to the heart. Pulmonary, including the heart and lungs. And Systemic, to the rest of the body.

When blood leaves the veins of the systemic circuit, it enters the right atrium, an upper chamber of the heart, and then the right ventricle, a lower chamber.

From here, blood travels a short distance into the pulmonary circuit through the pulmonary artery to the lungs where it is oxygenated, and then transported back by the pulmonary vein to the opposite side of the heart, the left atrium. It then flows into the left ventricle, which due to a thicker wall can pump blood much farther through the major artery, the aorta, to the entire body.

The aorta branches off, first to the coronary arteries providing the coronary circuit with blood. And then into other smaller arteries that bring the blood to the systemic circuit. The carotid arteries to the brain. The brachial arteries to the arms. The thoracic arteries to the trunk and gut. And the iliac arteries to the legs.

These arteries divide into even smaller vessels called arterioles and then capillaries going deep into muscles and providing all cells in the body with oxygen and nutrients while removing waste products.

The smallest capillaries converge into venules and veins to return blood. The jugular vein from the brain and head. The superior vena cava from the upper body parts. And the inferior vena cava from the lower body parts. All converge back to the heart and lungs where gas is exchanged and waste is finally removed.

22.5: Anatomy of the Circulatory System

The human circulatory system consists of blood, blood vessels that carry blood away from the heart, around the body, and back to the heart, and the heart itself, which acts as a central pump. The systemic circuit supplies blood to the whole body, the coronary circuit supplies blood to the heart, and the pulmonary circuit supplies blood flow between the heart and lungs.

Blood travels from the right atrium to the right ventricle of the heart through the tricuspid valve, then from the right ventricle to the pulmonary artery through the pulmonary valve. Pulmonary veins then carry the blood to the left atrium of the heart, from which it is carried to the left ventricle through the mitral valve. Finally, the left ventricle pumps blood to the aorta (the largest artery in the body) through the aortic valve.

Arteries, which carry blood away from the heart, split and get progressively smaller, becoming arterioles and eventually a series of capillaries, the sites of gas exchange. Capillaries converge to become larger venules, and eventually merge into veins, which bring blood back to the heart.

Double Circulation and the Pulmonary Circuit

Humans have a double circulatory system, in which blood travels through the heart twice via the pulmonary and systemic circuits. First, the heart receives deoxygenated blood in its right side and then pumps it to the nearby pulmonary circuit, the capillaries that are involved in gas exchange. Here, oxygen enters the blood, while carbon dioxide leaves the blood.

The blood is then returned for its second trip through the heart. After passing through the left of side of the heart, blood is pumped out to the entire body through the systemic circuit, all the way to the toes, the fingers, the head, and everywhere in between. The strength required for this task makes the left side of the heart much thicker than the right side, giving the heart a lopsided, non-symmetrical appearance.

Coronary Circulation

The right side of the heart receives deoxygenated blood. So, how does the heart get oxygenated blood? There is a network of arteries and veins surrounding the heart tissue in a crown-like manner that are together known as the coronary circuit. These bring oxygenated blood to the heart and carry deoxygenated blood away.

Capillaries vs. Venules

The walls of the blood vessels reflect their function. Capillary walls are very thin to facilitate gas exchange, whereas arteries and veins are thicker and more complex. Arteries are the thickest blood vessels and are very strong, with two additional layers of tissue (compared to capillaries) to accommodate the high pressure of blood pumped from the heart. Dilation and contraction of the muscle tissue of the arteries are regulated by hormones, directing blood to or from specific areas. Blood returns to the heart at a much lower pressure, so veins have walls that are much thinner than those of arteries.

Patwardhan, Kishor. &ldquoThe History of the Discovery of Blood Circulation: Unrecognized Contributions of Ayurveda Masters.&rdquo Advances in Physiology Education 36, no. 2 (June 1, 2012): 77&ndash82. [Source]

Golbidi, Saeid, and Ismail Laher. &ldquoExercise and the Cardiovascular System.&rdquo Cardiology Research and Practice 2012 (2012). [Source]

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Is the human transport system the same as the circulatory system?

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Study Population and Follow-up Period

The Nordic countries have a long tradition of highly comprehensive population registers, which are based on individual data. Every resident in the Nordic countries has their own unique personal identification number which enables linkage of data between registers. The universal population register coverage makes it possible to follow entire populations in the three countries with a combined total population of 20 million people. For this study, the population at risk consisted of all persons with a hospitalization with a main diagnosis of bipolar disorder or schizophrenia to a psychiatric ward in Denmark, Finland, or Sweden. Information on psychiatric hospitalization was extracted from population based discharge registers.

The follow-up for the population at risk began on 1 January 2000 or on the cohort member’s 15th birthday, whichever came last. The follow-up ended on 1 January 2007, or at the day of death, whichever came first. We choose the year 2000 as the starting point as we did not want to include information from a period with a much shorter life expectancy in the Nordic population.

Assessment of Psychiatric Illness and Cause of Death

Patients included were diagnosed with schizophrenia or bipolar disorder. Thus, individuals diagnosed with schizoaffective disorder were not included, neither in the schizophrenia nor the bipolar disorder diagnostic groups.


Data on psychiatric hospitalizations of the entire Danish population were drawn from the Danish Central Psychiatric Case Register [13]. The register includes data on all psychiatric inpatient admissions in Denmark since April 1, 1969. Only patients admitted to a psychiatric ward were included. In Denmark, the diagnostic system used until December 31, 1993 was ICD-8 [14], and from January 1, 1994, ICD-10 [15]. We identified all persons with schizophrenia (F20/295 excluding 295.49 and 295.79) or bipolar affective disorder (F30, F31/296.19, 296.39). The Danish Cause of Death Register [16] contains computerized information about all deaths of Danish citizens and residents, including date and causes of death. The validity of the register is high [16].


Finnish data on psychiatric hospitalization was drawn from The Finnish Hospital Discharge Register (FHDR) which includes information on all inpatient hospitalization since 1969. The ICD-9 classification was used during the period 1987–1995 and the ICD-10 from 1996 onwards. We identified all persons with schizophrenia (F20/295 excluding 295.4 and 295.7) or bipolar affective disorder (F30, F31/296.2 to 296.7). The validity of the FHDR has been found to be of good quality [17]. The Cause of Death Register in Finland has information on all causes of death for persons living in Finland. The completeness of the register is excellent since all diagnosis routinely passes a validation by Statistics Finland [18].


The Swedish Hospital Discharge Register covers all hospital admissions and discharges in all regions in Sweden since 1987. The register includes data since 1964, but some regions are not included for years 1964–1986. The ICD-9 diagnostic system was used during the period 1987–1996 and ICD-10 from 1997 onwards. We identified all persons with schizophrenia (F20/295 excluding F, H) or bipolar affective disorder (F30, F31/296 excluding B, W, and X). The Swedish register has a high level of completeness [19]. The Swedish Cause of Death Register contains information about all deaths in Sweden and the coding error is estimated from a sample of deaths and was only 3% [20].

In all three countries persons were categorized as having bipolar disorder/schizophrenia from the date of their admission to a psychiatric hospital with such a diagnosis. Patients with a diagnosis of both schizophrenia and bipolar disorder were categorized as having schizophrenia from the day of first diagnosis of schizophrenia. Because data in Sweden was not complete before 1987, we only used information on schizophrenia and bipolar disorder from 1987 and onwards in all countries. We thus had a 13 year period from 1987 to 1999 to find persons with a schizophrenia/bipolar disorder before start of follow-up. All persons with a first contact of schizophrenia/bipolar disorder in 2000 or thereafter were included at the day of first contact with a diagnosis. E.g. if a person had his/her first schizophrenia or bipolar diagnosis on July 31, 2004 the person was not in the population at risk from January 1, 2000 to July 31, 2004 but was in the population at risk from August 1, 2004 onwards.

The underlying cause of death was used in all the three Cause of Death Registers. Over-all mortality was examined: ICD-10 A00-Y98, deaths from diseases and medical conditions ICD 10: A00-R99, and deaths from external causes ICD-10: V01-Y989. Cause-specific death of Chapter IX: Diseases of the circulatory system (Chapter I), was defined as ischaemic heart disease (I20–25), cerebrovascular disorder (I60–69) and a miscellaneous group of Chapter I (I00-I20, I26-I59, I70-I99).

Statistical Analyses

In the cohort study, standardized mortality ratios (SMR) was calculated, comparing the mortality rates among persons with bipolar disorder or schizophrenia with the rate in the general population [21]. In order to make a reliable comparison of SMRs between the countries the crude mortality rates were age-standardized by using the Nordic standard population in 2000 [22]. I.e. for each 5-year group we had the percentage of the population in that age group. We then insured that the population we examined had the same age distribution as the Nordic standard population in all three countries. That is, we standardized the SMR in each of the three countries to the same age distribution to ensure that the differences in SMR was not merely due to differences in the age distribution in the three countries, see chapter 3 in Rothman et al. [23]. If no age standardization is performed it is difficult to compare SMRs. If, as an example the Swedish population is much older than the Danish population, it would appear that the persons in Sweden die at a higher rate, simply because the mortality rate is higher in older person. When we standardize we only compare persons with the same age to one another. Note that the 5 year age group was only used in the standardization of the SMRs. Cause specific SMRs were calculated for each specific subgroup of mortality in the same way. We calculated 95% confidence intervals using the approximation described in the book by Breslow and Day [24].

For persons with schizophrenia or bipolar disorder, we calculated life expectancy at 15 years by using Wiesler's method with one-year age stratification [25], [26]. Life expectancy at a given age represents the average number of years of life remaining if a group of persons at that age were to experience the mortality rates for a particular year over the course of their remaining life. Life expectancy at 15 years is a summary measure of the age specific all-cause mortality rates in an area in a given period [25], [26]. The life expectancy among these persons was compared to that of the general population, and the differences in life expectancies were calculated. The probability of getting a diagnosis of schizophrenia or bipolar disorder before the age of 15 is virtually zero in the Nordic countries, and thus the reason for starting follow-up at that time point in life. If a person died half way through the follow-up period, they only contributed half the period to the time at risk.

The number of deaths that would not occur if the group of persons with a psychiatric disorder had the same mortality as the general population were calculated for each subgroup of psychiatric disorder, sex and country by the formula: N*((SMR-1)/SMR), where N is the number of deaths and SMR is the SMR of the excess mortality [27].


All personal identification numbers were anonymized before the analysis of data began. The study was approved by ethical committees and data protection agencies in each of the three countries. According to the legislation in the three countries, no informed consent from participants was needed because data were analyzed anonymously.

The study was approved by the Danish Data Protection Agency (2000-41-0307).

The permission to create the Finnish research database was given by STAKES (National Research and Development Centre for Welfare and Health, currently THL National Institute for Health and Welfare), Statistics Finland and National Social Insurance Institution. The Data Protection Ombudsman (Tietosuojavaltuutettu) gave his statement before the study data was created, as requested by the national legislation on data protection.

We have received ethical permission for this study from the ethics committee in Gothenburg, Sweden (Dnr 130-08).

Watch the video: THE CIRCULATORY SYSTEM. Educational Video for Kids. (January 2022).