The Human Circulatory System

Blood Cells

Blood Cells

The primary area of anatomical studies for those seeking to obtain the rank of Purple Belt is the human circulatory system. This system is comprised of many different organs throughout the body that function to deliver nutrients, collect waste, transport oxygen throughout the body, and collect carbon dioxide for eventual expulsion via respiratory action.

In this part of your curriculum, you will study the primary organs of the circulatory system, the main circulatory loops in the human body, blood, and blood pressure regulation.

Student Responsibilities

Much of the material in anatomical studies is provided at an adult level. Human anatomy is quite complex and requires significant detail to obtain a sound appreciation for how the human body functions (to the extent that we cover it). If you (or your child) are under the age of sixteen then your primary responsibility is to know the function and names of the major organs in human anatomy. You need not demonstrate knowledge beyond this elemental understanding of this anatomical system.

If you are sixteen years or older then you should have a thorough knowledge of the material presented below. You need not know more than what is presented in the textual content below, but you are naturally free to study these materials further using other medical and anatomical reference materials. We do not limit what you might learn to what we present below.

The Human Heart

The heart is a muscle located within the left front side of the chest. This vital organ pumps blood throughout the body. In humans (and some other animals and birds) the heart is composed of four chambers. The four chambers are the Left Atrium, the Right Atrium, the Left Ventricle, and the Right Ventricle.

The right side of the heart (the Right Atrium and Right Ventricle) are used to pump low oxygenated blood to the lungs where the blood absorbs oxygen and releases carbon dioxide.

The left side of the heart (the Left Atrium and Left Ventricle) then pumps oxygenated blood returning from the lungs out via the arteries to the remainder of the body where nutrients and oxygen are delivered to the cells of the body and carbon dioxide is absorbed. The blood then collects into veins where it is ultimately delivered back to the right atria of the heart.

As can be seen in the diagram below, the right side of the heart (shaded in blue) receives blood coming back from the body that is low in oxygen and rich in carbon dioxide. This blood flows naturally into the right atrium where it collects until the heart muscle relaxes. When the heart muscle relaxes the atrioventricular valve opens allowing the accumulated blood to flow into the right ventricle. When the heart muscle contracts, the atrioventricular valve is forced closed and the semilunar valve is forced open. The contraction then forces blood out of the ventricular chamber and into the pulmonary artery.

The blood then travels via the pulmonary artery to the lungs where carbon dioxide is released and oxygen is absorbed by the blood cells. The blood then flows back to the heart via the pulmonary veins and enters the left atrium where it collects until the next relaxation of the heart muscle. When the muscle relaxes the atrioventricular valve opens allowing blood to flow into the left ventricle. When the heart again contracts the atrioventricular value closes and the semilunar valve opens, forcing oxygenated blood into the aorta. Blood then flows to the remainder of the body via various arteries downstream of the aorta.

For the sake of clarity, it is important to point out that a single contraction of the heart muscle compresses both ventricles at the same time. This makes the human heart, and a four-chamber heart in general, a very efficient pump. Other animals have a less efficient pumping system. Fish have a two-chamber heart while reptiles have a heart with three chambers.

Circulatory Loops

So, for the sake of brevity, the right side of the heart receives blood from the body and sends it to the lungs. The left side of the heart receives blood from the lungs and sends it to the body. These are referred to as the pulmonary and systemic circulatory loops, respectively.

There is a separate circulatory system that delivers nutrients and oxygen to the heart and removes waste and carbon dioxide. The left and right coronary arteries, connected via the aorta, feed blood to the left and right sides of the heart. The coronary sinus is a vein that delivers blood containing waste products back to the vena cava.


The Pericardium is a double-walled sac that surrounds the heart. The pericardium helps protect the heart from infection and physical shock. It consists of multiple layers of tissue of various types that help it perform its several functions. The space between the outer two layers of these tissues is filled with a fluid called the serous-fluid that helps protect the heart from sudden movement and shock. The pericardium also functions to fix the heart in place within the chest cavity. The pericardium also lubricates the heart to reduce friction.


Within the human body, blood is composed of several constituent parts. The largest part, by volume, is blood plasma. This is a liquid composed primarily of water, salts, and blood plasma proteins. It comprises about 54% of all blood volume. Plasma carries proteins, nutrients, vitamins, and hormones and also transports wastes that are later secreted by the body. This liquid also facilitates the transport of red blood cells through blood vessels.

Red blood cells are the fundamental cells of the blood and comprise about 45% of the blood volume. They contain hemoglobin that is useful for absorbing and releasing oxygen. They also transport carbon dioxide back to the lungs where it is exhaled.

Red blood cells are unique in that they do not contain a nucleus. This allows them to conform readily to narrow passages and to modify their shape as necessary as they move about the body. This also limits their lifespan. A red blood cell lives on average about 120 days.

This means red blood cells must constantly be replaced. New red blood cells are constantly produced by the marrow in flat bones, including the ribs, shoulder blades, skull, and hip. They are also produced in some long bones such as the femur and humerus.

The remaining two elements of the blood are also produced by the bone marrow. White blood cells fight infections, destroy foreign cells, and attack bacteria. They are a key element in the body’s immune system. These cells typically have a lifespan of only a single day.

Platelets, the last blood element, facilitate blood clotting and the healing of wounds. Platelets are fragments of bone marrow tissue and are not truly cells in their own right.

The kidneys constantly monitor the amount of oxygen they receive from the blood. If they detect a low level of oxygen in the bloodstream they secrete a hormone called erythropoietin that stimulates the bone marrow to produce additional red blood cells. The liver also produces erythropoietin, especially in young people, but the amount produced declines significantly as a person reaches adulthood.


Human Circulatory System

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Arteries carry blood from the heart to other areas of the body. This is a large branching system in which smaller arteries branch off from larger arteries as blood is transported throughout the body.

The primary purpose of an artery is to carry oxygen-rich blood away from the heart and deliver it throughout the body. The diagram at the right depicts most arteries in red, but there are two important exceptions.

The left and right pulmonary arteries are depicted in blue. This is because the blood they contain is being pumped away from the heart and toward the lungs, but this blood has a reduced oxygen content. You may notice that the oxygen-rich blood returning to the heart from the lungs via the pulmonary veins is colored red. So the red vessels in the diagram indicate oxygen-rich blood. Except for the pulmonary veins, this usually indicates an artery.

Each artery is a blood vessel with three primary layers. The outermost layer, called the tunica externa, is composed primarily of connective tissues comprised of collagen fibers. The middle layer, called the tunica media, is composed of smooth muscle cells. The innermost layer is called the tunica intima and is made up of endothelial cells that form the smooth tube through which blood cells flow.

Branching off from the aorta near the top of its arch are the brachiocephalic artery, the left common carotid artery, and the left subclavian artery. The brachiocephalic artery very quickly splits into the right common carotid artery and the right subclavian artery. The aorta then curves downward, passing down through the chest (the thorax) near the vertebrae column, and descends into the trunk of the body. This part of the aorta is referred to as the descending aorta or the descending thoracic aorta.

The subclavian arteries run just behind (posterior to) the clavicle (collar bone) and supply blood to the arms, chest, and head. An early branch from each subclavian artery called the vertebral arteries travels up through the sides of the neck and then up through the vertebrae of the neck and into the brain. Smaller branches from the two vertebral arteries feed muscle groups in the neck area. After they enter the head the two vertebral arteries merge into the single basilar artery which supplies much of the brain.

The carotid arteries proceed up on either side of the neck until they are slightly above mid-point in the neck. In this area, each carotid artery splits into two separate arteries called the internal carotid artery and the external carotid artery. The internal carotid arteries move inward and further up the neck, eventually entering the skull and providing blood to the brain. The external carotid arteries remain closer to the surface and supply blood to the neck and face.

Where the carotid artery splits into the internal and external carotid arteries is an area referred to as the carotid sinus. The carotid sinus contains baroreceptors that the body uses to monitor and maintain blood pressure in the head and neck area. The baroreceptors respond to both increases and decreases in blood pressure. Hering’s nerve passes these baroreceptor impulses on to the vagus nerve and ultimately to the vasomotor area of the brain stem that helps regulate blood pressure.

There are also baroreceptors in the aortic arch. In a later section, we will discuss how the various baroreceptors help to regulate blood pressure.

As the descending aorta passes through the abdomen it supplies blood to many of the organs and tissues in this region of the body. This artery ends as it divides into the left and right common iliac arteries. The common iliac arteries then branch at the formation of the internal iliac artery and the external iliac artery.

The left and right internal iliac arteries supply blood to the lower back, hips, muscles in the rear, and various reproductive organs. The left and right external iliac arteries provide blood primarily to the legs. Numerous branching of these arteries occurs as they proceed further down and into the legs. The primary arteries are delineated in the Circulatory System diagram above.

Generally speaking, arteries continue to branch and divide. Each branch is typically smaller than its predecessor. As the branching continues the arteries can become quite small. At the end of this branching, the arteries are extremely small and are referred to as arterioles.

Arterioles have a muscular outer lining that is used to aid in blood pressure regulation (discussed later). Arterioles connect to capillaries to deliver blood to individual cells in the body.


Capillaries are extremely small blood vessels. They are so small that only a single blood cell at a time can pass through them. These vessels have a thin membrane that allows oxygen, fluids, and nutrients to pass out of the bloodstream (from blood cells and plasma) and into the surrounding tissues. This thin membrane also allows carbon dioxide, fluids, and waste to pass from the surrounding tissues into the bloodstream. Capillaries are, in effect, an exchange mechanism where life-giving nutrients, fluids, and gasses are provided to bodily tissues while wastes and exhaust gasses are collected for future expulsion.

There are three primary types of capillaries in the body. These are the continuous capillaries, the fenestrated capillaries, and the sinusoid capillaries.

By OpenStax College [CC BY 3.0 (], via Wikimedia Commons

Continuous capillaries are the most abundant type in the body and are generally found in skeletal muscles and the nervous system. These capillaries allow only small molecules (such as water) and ions to pass between the capillary walls and surrounding tissues.

Fenestrated capillaries are a bit larger and have pores along the capillary walls that allow larger molecules and smaller proteins to be exchanged between the capillary and the surrounding tissue. These capillaries are primarily located in the intestines, pancreas, kidneys, and endocrine glands.

Sinusoidal capillaries are the largest and, in addition to pores, have multiple large gaps in their walls that allow large molecules, cells, and protein serums to move between the capillary and surrounding tissues. Red and white blood cells move readily in and out of these capillaries via these gaps. As a result, sinusoidal capillaries are found in bone marrow, lymph nodes, the liver, and the spleen where large volume transfers of cells between the capillaries and organ tissues are required.

Some areas of the body require large numbers of capillaries to facilitate high nutrient and waste exchange rates. Large muscles and organs that require significant blood flow are supported by large concentrations of capillaries referred to as capillary beds. Capillary beds are usually three-dimensional structures to ensure all areas of the associated tissue have sufficient blood flow.

Some areas of the body, such as ligaments and other connective tissue have very low blood flow requirements and therefore have limited capillary support. Injuries to connective tissues often require a longer time to heal than other parts of the body due, in part, to the reduced blood flow in those tissues.

Arterioles feed blood into each capillary. The capillaries facilitate nutrient, gas, and waste exchanges and then pass the blood cells on to venules, which are the smallest veins in the body. So capillaries sit between the arteries and the veins in the body.


The veins, in general, collect blood containing waste material and gasses and return them to the heart. The blood pressure that was present in the arteries was completely depleted across the capillaries and no significant residual blood pressure remains in the veins. This means the veins must use an alternate mechanism for transporting blood back to the heart.

The primary mechanism is squeezing the veins through muscle movement. Overall this mechanism is called the venous pump or the skeletal-muscle pump. Movement of the muscles exerts pressure on the vessels causing them to constrict, forcing blood to move away from its current location. To ensure that blood moves in the direction of the heart, veins contain valves that close when the vein is relaxed so that blood flows only toward the heart and cannot (under normal conditions) flow in the opposite direction.

Abdominal, digestive, and respiratory muscles are primary elements of the venous pump. These muscles are consistently active even when you are at rest. If you are standing then the legs also help move blood with relative ease. If you sit for long periods then blood is less readily transported from the legs back toward the heart. If you exercise then large portions of the body are active and the venous blood flow is increased during the exercise period.

After blood passes through a venule it is delivered to a very small vein. This vein ultimately merges with a larger vein and this process continues until the blood reaches either the anterior vena cava or the posterior vena cava. Blood returning from the head and the upper body returns via the anterior vena cava. Blood returning from the abdomen and lower body returns via the posterior vena cava. These largest veins of the body connect directly to the right atrium of the heart.

Veins often run parallel to arteries in the body and serve to return blood from the body back to the heart. Veins are often located near the surface while arteries tend to be found deeper within the body. Exceptions can be easily found to this rule of thumb, but it is generally true.

When comparing the circulatory system of two different individuals the arteries will likely be in nearly the same locations. The veins however will show significant variability regarding their location. Vein locations are highly individualized (this helps explain why the medical technician has a hard time finding the vein in your arm when attempting to draw blood).

Hepatic Portal Circulation

Blood flow through the digestive system is in many ways quite different than the blood flow in other areas of the body. This is primarily because this system is used to both collect and process nutrients absorbed through the digestive tract.

This circulatory system is composed in large part of veins that collect blood from the digestive tract. The veins used to collect this blood cover nearly the entire digestive tract from the lower part of the esophagus through nearly all of the large intestine and colon.

Arteries deliver blood to the digestive tract that then flows across large capillary beds in the various organs of the digestive system. Fluids, digested nutrients, oxygen, and chemicals are absorbed via the capillaries. As elsewhere in the body blood then flows from these capillaries into a network of veins. These are the hepatic veins, which are unusual in two ways. Firstly, they do not transport blood back to the heart and secondly, they do not contain any valves. In this sense, these blood vessels are not truly veins. But since they carry deoxygenated blood under low pressure they are usually classified as veins.

The veins from the digestive organs then join veins coming from the pancreas and spleen. This large volume of blood is then delivered to the liver. Within the liver, additional capillary beds allow another exchange of nutrients, fluids, chemicals, and gasses. The tissues in the liver process these compounds in various ways, sending some off as waste, changing the chemical composition of others, and using some to fuel the tissues in the liver. The blood in the capillary beds within the liver is then conveyed into a large network of veins that conveys the blood out of the liver and into the inferior vena cava which delivers the blood to the right side of the heart. This is nutrient-rich blood that will eventually be delivered throughout the body via the pumping action of the heart.

The liver does receive blood from arteries as well. This provides oxygenated blood that the liver requires to perform many of its functions. But the vast majority of the blood in the liver (about 75%) comes from the hepatic vein system. The liver regulates the delivery of sugars, proteins, and fats that enter the bloodstream and removes various toxins from the blood. It also converts many of the nutrients absorbed in the digestive tract into forms that can be utilized by the cells of the body. Your liver also produces various proteins and breaks down alcohol and other chemical compounds into water-soluble forms that can be utilized in the body.

Blood Pressure Regulation

Throughout the day the circulatory system must make changes to help an individual maintain fairly consistent blood pressure. Many factors contribute both to the need for these changes and the mechanisms used by the body to produce them.

If you are at rest then you will typically have a low heart rate and relatively shallow breathing. If you are exercising heavily then your heart rate and respiratory rate will both increase. The heart rate increases to help supply the oxygen and nutrients your skeletal muscles need to sustain your exercise. The respiratory rate increases to help bring in additional oxygen and expel the increased carbon dioxide produced while exercising.

But this is not the only situation that will cause your body to adjust your blood pressure. If you have been sitting for a while and then suddenly stand you may experience a sudden drop in blood pressure. If this change in pressure is not adequately compensated in time then you may faint from lack of blood to the brain. Your body is good at making these abrupt changes in blood pressure to reduce the chances that you will suffer from blood flow deficiencies, and conversely, from excessive blood accumulations in the body.

There are four primary ways the body seeks to control blood pressure. These are:

  • Heart rate
  • Heart volume
  • Vascular resistance
  • Blood volume

Heart rate is the number of times your heart beats in a given unit of time, usually measured as beats per minute. The faster the heart beats the more blood it can move throughout the body. Your heart rate goes up as you exercise and then declines again as you assume a more restful state. This is the most obvious way in which you can sense and detect blood pressure regulation in the body.

Heart volume refers to how hard the heart compresses during each heartbeat. At rest, the heart muscle may constrict only moderately. When running the heart muscle may constrict more fully to move more blood into the arteries on each compression cycle.

The symbol Q is often used to represent the blood flow rate and is a product of heart rate and heart volume.

As you may recall, arteries and arterioles have a muscular layer in their lining walls. These muscles can be caused to constrict or dilate to either narrow or widen the size of these blood vessels. This process varies the resistance of blood flow through the arteries and is referred to as vascular resistance.

A formula often used to express changes in blood pressure is:

∆P = Q * R

Where ∆P is the change in blood pressure, Q is the blood volume, and R is vascular resistance. By this formula you can see that changing blood pressure can be accomplished by modifying the heart-output, changing the resistance offered by the arteries, or both.

Two different nervous systems are controlled by the upper spinal column and the brain stem. These are the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS). These nervous systems are not controlled by conscious thought but are instead part of the Autonomic Nervous System (ANS), a system that is responsible for involuntary bodily activities such as breathing, coughing, sneezing, vomiting, and blood pressure management. The SNS and PNS are separate systems to perform opposing functions. As a result the nervous system generally does not use both systems concurrently when performing some function.

As discussed earlier there are baroreceptors in the aorta and the carotid sinus that send signals to the ANS regarding current blood pressure. The autonomic nervous system then makes any necessary adjustments to heart rate, heart compression, and arterial resistance to keep blood pressure at a normal level.

There are also low-pressure baroreceptors found in larger systemic veins, the pulmonary vessels, and the right atrium and ventricles of the heart. These low-pressure baroreceptors detect low blood volumes and initiate processes, often involving the kidneys, to reduce or retain water and salts in the body. If the blood volume is too high, the kidneys will secrete more fluids. If the blood volume is too low, the kidneys will attempt to retain bodily fluids. Adjusting blood volume is another way the body helps to regulate blood pressure. This is a longer-term regulatory mechanism than the short-term system provided by the arterial baroreceptors.

When you exercise heavily or are under sudden stress the SNS sends signals that tend to increase blood pressure. The heart rate and heart compressions generally increase and the arteries, and especially the arterioles, may compress to restrict blood flow, thereby increasing resistance.  The SNS is a fast response system using fast nerve tissues and can ramp up cardio activities very quickly.

Muscles that are involved in exercise require more blood flow than other areas of the body. Through local chemical reactions, the arterioles that provide blood to these muscles are dilated to increase the flow of blood to these specific muscles or muscle groups. Other arterioles in the body may constrict to allow the overall blood pressure in the body to remain at a constant average value.

When the SNS begins to increase blood pressure it will result in decreased digestion and decreased blood flow to areas such as the skin and inactive skeletal muscles as arterioles constrict throughout the body. After prolonged periods of exercise, many of these areas may subsequently reestablish increased blood flow. For example, the skin may see increased blood flow as the body attempts to expel excess heat.

The PNS is a slower acting system that works to slow down the heart rate, reduce heart compression, and dilate arteries and arterioles. All of these activities lower blood pressure and return the body to a more relaxed state. The digestive system is fully activated as blood flow is increased to these organs.

There are a great many other feedback and control mechanisms that can affect blood pressure. We have not talked about hormonal changes that come about due to stress and how these can change blood pressure. We have also not talked about how planned exercise can send signals to the ANS so that the body prepares for your exercises before they begin. All of these (and others) are things you may wish to explore further should you wish to know more about blood pressure regulation in the body.

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