This page has  Four Sections:

  • Diagram of Blood Flow Through the Heart
  •  Vascular System Anatomy and Physiology
  • Red Blood Cell Anatomy and Physiology
  • Lung Anatomy and Physiology

Schematic of Blood Flow Through the Heart

We will be using some basic diagrams to explain the flow of blood through the 4 chambers of the mammalian heart. This will dramatically help in your understanding of the flow of blood. It will also help in understanding the anatomy later in this page when we show you actual anatomic structures of the dogs heart.

It might be a good idea to come back to these diagrams when we go into more detail later. They are kept simple on purpose so that you can understand the sometimes complicated concepts we will be explaining.

Blood that has supplied the cells with oxygen, and now contains carbon dioxide to be eliminated from the body when we exhale, flows into the heart from 2 directions. From the head, it flows through the anterior vena cava (AVC) into the right atrium. From the back end of the body it flows through the posterior vena cava (PVC) and into the right atrium.

When an adequate amount of blood has filled the right atrium (this takes only milliseconds) it contracts, and blood flows through the tricuspid valve (also called the right atrio-ventricular (A-V) valve) and into the right ventricle.

The tricuspid valve, like the other valves in the heart, is a one-way valve. In a healthy heart blood can flow only from the atrium into the ventricle. A heart murmur, which is explained later, is a turbulence of blood flowing through one of the heart valves (there are 4 of them), like the tricuspid valve. A murmur is detected with a stethoscope when ausculting the chest.

The blood that is now in the right ventricle rapidly mixes with the small amount of residual blood that remains in the right ventricle from its last contraction. When the right ventricle is filled adequately (again, this takes only a few milliseconds), it contracts and the blood flows through another valve called the pulmonic valve, into the pulmonary artery, and eventually into the lungs.

In the lungs the blood rids itself of carbon dioxide and absorbs a fresh supply of oxygen during exhalation and inhalation. The blood goes from poorly oxygenated to freshly oxygenated, and is now ready to supply the cells of the body with fresh oxygen all over again. Now it just has to get to all those billions of cells.

Diagram of blood flowing into right atrium

The freshly oxygenated blood in the lungs flows through the pulmonary vein and into the left atrium. Just like in the right atrium, when there is enough blood present, the left atrium contracts and the blood flows through the mitral valve into the left ventricle. Remember this mitral valve since we will be talking about it later in a common heart problem in the geriatric pet called endocardiosis. The mitral valve is a one-way valve also.

The blood that enters the left ventricle mixes with residual blood that remains from the last contraction. When the left ventricle is adequately filled with blood it contracts and ejects its blood through the aortic valve and into the aorta. Once in the aorta, a branch called the brachiocephalic trunk, supplies the head, while the rest flows down the descending aorta and to the rest of the body.

A small branch (not shown in this diagram) off the aorta supplies the heart through the coronary artery. It is sometimes forgotten that the heart is made up of cells that need a blood supply bringing oxygen and nutrients to the heart cells so they can do their job.

It is in people that these blood vessels to the heart (called the coronary arteries) get blocked through the process of atherosclerosis and can cause a myocardial infarction (MI) that leads to death of a heart cell. The main coronary artery branch that is affected is the Left Anterior Descending artery (LAD), also know as the widow maker since blockage of this artery leads to death of heart cells due to lack of oxygen. When enough heart cells die the normal electrical activity of the heart is disrupted, and your heart goes into ventricular fibrillation. The end result is a heart attack.

Diagram of blood flow into the left side of the heart


The right and left hearts are doing their work at the same time, so coordination of all this blood flow is critical. This is especially apparent when you realize that a typical dog or cat heart is beating between 100 and 200 beats every minute. This means that for the average dog or cat, everything in this diagram below happens twice each second.

Think of the coordination needed in a bird whose heart rate is 350 beats per minute! The heart valves have to open and shut very rapidly, the atria and ventricles have to expand and contract very rapidly also as they fill up with and eject blood, and the rest of the body has to cooperate in the first place by bringing an adequate amount of blood into the heart through both vena cava’s.

In addition, the respiratory system has to cooperate by inhaling air, exhaling air, and exchanging carbon dioxide and oxygen. It’s a wonder that all of this can even be coordinated.

Diagram of blood flow into right and left hearts

When the heart muscle contracts and ejects blood it is called systole. When the heart muscle is relaxing in between systoles, and filling up with blood in readiness for the next contraction, it is called diastole. Diastole is important to the heart muscle itself, since it is during diastole that blood flows into the coronary artery to supply the myocardium.

With such a complicated system it is no wonder that things can go wrong. We will learn more about heart pathology later, first there is more to learn about anatomy and physiology of the cardiovascular system. Stay with us…..

Vascular anatomy and Physiology


These vessels direct blood away from the heart and towards the cells of the body. They tend to lay deep in the body tissues, partially to protect them from trauma. arteries have several layers; a tough outer layer, a middle layer of smooth muscle, and an inner layer of very smooth cells. The tough outer layer allows the artery to withstand the high pressure that occurs with each beat of the heart. Most normal people and pets have a blood pressure that ranges around 120-170 mm of Hg (mercury) when the heart contracts (systole).

Let’s take a temporary detour from dogs and cats. We are an exotics practice, and comparative anatomy is a big part of what we need to understand daily at work. A giraffe has a blood pressure that goes up to 240 mm of HG. It needs this high pressure since the blood has to flow upwards a long distance against gravity.

It also has one-way valves in its arteries to keep the blood flowing upwards after each heartbeat. If it did not have these valves, after each beat of the heart the blood would flow backwards to the heart and not up to the brain where it is needed.

Dr. P took the following picture in the Selous, Tanzania. You can learn more about all his wildlife trips from this link.

Baby giraffe sitting on the ground

This baby Masai giraffe has no idea its stays conscious due to its high blood pressure supplying its brain with oxygen. Without a blood pressure of 240mm of Hg the blood would not make it up to the brain and it would pass out.

The high blood pressure needed to bring blood to a brain so high off the ground causes problems when a giraffe lowers its head to drink. This high blood pressure, along with the added effect of gravity, can literally cause blood vessels in the brain to burst. This would lead to coma and death. Nature has adapted to this with a special set of blood vessels near the brain that absorb this extra pressure when the head is lowered.

Adult giraffe drinking water

They have long prehensile tongues that aid in drinking

The smooth inner layer of the artery gives red blood cells and the fluid surrounding the red blood cells (called plasma), a friction-free pipe to get to all of the cells of the body.

The muscular wall (the middle layer) of the artery helps the heart pump the blood. When the heart beats, the artery expands as it fills with blood. When the heart relaxes, the artery contracts, exerting a force that it strong enough to push the blood along. This rhythm between the heart and the artery results in an efficient circulation system.

The smooth muscle in the walls of arteries also allows them to selectively constrict and dilate. This is very important, because blood does not flow to all organs in the same amount consistently. For example, when you eat a meal, the arteries to your intestines dilate and more blood flows to them to aid in digestion.

Or, when you are exercising, the arteries to your intestines will constrict and the arteries to your muscles will dilate. This process goes on continuously and in a highly refined process throughout life, all depending on the physiologic needs of individual cells at a specific moment in time.

The main artery from the heart is called the aorta. It is large, and has a thick wall because of the high pressure of blood that is flowing through it. The ascending aorta supplies the head with blood through arteries called the brachiocephalic trunk, eventually branching to the carotid arteries.

The descending aorta goes through the thoracic cavity and supplies the rest of the body from within the abdomen. A branch of the descending artery, called the coronary artery, supplies the heart. Let’s not forget the fact that the heart is an organ with millions of cells also, and they need oxygen and nutrients also if they are to perform their job.

This long white structure in the picture below is an actual picture of a dog’s aorta as it comes off the heart (you cannot see the heart) on the left. The blood is flowing from left to right as it goes to the back of the body. The aorta is firmly embedded in a structure called connective tissue. This tissue gives it stability, and prevents it from tearing or rupturing during movement. A weakening anywhere along the aorta can lead to a bulge called an aneurysm. If an aneurysm ruptures death is almost instantaneous.

Picture of the aorta in a dog's chest

The aorta is one of many blood vessels in the chest

This radiograph of a cat’s chest shows the aorta as it leaves the heart in the same way as the picture above. It is the grayish linear object with arrows on top. The arrows show the direction of blood flow to the back of the body. The aorta disappears on the radiograph after the last arrow on the right. You cannot see the branch of the aorta, called the brachiocephalic trunk, that supplies the head. This trunk branches off near the first arrow in the lower left.

X-Ray of a cat's chest showing the aorta

It is interesting how the aorta appears and then disappears


As an artery gets further from the heart it gets smaller and eventually becomes an arteriole. An arteriole is smaller in diameter than an artery, and is found closer to the target organ. For example, a branch off of the descending aorta, called the renal artery, supplies the kidneys. As the renal artery enters the kidneys it breaks up into many small branches called arterioles.

The arterioles also are lined with smooth muscle, allowing further refinement of blood flow to a target cell. It is the renal artery and its branches that is involved when a pet is dehydrated or gets acute renal failure.

Blood vessels surrounding capsule of a kidney

These small blood vessels surrounding the kidney give you an idea of how they can branch into smaller and smaller sizes as they enter an organ


At the level of the cell the arteriole branches into even smaller vessels called capillaries. They do not contain smooth muscle, and cannot selectively constrict or dilate like arteries and arterioles. They are very small in diameter, so only one red blood cell can pass at a time. In fact, the capillary is so small that red blood cells literally have to squeeze their way through in many cases.

The wall of capillaries are only one cell thick, all for a reason. It is at the capillary level that oxygen flows from hemoglobin, contained in the red blood cell, into the actual kidney cell or liver cell. At the same time, the hemoglobin picks up carbon dioxide that is coming out of the cell.

This red blood cell, whose hemoglobin is now saturated with carbon dioxide instead of oxygen, eventually flows back to the lungs to rid itself of carbon dioxide and take on a new load of oxygen for delivery to some other cell in the body.

After about 90 days the red blood cell wears out and is metabolized by the body. The spleen is a big part of this, which is why we can see red blood cell abnormalities in spleen disease.

The capillaries have many other functions besides the exchange of oxygen and carbon dioxide. The yellow around the red blood cells in the diagram above is fluid, called plasma, that flows in the bloodstream along with red blood cells. This fluid contains nutrients like fats, carbohydrates, proteins, and electrolytes that the cell needs to function. It also contains hormones, antibodies, clotting factors, and drugs that we might administer. This fluid also flows though the wall of the capillary and into the cell.

Later on you will learn about pulmonary edema, which is fluid buildup in the lungs. In the case of the diagram above, the cell is the actual lung (called the alveoli) that becomes filled with excess fluid. This occurs because the fluid in the capillary is under higher pressure than normal, so more fluid flows out of it and into the actual lung cell. We will explain why this happens later in the pathophysiology section.

Some capillaries have specialized functions. If they line the intestines they will absorb nutrients like fats, carbohydrates, and proteins from the inside of the intestines directly into the bloodstream. If these capillaries line the kidneys they will help excrete waste products and regulate the metabolism of electrolytes. If they line the liver they will help in the metabolism of nutrients and the distribution of hormones.


As the capillary leaves the individual cells it is assigned to supply, and starts the journey back to the heart, it becomes a venule. Venules are small veins, and have a job similar to arterioles, although there are many more venules than arterioles. Their numerous branches drain an organ, eventually coalescing into veins on their trip back to the heart. The lymphatic system also is a part of draining fluid from cells back to the heart.


As the venules coalesce they eventually form veins and continue on their way through the cardiovascular system. Veins have 3 layers just like arteries, although each layer is thinner and not as strong. They don’t need to be as strong because the blood is under much lower pressure in the venous system. The blood in the veins is darker in color compared to the blood in the artery because they contain less oxygen. About 2/3 of the blood in the body resides in the veins at any one time.

Sometimes these veins return the blood back to the heart abnormally. One of the most common ones is called a portosystemic shunt (PSS), also known as a liver shunt. Our liver page has details about this disease.

Those veins in the back of the body eventually drain forward into the posterior vena cava, and into the right atrium of the heart. The veins that drain the head and upper part of the body eventually drain backwards into the anterior vena cava and into the right atrium of the heart. You learned about this in the diagram at the beginning of this page.

The pressure in the veins is much lower than in the arteries and arterioles. This can be a problem in the extremities. For example, if you stand for a long period of time, the blood in the veins of your lower legs needs to push against gravity to get back up into your heart. There is not enough pressure in these veins to do this by themselves.

To help get this blood back into the heart the veins have one-way valves so blood always flows towards the heart. Also, the skeletal muscle surrounding these veins continuously contracts in small amounts, further pushing blood in the right direction. You can see this yourself in your leg veins. Cross one of your legs and watch your calf muscle closely. You can see the small muscular contractions helping the veins.

Veins tend to lay at the surface of the skin and are easily visualized. In addition to the functions described above, they are part of the thermoregulatory mechanism of the body. When they are at the surface and are dilated they rid the body of excess heat. This is taken to an extreme in large animals like African elephants that have a large volume of body in relation to surface area, and live in a hot environment.

Large veins in elephant ear

The huge veins with flopping ears keep them from overheating in their hot environment

As you can imagine, nature is not always this simple. There is a special set of veins in the body called a rete mirable. It is a conglomeration (for lack of a better word) of arteries and veins, usually running adjacent but flowing in opposite directions, that allows for heat exchange. A classic example is the pampiniform plexus, one of the thermoregulatory mechanisms of the testicle.

In mammals, the core temperature of the testicle has to be a few degrees cooler than core body temperature for sperm to be fertile. The warmer arterial blood coming from the body and supplying the testicle flows directly past the slightly cooler blood that is in the venous system draining the testicle. This allows the cooler blood from the venous system to absorb some of the heat from the warmer arterial blood, thus slightly cooling the arterial blood that enters the testicle.

In the picture below from our Canine neuter page our surgeon is holding the body of the testicle with his fingers. The white horizontal structure below his finger, that deviates vertically, is the vast deferens. This is where semen flows out of the testicle.

Testicle showing blood supply

 The pampiniform plexus is circled in blue. It is difficult to differentiate the arteries from the veins because they are all wrapped together.

Red Blood Cell Anatomy and Physiology

Red blood cells are continually manufactured in the bone marrow, and are the only cells that do not contain a nucleus in mammals. Instead, they contain hemoglobin, which is the molecule that exchanges oxygen and carbon dioxide. Each red blood cell contains about 250 million hemoglobin molecules.

The hemoglobin molecule imparts the red color to red blood cells. It is the element “iron” in the center of the hemoglobin molecule that attracts oxygen and carbon dioxide. The oxygen/carbon dioxide exchange occurs in the lungs at a section called the alveoli.

Red blood cells are small and easily deformed, a prerequisite if one of your jobs is to squeeze through a capillary. One cubic centimeter (cc) of blood contains 5-10 million red blood cells in a typical adult dog or cat. To give you some perspective, a teaspoon of any fluid contains 5 cc’s. If you fill that teaspoon with blood that contains 10 million red blood cells per cc, you get 50 million red blood cells in a teaspoon.

Normal dog cbc

Our routine CBC on a blood panel measures these red blood cells and hemoglobin

Multiply that number by the thousand’s of cc’s of blood in a typical dog or cat and you get a number in the trillions. This means there are trillions of red blood cells in circulation at any one time! To take this calculation even further, if you multiply this number by the 250 million hemoglobin molecules in each red blood cell, you get a real big number.

The point of this exercise is to give you an idea of the tremendous amount of underlying microscopic anatomy and physiology that occurs in the heart, lungs, and blood vessels in order to deliver oxygen to the cells. An awful lot is going on that we just don’t see to make the whole system work. Knowledge of all of this is a testimonial to the people that have studied this their whole lives to understand it and explain it to the rest of us.

Oxygen delivery to cells is a critical part of the normal physiology of every aerobic organism. If this system gets disrupted there will be significant problems, and even death of the cell. As a result, the body has several mechanisms to cover for problems.

Other organs can manufacture a small amount of red blood cells if the bone marrow is having a problem. Red blood cells are also stored in the liver and the spleen in case of an emergency or immediate need. A good example here is when you cut yourself and you lose an extensive amount of blood. The blood in these storage organs releases red blood cells into the circulation.

A low number of red blood cells is called anemia. If the level of red blood cells becomes significantly low the cardiovascular system cannot supply the cells adequately with oxygen. This affects the function of these cells, and the organ involved does not work well.

Anemia blood panel

This is what anemia looks like on a blood panel called a CBC (Complete Blood Count)

Anemia is not a disease per se, it is a sign of a disease somewhere in the body. Numerous disease processes can cause anemia. Whenever we tell a client their pet has anemia the first thought that comes to their mind is a vitamin or nutritional deficiency. Even though these nutritional anemia’s exist, they are rarely the cause of the anemia.

An excess of red blood cells, called polycythemia, can also be a problem. There can be so many red blood cells that they actually form a sludge that cannot flow through the narrow diameter of the capillary. Polycythemia  has several causes, all of them hard to determine.

We monitor the amount of red blood cells by performing a test called  hematocrit (HCT), which is a measure of the percentage of red blood cells in a sample of blood. If it gets too high we literally remove excess blood from the body with a catheter and syringe.

Lung Anatomy and Physiology (pulmonary physiology)

The emphasis on this page is on the heart and blood vessels and not the lungs, since this would make this page far too complicated. We will show you a few pictures of the lungs since they are intimately involved with the cardiovascular system. The lungs have an extensive blood supply, which is necessary if they are to replenish the oxygen of the millions of red blood cells presented to them every second. As you know from the diagram above, the blood that enters the lungs comes from the right ventricle of the heart.

These lungs are in the thoracic cavity of a dog. They are sponge-like because they are filled with millions of air-filled pockets where carbon dioxide and oxygen exchange. Normally they are much more inflated than this, but are collapsed because the thoracic cavity is open. They have a tremendous blood supply within them, which is needed for the gas exchange that occurs here.

The large veins that drain the freshly oxygenated blood from the lungs back to the heart (the left atrium) are visible in this picture as the three vertical and blue colored vessels.

Lungs and blood vessels in a dog's chest

The size of these blue veins bringing blood back to the heart attest to how much blood flows through them

The lungs have an extensive network of passages and blood vessels. This barium radiograph illustrates just some of those breathing passages. The barium outlines 3  major bronchi (not the 3 major blood vessels above), the large white breathing passages below. All the fainter whitish area in the lungs below are barium in the small bronchi and alveoli. You can see how extensive this network is.

Lungs filled with barium

The dark cylinder from the left is the trachea (windpipe), the white area are the mainstem bronchi. The rest are the multitudes of small vessels in the lungs. 

Since we are an exotics practice, we thought it might be fun to show you the air filled lungs of a bearded dragon.

The heart is the purplish and round structure to the upper left of the picture. The lung is the air filled and mesh appearing structure in the middle of the picture.

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Heart Anatomy & Physiology

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