The Cardiovascular System

The cardiovascular system is responsible for transporting nutrients and removing gaseous waste from the body. This system is comprised of the heart and the circulatory system. Structures of the cardiovascular system include the heart, blood vessels, and blood. The lymphatic system is also closely associated with the cardiovascular system.

The Heart

The heart is a muscular pumping organ located medial to the lungs along the body’s mid-line in the thoracic region. The bottom tip of the heart, known as its apex, is turned to the left, so that about 2/3 of the heart is located on the body’s left side with the other 1/3 on right. The top of the heart, known as the heart’s base, connects to the great blood vessels of the body: the aorta, vena cava, pulmonary trunk, and pulmonary veins.


The heart sits within a fluid-filled cavity called the pericardial cavity. The walls and lining of the pericardial cavity are a special membrane known as the pericardium. Pericardium is a type of serous membrane that produces serous fluid to lubricate the heart and prevent friction between the ever beating heart and its surrounding organs. Besides lubrication, the pericardium serves to hold the heart in position and maintain a hollow space for the heart to expand into when it is full.

Structure of the Heart Wall

The heart wall is made of three layers: epicardium, myocardium and endocardium.


The epicardium is the outermost layer of the heart. Thus, the epicardium is a thin layer of serous membrane that helps to lubricate and protect the outside of the heart. Below the epicardium is the second, thicker layer of the heart wall: the myocardium.


The myocardium is the muscular middle layer of the heart wall that contains the cardiac muscle tissue. Myocardium makes up the majority of the thickness and mass of the heart wall and is the part of the heart responsible for pumping blood. Below the myocardium is the thin endocardium layer.


Endocardium is the simple squamous endothelium layer that lines the inside of the heart. The endocardium is very smooth and is responsible for keeping blood from sticking to the inside of the heart and forming potentially deadly blood clots.

The thickness of the heart wall varies in different parts of the heart. The atria of the heart have a very thin myocardium because they do not need to pump blood very far—only to the nearby ventricles. The ventricles, on the other hand, have a very thick myocardium to pump blood to the lungs or throughout the entire body. The right side of the heart has less myocardium in its walls than the left side because the left side has to pump blood through the entire body while the right side only has to pump to the lungs

Chambers of the Heart

The heart contains four chambers: the right atrium, left atrium, right ventricle, and left ventricle. The atria are smaller than the ventricles and have thinner, less muscular walls than the ventricles. The atria act as receiving chambers for blood, so they are connected to the veins that carry blood to the heart. The ventricles are the larger, stronger pumping chambers that send blood out of the heart. The ventricles are connected to the arteries that carry blood away from the heart.

Valves of the Heart

The heart functions by pumping blood both to the lungs and to the systems of the body. To prevent blood from flowing backwards, back into the heart, a system of one-way valves are present in the heart. The heart valves can be broken down into two types: atrioventricular and semilunar valves.

Atrioventricular valves

The atrioventricular (AV) valves are located in the middle of the heart between the atria and ventricles and only allow blood to flow from the atria into the ventricles. The AV valve on the right side of the heart is called the tricuspid valve because it is made of three cusps (flaps) that separate to allow blood to pass through and connect to block regurgitation of blood. The AV valve on the left side of the heart is called the mitral valve or the bicuspid valve because it has two cusps. The AV valves are attached on the ventricular side to tough strings called chordae tendineae.

Semilunar valves

The semilunar valves, so named for the crescent moon shape of their cusps, are located between the ventricles and the arteries that carry blood away from the heart. The semilunar valve on the right side of the heart is the pulmonary valve, so named because it prevents the back-flow of blood from the pulmonary trunk into the right ventricle. The semilunar valve on the left side of the heart is the aortic valve, named for the fact that it prevents the aorta from regurgitating blood back into the left ventricle. The semilunar valves are smaller than the AV valves and do not have chordae tendineae to hold them in place.

Blood Vessels

Blood vessels are intricate networks of hollow tubes that transport blood throughout the entire body. Blood travels from the heart via arteries to smaller arterioles, then to capillaries or sinusoids, to venules, to veins and back to the heart. Through the process of microcirculation, substances such as oxygen, carbon dioxide, nutrients, and wastes are exchanged between the blood and the fluid that surrounds cells.


The blood delivers nutrients to cells and removes wastes that are produced during cellular processes, such as cellular respiration. Blood is composed of red blood cells, white blood cells, platelets, and plasma. Red blood cells contain enormous amounts of a protein called haemoglobin. This iron-containing molecule binds oxygen as oxygen molecules enter blood vessels in the lungs and transport them to various parts of the body. After depositing oxygen to tissue and cells, red blood cells pick up carbon dioxide (CO2) for transportation to the lungs where CO2 is expelled from the body.

How Does Blood Flow Through the Heart?

The right and left sides of the heart work together. The pattern described below is continuously repeated, causing blood to flow continuously to the heart, lungs, and body.

Right Side of the Heart

• Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart.
• As the atrium contracts, blood flows from your right atrium into your right ventricle through the open tricuspid valve.
• When the ventricle is full, the tricuspid valve shuts. This prevents blood from flowing backward into the atria while the ventricle contracts.
• As the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

Left Side of the Heart

• The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium of the heart.
• As the atrium contracts, blood flows from your left atrium into your left ventricle through the open mitral valve.
• When the ventricle is full, the mitral valve shuts. This prevents blood from flowing backward into the atrium while the ventricle contracts.
• As the ventricle contracts, blood leaves the heart through the aortic valve, into the aorta and to the body

How does blood flow through the lungs?

Once blood travels through the pulmonic valve, it enters your lungs. This is called the pulmonary circulation. From your pulmonic valve, blood travels to the pulmonary artery to tiny capillary vessels in the lungs. Here, oxygen travels from the tiny air sacs in the lungs, through the walls of the capillaries, into the blood. At the same time, carbon dioxide, a waste product of metabolism, passes from the blood into the air sacs. Carbon dioxide leaves the body when you exhale. Once the blood is purified and oxygenated, it travels back to the left atrium through the pulmonary veins.

Components of blood

Red Blood cells (erythrocytes)

Disc-shaped cells, there are responsible for the carrying of oxygen to all working tissues. The oxygen binds to the haemoglobin in the red blood cells, and is transported in this manner. Carbon dioxide is returned from the working tissues, by the same path.

White Blood Cells (leukocytes)

Broken down into granular and agranular leukocytes, the primary function of these cells is the immune response. They provide what is required to battle infections, as well as inflammation, and work to repair any physical damage to tissues. They also break down foreign bodies, for them to be removed from tissues and blood.

Platelets (thrombocytes)

These serve to increase viscosity of the blood. As result of this function, they are used by the body to cause clotting, when combined with vitamin k. this happens when any damage to tissues occurs, and blood flow needs to be slowed or stopped.


Making up a very large percentage of the blood, its chief purpose is transport. Almost completely comprised of water; hormones and nutrients, as well as all our waste by-products can be found dissolved in the plasma. And, these are carried throughout the body in this manner.

Functions of the Cardiovascular System

  1. Transportation – the cardiovascular system is responsible for moving blood to all of the tissues of the body. In so doing, it transports fuels, nutrients and oxygen, as well as hormones, to the working tissues. It also removes waste byproducts.
  2. Regulation – maintenance of several homeostatic conditions is a responsibility of the CVS. Directing blood flow to the skin, or restricting it from the skin, will aid in temperature regulation. The pH balance is another example, where the basic solution of the blood can be a buffer against acidic byproducts of working tissues. Finally, the osmotic concentration of the body is balanced through the content of the blood plasma, allowing an isotonic environment to be maintained.
  3. Protection – the white blood cells are responsible for removal of debris, as well as fighting different pathogens introduced to the body. Also, stored antibodies can be sent to fight off previously encountered infections and inflammation. Finally, scabs are formed to restrict blood flow from leaking out of the body, due to injury.

To achieve these, the system follows two distinct loops, with the heart at the centre of each. These flow loops can be considered to be the pulmonary loop, and systemic loop.

The pulmonary loop takes deoxygenated blood from the heart, to the lungs, via pulmonary arteries, and returns oxygenated blood to the heart, via pulmonary veins.

The systemic loop takes oxygenated blood to all of the working cells of the body, via the aorta and its subsequent branches, and returns deoxygenated blood to the heart via the superior vena cava (from the upper body) and inferior vena cava (from the lower body). The vessels that carry the blood, in these loops, will be further discussed, a little later.

Adaptations to Physical Activity

Adaptations of the Heart

As mentioned, the heart is a pump. And, this pump is fully muscular. While highly specialized, this muscle still behaves like any other, in that it uses fuel to contract, and relaxes between contractions. It also creates waste, which must be removed. Another similarity to other muscle is that it adapts to exercise. And, just like other muscle, these adaptations are both acute, and chronic.

Acute adaptations are stroke rate, and stroke volume. In other words, how fast the heart beats (with we notice as the heart rate) and how much blood is pumped with each stroke (or beat). The chronic adaptation most notable is hypertrophy of the myocardium. In athletes, we notice this mostly around the left ventricle.

Heart Rate

In times when the body needs more blood, the rate increases. This can be for a number of reasons. Exercise, when the working muscles need blood for oxygen and fuel, as well as removal of waste, is one example. Similarly, after a meal the gastro-intestinal tract will require more blood, both for absorption of nutrients, as well as for the smooth muscle involved in peristalsis. Conversely, times of rest will lead to decreased heart rate. Relaxing on the couch, watching television, could be one example of this (provided one isn’t watching a closely contended sports game). But, the most rested a person gets is when they are asleep. As a result, this is also when the heart rate will be recorded at its lowest.

Heart rate is also one of the easiest ways to note adaptations in fitness. As we’ve mentioned already, the heart muscle becomes stronger, and better at pumping blood. This leads to more blood being pumped with each beat, leading to a decreased heart rate (if more blood is sent with each beat, fewer beat are required to send the same amount of blood). And, as will be shown below, as tissues become more efficient at using the blood pumped to them, the demand decreases. This compounds the decrease in heart rate, as now the amount of blood demanded by working tissues is less.

Blood Pressure

Blood pressure is the amount of force placed on the blood vessel walls by the blood inside it. However, when we measure blood pressure, we can only measure the pressure exerted by the blood vessels back onto the blood. Luckily, these have to be the same, or the vessels would burst. Blood pressure comes about because there is a continuous flow of blood, through the vessels. This flow, however, happens at different rates, depending on when the heart beats, and when it is as rest. This is why we get two readings for blood pressure. The systolic reading (top number) is when the heart beats, while the diastolic reading (bottom number) is when the heart is at rest, or between beats.

If the heart rate increases, the amount of blood being pumped through the blood vessels will obviously increase. But, the rate at which blood moves through the vessels is often not as fast as the increased heart rate. This leads to a slight back-log, for lack of a better description. In other words, there is now more blood in the vessels, than usual. As a result, there will be more pressure placed on the vessels walls, by the blood (blood pressure will increase). As the heart rate steadies, and decreases, the blood pressure will follow the same pattern. However, there is one small difference thrown into the mix, when exercise was the cause of the increased heart rate. After cessation of an exercise session, venous return is still increased. This means that blood is being removed from vessels at a faster rate. This, coupled with a slowing heart rate pumping less and less blood into the vessels, leads to a drop in blood pressure below the normal resting levels.

As with all other tissue, the heart requires blood flow. This is to supply it with nutrients, as well as remove waste. It has a collection of arteries and veins, known as coronary circulation. This is the same system worked on in bypass surgery, and which is the cause of conditions such as angina pectoris, and can lead to myocardial infraction (heart attacks).

Another potential chronic disease related to the heart is congestive heart disease. This is at the other end of the scale. As opposed to hypertrophy of the heart muscle, this is when the entire heart increases in size, within the sheath that encloses it (known as the pericardium). When this happens, the amount the heart can expand and contract is limited. As a result, the amount of blood it can pump is greatly restricted. At rest, this is usually not even noticeable. However, as soon as there is increased demand, the heart cannot meet this demand.

Adaptations of the Circulatory System

As mentioned earlier, blood is carried in circulatory loops. These loops consist of blood vessels which carry the blood to all of the tissues of the body, and decrease in size the further from the heart. Moving away from the heart, blood travels in arteries. These branch off into smaller arterioles, which then continue to branch off, and decrease in size, until the blood reaches the capillary beds, in the tissues. This is where the exchange of nutrients, water, waste etc. takes place. As blood travels back to the heart, the vessels do the opposite of the arteries. The smaller venules combine and increase in size, until they reach the veins, and eventually reach the heart. While the heart pumps blood along the arteries, the return to the heart is achieved through contraction of skeletal muscle. This squeezes the blood along the veins. To prevent back-flow, veins have flow valves, responsible for ensuring the blood only flows in the right direction.