Overview of the Cardiovascular System

The cardiovascular system involves the heart, and the blood vessels. There are different types of blood vessels:

• Artery – transports blood from the heart to the tissues and organs. Each time the heart beats the blood gets pumped through the arteries in the body. If the artery is palpated then a pulse can be felt each time the heart beats.
• Arteriole – connects the arteries with the capillaries.
• Capillary – numerous small vessels within the tissues. Capillaries have thin walls to enable the exchange of substances between the blood (in the capillary) and the cells of the tissue. Substances that are exchanged include oxygen, nutrients and waste products.
• Venule – connects the capillaries with the veins.
• Vein – returns the blood from the tissues back to the heart. No pulse can be felt in veins, as the pulse is diffused in the network of small capillaries. Instead veins have valves to keep blood flowing in one direction. The valves allow blood to pass through them, but prevent blood from flowing backwards. In this way the blood is directed back to the heart.

Heart Anatomy

There are 4 chambers in the heart: the left and right atrium, and left and right ventricle. Each chamber has a valve allowing blood to pass through, but preventing it from flowing backwards. On the left side there is the mitral valve and aortic valve, and on the right side there is the tricuspid valve and pulmonary valve.

The Cardiac Cycle

The cardiac cycle begins with all four chambers relaxed. Blood flows into the atria, and then passively from the atria into the ventricles. The next stage is the contraction of the atria, causing additional blood to be pumped from the atria into the ventricles. The next stage is the contraction of the ventricles, causing blood to be ejected from the heart. Then, the ventricles relax and the cycle repeats itself.

A stethoscope can be used to listen to the heart sounds (this is called auscultation). The first heart sound (called S1) is produced when the mitral and tricuspid valves close (at the beginning of ventricular contraction). The second heart sound (called S2) is produced when the aortic and pulmonary valves close (at the beginning of ventricular relaxation). Abnormal heart sounds may be present in various cardiac diseases.

The Circulatory System

The function of the heart is to pump blood through the network of blood vessels, which is referred to as the circulatory system.

The circulatory system is split into two separate systems. There is the circulation going to the lungs, called the pulmonary circulation; and there is the circulation going to the rest of the body’s organs, called the systemic circulation.

Systemic Circulation
The pulmonary veins empty the blood into the left atrium. As the atrium contracts, the blood passes into the left ventricle. As the ventricle contracts, the blood is pumped into the aorta. The aorta branches into smaller arteries which transport the blood to all the tissues and organs (except the lungs) in the body.

In the tissues and organs, the arteries eventually join arterioles that join to capillaries. Oxygen and nutrients are transferred from the blood, through the capillary walls, to the tissue cells. At the same time carbon dioxide and other waste products are transferred from the cells, through the capillary walls, to the blood.

The capillaries then join venules that join to veins, which transport the blood back to the heart. The veins join the inferior and superior vena cava, which transport the blood to the right atrium of the heart. 

Pulmonary Circulation
The inferior vena cava and superior vena cava empties the blood into the right atrium. As the atrium contracts, the blood passes into the right ventricle. As the ventricle contracts, the blood is pumped into the pulmonary artery. The pulmonary artery transports the blood to the lungs.

The pulmonary artery splits into the left and right pulmonary arteries, which then branch into smaller and smaller arteries, until they eventually join arterioles that join to capillaries. A network of capillaries covers each alveolus. Gas exchange takes place between the alveoli and the capillaries. Oxygen is transferred from the alveolus, through the capillary wall, to the blood; and carbon dioxide is transferred from the blood, through the capillary wall, to the alveolus.

The capillaries then join venules that join to veins, which transport the blood back to the heart. The pulmonary veins enter the left atrium of the heart, to begin the process again.