Human circulatory system mainly consists of a heart, a system of blood vessels, and blood. Other specialized organs, like lungs and kidneys play vital roles for the exchange of materials between blood and the external environment. It is also known as cardiovascular system.

What makes the heart such an important organ? How is heart connected to other constituents of the human circulatory system? What constitutes this system?

The cardiovascular system consists of systemic and lymphatic system. Systemic circulation is a major part of the circulatory system.

Invertebrates have an open circulatory system, in which fluid passes freely throughout the tissues or defined areas of tissue. All vertebrates, however, have a closed system consisting of blood vessels.

Components of Human Circulatory System

Blood

Blood is a liquid connective tissue that circulates in a closed system of blood vessels present in a body everywhere. Human blood consists of blood cells (RBCs and WBCs) and blood platelets, and liquid component—plasma.

The corpuscles comprise about 45% and the plasma about 55% of the blood volume. Plasma is a mixture of water, proteins, and dissolved substances (Figure 1).

Figure 1. A section of blood vessel showing the components of blood
Figure 1. A section of blood vessel showing the components of blood

Blood Vessels

Blood vessels are made of layers of connective tissue and muscle and they are lined with endothelium internally. Blood vessels form a complex network of hollow tubes present throughout the body that not only delivers oxygen and valuable nutrients to body cells but also removes waste from the cells.

There are four major types of blood vessels that are connected to each other forming one continuous closed system (Figure 4).

i. Arteries: These are wide, elastic and thick-walled vessels that carry blood away from the heart to organs and the limbs of the body. The largest artery is called the aorta. Arteries are further divided into small arterioles. Arteries are composed of three coats: tunica adventitia, tunica media, and tunica intima (Figure 2).

Figure 2. Anatomy of an artery
Figure 2. Anatomy of an artery

ii. Veins: These vessels bring back blood from the tissues and organs to the heart. Veins do not have thick walls as blood in veins flow under less pressure than that in arteries. However, veins can accommodate more blood. Several small branches that collect the blood from the cells and organs are called venules, which unite to form veins, which in return blood to the heart (Figure 3).

Similar to arteries, veins are also composed of three coats: tunica adventitia, tunica media, and tunica intima. Veins have valves that allow blood to flow only in one direction. These valves are semilunar, pocket-like folds which are formed by the folding of the intima layer of the veins.

Figure 3. Anatomy of an artery and a vein
Figure 3. Anatomy of an artery and a vein

iii. Capillaries: Arteries branch out into smaller and thinner blood vessels called arterioles. Arterioles further divide into smaller vessels to form capillaries, which provide blood to all the cells. The average diameter is 8 micrometers with a very thin wall of 0.2 micrometers. They are permeable and are the site for exchange of gases, nutrients and waste between circulation and body tissues.

iv. Sinusoids: Sinusoids are very small vessels located within the liver, spleen, and bone marrow.

Figure 4. Blood vessel types anatomical diagram
Figure 4. Blood vessel types anatomical diagram

Heart

A heart is a hollow, highly muscular structure located in the thoracic cavity above the diaphragm in between the two lungs. Heart is protected by rib cage. Heart is surrounded by two layered tissue membrane called pericardium. The space between the two layers is filled with fluid called pericardial fluid.

Functions of the circulatory system

The circulatory system performs the following functions.

  • It carries food substances to the body cells.
  • It carries absorbed oxygen to every cell of the body.
  • It brings back carbon dioxide to the lungs.
  • It removes harmful chemical substance from the body.

Table showing differences between artery, vein, and capillary

FeaturesArteryVeinCapillary
Structure of wallOuter coat is thin middle coat is thickOuter coat is thick middle coat is thinOuter coat is thick middle coat is thin
Wall layersTunica adventitia, tunica media, and tunica intimaTunica adventitia, tunica media, and tunica intimaTunica intima
LumenLargeSmallSmall
FunctionCarry blood away from heart and nutrients, oxygen to cells of the body

Carries oxygenated blood except pulmonary artery

Carry blood towards the heart and carbon dioxide away from the body tissues

Carries deoxygenated blood except pulmonary vein

Links artery to vein and helps in diffusion of gases and nutrients

Carries both oxygenated and deoxygenated blood

Semilunar valvesAbsentPresentAbsent
Pressure of blood flowHighLowLow

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