Talking strictly in the sense of cardiovascular system, the major parts are blood, blood channels, and heart, but considering lymphatic system as its component, lymph, lymph nodes and lymphatic vessels have to be included.

Blood

Forming 7% of human body weight and a type of special body fluid, blood is one of the most important parts of circulatory system. It serves as a medium for the transportation of circulatory and metabolic substances to every living cell in the body and brings back any waste materials produced therein including carbon dioxide and toxins.

Apart from its watery content, the circulatory fluid is composed of blood cells and plasma which contribute to the volume by 55%. Red blood cells, white blood cells and platelets constitute most of the remaining part of blood, but oxygen, carbon dioxide, nitrogen, chemicals, nutrients and metabolic wastes are always present in varying concentrations.

Red blood cells contain a globular protein, called hemoglobin that can bind oxygen on its surface and release it wherever it is needed. About one third of each red blood cell is composed of this oxygen binding protein the deficiency or destruction of which causes serious diseases, often leading to death.

White blood cells act as defense force of body against any attack from the foreign substances and disease-causing germs. Whenever a harmful microbe or substance enters the body, the armed forces of leukocytes immediately surround and kill or neutralize it.

Platelets (also called thrombocytes) are nucleus-less bodies that not only cause blood clotting but also serve as a natural factor for growth and contribute to the maintenance of homeostasis.

Chemical substances in the blood include hormones, amino acids, vitamins and electrolytes which perform various assigned tasks in the body, like growth, biological catalysis, synthesis of new cells and & products, and so on. Now, let’s have a look about the structure and functioning of other parts of circulatory system.

Blood Vessels

Vessels are those parts of circulatory system in humans through which the blood flows. Three types of circulatory circuits are formed by these tubes which include systemic, coronary and pulmonary systems.

The vessels that carry blood from heart and deliver it to various parts of the body are termed as arteries, while an extensive system of veins collects blood from all cells of body and carries it back to heart. Owing to the relative concentration of oxygen in blood, the vessels have two different colors and can easily be distinguished from each other.

Arteries contain oxygenated blood which makes them appear reddish, while veins are bluish in appearance because of deoxygenated blood flowing therein.

When they leave heart, the size of arteries and veins is larger which gradually decreases as they move away from it. Finally, they attain the size of extremely narrow tubes, called capillaries that are only 1 millimeter in diameter.

The two types of capillaries containing oxygenated and deoxygenated blood meet end-to-end with each other. At this point, arteries deliver oxygenated blood while veins carry the oxygen-depleted blood back to heart.

Containing clear body fluid, the lymph vessels, similarly, form a system of lymphatic canals and act as complementary to the overall performance of circulatory system.

After division and subdivision, they are reduced to the size of lymphatic capillaries which collect the interstitial fluid and transport it to lymph nodes where, after processing, it is mixed back with blood.

In most of the cases, oxygenated blood travels in arteries and deoxygenated blood flows through veins, but there are some exceptions, like pulmonary artery and pulmonary vein contain deoxygenated and oxygenated blood, respectively. Pulmonary artery transports oxygen lacking blood from heart to lungs while pulmonary vein brings the oxygen rich blood back to heart.

Heart

Among different parts of circulatory system, heart is considered the chief organ. Beating about 1.6 billion times in the average lifespan of a human being, it never gets tired or stops working for a moment till death.

This vital organ is made up of cardiac muscles which are specialized to perform continuous activity of repeated contractions and retractions. The hollow chambers are used to collect and pump the blood to all parts of the body.

The blood pumped by heart moves into two separate circuits; one cycle consists of pumping it to lungs and receiving back, while, in the other circuit, blood is pumped to and recollected from each organ of the body. In this process each cell of body is supplied with respiratory gases, nutrients and other metabolites, and disposes of its metabolic waste products.

Structure and Function

Pericardium is a double membrane structure that encloses heart and renders it flexibility and a sac-like shape. Two atria and the same number of ventricles form four inner cavities of the heart. Upper two chambers are atria while the lower ones are called ventricles which form the receiving and discharging pouches, respectively.

The right atrium is connected with right ventricle through a valve, called tricuspid valve which maintains the flow of blood in one direction, thus preventing its back flow. Likewise, the other AV (atrio-ventricular) valve regulates the flow of blood between left atrium and left ventricle. There are also two other valves involved in the double circulation system which are pulmonary and aortic ones.

Deoxygenated blood is collected from each and every cell of the body by an extensive network of veins, which is then transported to lungs for oxygenation. The blood thus received from the lungs is rich in its oxygen content that is then pumped and supplied to all the parts of body through another branching system of vessels called arteries.