Functions of the Circulatory System You've Always Wanted to Know

Circulatory System Functions
The human circulatory system functions through the heart, blood and a network of blood vessels, to enable the transport of various substances throughout the body. It is mainly concerned with the transport of nutrients, gases, blood cells, hormones and metabolites throughout the body. The following article gives an elaborate account of these functions.
Heart, blood and blood vessels are the three crucial components of the circulatory system. The major function of this organ system is to carry oxygen and nutrients to every cell and tissue of the body. It also transports carbon dioxide and waste materials from different tissues to the appropriate excretory organs. Each structural component plays a unique role in order to collectively fulfill these functions.
Heart is the central organ responsible for collecting oxygenated blood from the lungs, and pumping it throughout the body with the help of an intricate network of blood vessels. Blood, the fluid connective tissue of our body, is the main carrier of various important molecules, namely, oxygen, carbon dioxide, glucose, antibodies, urea, etc. It also serves as a medium for the transport and interaction of immune cells, and other components of the immune system.
To successfully perform these functions, the circulatory system has to work in close association with every other organ system of the body. Given below is a detailed account of the functions performed by each component of the circulatory system.
The human heart is a fist-sized organ contained inside a sac called pericardium. It is divided into four chambers - two upper chambers termed atria, and two lower chambers called ventricles. The structure of a human heart has been illustrated below.
human heart
1 - Superior vena cava
2 - Right atrium
3 - Tricuspid valve
4 - Right ventricle
5 - Pulmonary valve
6 - Pulmonary artery
7 - Pulmonary vein
8 - Left atrium
9 - Bicuspid valve
10 - Left ventricle
11 - Aortic valve
12 - Aorta
Through rhythmic contractions and relaxations of cardiac muscles, the two major functions performed by the heart are:
1] Collecting deoxygenated blood from the body, and pumping it to the lungs for oxygenation
This is achieved through the pulmonary circuit which includes the right atrium and ventricle. Deoxygenated blood enters the right atrium through the superior vena cava, and is pumped into the right ventricle through the tricuspid valve. Ventricular contraction pumps this blood to the lungs through the pulmonary valve and pulmonary arteries.
2] Receiving oxygenated blood from the lungs, and pumping it to the entire body
This is achieved through the systemic circuit which includes the left atrium and ventricle. Oxygenated blood from the lungs in received in the left atrium through pulmonary veins, and is pumped into the left ventricle through the bicuspid (mitral) valve. Ventricular contraction then pumps this oxygenated blood into the aorta through an aortic valve, and is supplied to the entire body.
streaming blood
Blood is a type of fluid connective tissue, and comprises an array of cells suspended in a viscous liquid called plasma. Plasma is made up 90-92% of water along with nutrients, salts, gases, hormones and proteins like signaling molecules, enzymes, antibodies, waste products, and other metabolites. Red blood cells, platelets and white blood cells form the cellular components of blood.
Blood serves as a transport medium for cells and molecules of the body. It transports:
  • Oxygen from the lungs to every single cell of the body
  • Carbon dioxide from each cell to the lungs
  • Nutrients and glucose from the digestive system to the liver and other organs
  • Metabolic wastes from different organs to the kidneys
  • Hormones from the endocrine glands to different effector organs
Apart from transportation, the components of blood play an important role in several homeostatic mechanisms including the regulation of body temperature, food and water balance, iron content, osmotic balance of cells, etc.
The white blood cells or leukocytes present in blood are involved in imparting immunity against various pathogens. These cells participate in the primary immune response of the body. Moreover, antibodies, cytokines, and other molecules of the immune system are also present in blood where they interact with foreign bodies, pathogens and target them for destruction by the immune cells.
Blood Vessels
Blood Vessels
Blood vessels serve as the means for accomplishing the various transport functions of blood. The network of blood vessels provides a route for blood to reach every organ of the body. Arteries, veins and capillaries are the main components of this network.
Arteries are the vessels involved in transport of oxygenated blood from the heart to different organs (exception: pulmonary artery), through its branches called arterioles.

Veins, in contrast, collect deoxygenated blood from the different organs, through branches termed venules, and carry it back to the heart (exception: pulmonary vein).

Capillaries are the thinnest blood vessels, and connect the venules and arterioles. It is the capillaries which serve as the actual site for exchange of nutrients between blood and tissues.

Arteries ►► Arterioles►► Capillaries ►►Venules ►► Veins

Thus, blood from the heart enters the arteries, and further flows into the arterioles and capillaries. Nutrient and gaseous exchange occurs here. This blood, now in a deoxygenated state, flows into the venules and finally enters the veins to be transported back to the heart, and then to the lungs.
Cell Mediated Immunity
The circulatory system is a complex organ system that plays a major role in supplying oxygen and nutrients to all the cells and tissues of the body, as also collecting carbon dioxide and waste materials from the cells. It is also involved in transporting hormones, distributing heat throughout the body, transporting immune cells and chemical messengers of the immune system. The structural and functional integrity of every component of the circulatory system is equally important for a healthy survival.