Organs of Human Circulatory System

The circulatory system consists of the following parts:

  1. Heart, the central pumping organ
  2. Blood vessels, the connecting tubes: arteries, veins and capillaries.
  3. Blood, the circulating fluid, a connective tissue made of a fluid matrix and cells.
  4. Lymphatic system comprises of lymph nodes and lymph vessels.

1. Human Heart

It is a muscular organ made of cardiac muscle fibres. It is able to perform its function by coordinating contraction and, relaxation and opening and closing of a number of valves present inside the heart. This fist sized organ consists of 4 chambers, the two upper chambers - the atria and two lower chambers - the ventricles. Ventricles have thick muscular walls for pumping blood to longer distances. Heart is covered by a membrane - the pericardium.

2. Blood Vessels

The tubes transporting blood are called Blood Vessels. The wall of a blood vessel has three layers - tunica externa, tunica media and tunica interna. There are 3 kinds of blood vessels:

  1. Artery
  2. Capillary
  3. Vein

Artery transports blood away from the heart. Capillaries link arteries to veins for exchange of material between blood and tissues which also have capillaries. Veins are the site of transport of blood towards the heart.

Arteries divide into Arterioles and then into Capillaries. This way they come in contact with all the tissues and bathe the cells with blood plasma.

3. Blood

Blood is a red coloured, thick and slightly alkaline, fluid which keeps circulating in our body through the blood vessels. It transports substances in the body such as oxygen, nutrients, and hormones. It also carries waste to the kidney. It protects body against disease. It maintains normal body temperature.

Blood is a fluid connective tissue made of plasma and blood cells. Plasma is a pale yellow liquid consisting of blood proteins like albumin, globulin and fibrinogen. The cells of blood are Red Blood Corpuscles (RBC) and White Blood Corpuscles (WBC) and cell fragments, the Platelets. Blood cells are formed in the bone marrow.

Blood Group

The blood of an individual may belong to any one of the four blood groups, A, B, AB, and O. Blood group remains constant throughout lifetime as it is genetically controlled and is inherited from parents. These blood groups are due to the presence of special proteins present on the membrane of RBCs termed as antigens. Antigens present could be A, B both A and B or no Antigen may be present.

Blood Transfusion

When excessive blood is lost from the body either due to an accident, hemorrhage or during surgery (operation), doctors transfer blood from a healthy person (Donor) to the patient (Recipient). This is called Blood Transfusion.

Rh Factor

Presence or absence of another blood protein in addition of ABO antigens makes a person Rh+ or Rh–.

Blood Pressure

The flow of the blood in the arteries exerts a pressure on their elastic walls. This pressure is called blood pressure. The pressure of blood at the time of ventricular contraction is higher and is called systolic pressure. When ventricles are relaxed and are being filled by blood, there is a drop in pressure. This lower pressure is called diastolic pressure. These two pressures can be measured in the arteries of the arms.

The device used for measuring blood pressure is called Sphygmomanometer. A reading of 120/75 means that the person’s systolic pressure is 120 mm of mercury and diastolic pressure is 75 mm of mercury.

The difference between diastolic and systolic pressure can be felt as a throb in the arteries of the wrist. This throb at the wrist is called Pulse. The number of throbs felt at a particular point on the wrist (due to systole) per minute is called Pulse Rate. It is equal to the number of heart beats i.e. around 70 beats per minute for a normal adult.

4. Lymphatic System

The body has the presence of two kinds of circulating fluids - blood and lymph. Lymph remains unnoticed even if it oozes out at any point of injury because it is colourless.

The lymphatic system consists of a large number of lymph ducts, lymph nodes and lymph vessels. It lacks a pumping mechanism. Fluid is pushed by muscle movement.

The lymph nodes are scattered throughout the body. They are more concentrated in the neck, armpits and groins.