Blood circulation

 Process of blood circulation

Glucose and oxygen are the main components to produce energy in the body. Blood is the transport medium of both the above components to the cells and the waste out of the cells.

Blood is a special connective tissue. It is a red colour fluid. When blood is centrifuged and kept aside, there will be two different layers. The dark red layer consists of blood corpuscles while the pale yellow layer contains the plasma. On this basis, blood which is seen as a homogeneous fluid, contains a plasma and a suspension of corpuscles. When a slide with a blood smear observed through the microscope there will be several types of corpuscles in it.

Red Blood cells (Erythrocytes)

One cubic millimetre of human blood contain about five million of red blood cells. These red coloured and biconcave disc-like cells are clearly visible among the other corpuscles. They form in red bone marrow. The life span of RBC is about four months (120 days). Absence of nucleus in red blood cells provides a large surface area to absorb more oxygen. A pigment called haemoglobin is present in red blood cells. Haemoglobin binds with oxygen and form oxyhaemoglobin to transport oxygen to cells.

 White Blood cells

A type of corpuscle, larger than the size of red blood cells, but smaller in number is present in blood. They are with nuclei and form in bone marrow. They are colourless and known as white blood cells. The ratio between red blood cells to white blood cells is 600:1

Two Types of WBC present in blood

• Granulocytes 
• Non-granulocytes

 Granulocytes are further divided into three types

• Neutrophils
• Eosinophils
• Basophils

Non-granulocytes are in two types

• Lymphocytes
• Monocytes

One cubic millimeter (1 mm3 ) of human blood contains 4 000 - 11 000 number of WBC.

WBC destroy infectious particles that enter the body by phagocytosis. Therefore percentages of WBC increase above the normal levels in microbial infections. Investigation of WBC counts in blood helps to diagnose diseases.

 The function of WBC is to protect the body from infectious particles that enter the body. This is done by phagocytosis and by producing antibodies.

Platelets

In addition to RBC and WBC there are fragments of cells that cannot be considered as cells in human blood. These corpuscles without nuclei are known as platelets. One cubic milimetre of blood contains 150 000-400 000 platelets. They form in bone marrow. Life span of platelets is approximately 5-7 days. Due to diseases like Dengue and Leptospirosis, platelet count drops drastically. Platelets contain thromboplastin which help in coagulation of blood.

Blood plasma

92% of blood plasma is water. Other than water the second most abundant compound is protein. Nutrients, nitrogenous waste, hormones, enzymes, gases and ions are present in blood plasma.

Function of blood

Transportation of materials (digested end products, respiratory gases, excretory byproducts, hormones, mineral ions and proteins)

Protect body against pathogenic microbes by phagocytosis and by producing antibodies.

Maintenance of chemical coordination and homeostatis among tissues and organs

Blood Circulation

Arterial system consists of all the arteries in the blood circulatory system. It transports oxygenated blood. But pulmonary artery transports deoxygenated blood to the lungs. Venous system consists of all the veins in the blood circulatory system. It transports deoxygenated blood.

But pulmonary veins transport oxygenated blood from lungs to the left atrium.

Blood circulation of human

Double blood circulation

The circulation where blood flows through lungs is known as pulmonary circulation. The circulation where blood flows through the rest of other organs is known as systemic circulation. Right ventricle of the heart acts as the pump for the pulmonary circulation, and left ventricle for the systemic circulation. So it is clear that blood flows twice through heart before entering into systemic artery. In human, when the blood circulates once through the body it flows twice through the heart. This is called as double circulation. 

Heart beat

Atria and Ventricles of heart contract to pump blood out of the heart. These contractions and dilations of heart muscle are known as heart beat. The heart beat rate of a healthy person at rest, is 72 beats per minute. Pulse rate is also similar to heart beat rate.

Cardiac cycle

In one heart beat atria contract when ventricles dilate. Next ventricles contract, atria dilate. Contraction of atria is known as diastole (0.1 seconds) whereas contraction of ventricles is known as systole (0.3 seconds). After that atria and ventricles are in relax mode and it is known as intervening (0.4 seconds).

Cardiac cycle refers to a complete heart beat from its generation to the beginning of the next beat. The stages of cardiac cycle are as follows

1. Diastole - Atrial contraction
2. Systole - Ventricular contraction
3. Intervening - Atrial and Ventricular relaxation (complete cardiac diastole)