Blood Fluid & Their Circulation

In unicellular organisms, a single cell carries out all the life processes as the cell itself is the organism. In advanced form like the few-celled algae, protozoa, sponges, etc., the size of the organism ensures that all the cells are not very far from each other. The uptake of materials from the environment is through the general body surface and the transport within the cells is by diffusion. However, more advanced multicellular forms need a transportation mechanism. The sites of absorption and synthesis are very specific and are separated by a greater distance from the other parts of the body.

The actual movement of materials into the individual cells is by diffusion, osmosis or active transport.

1.0Transportation In Human Beings

The process of transporting the absorbed food, water and waste products from one place to another in the body is called circulation.

Blood circulation was explained by William Harvey.

In human beings, the circulatory (transport) system is divided into two system:

(i) Blood Circulatory System

(a) Blood

(b) Blood vessels

(c) Heart

(ii) Lymphatic System

(a) Lymph

(b) Lymph vessels

(c) Lymph nodes

2.0Cardiovascular System or Blood Circulatory System 

The circulatory system includes : Circulation of blood throughout the body is driven by the pumping action of heart- There are two main circulatory routes.  

• Pulmonary circulation (lungs) : The circulation is in between heart and lungs. 
• Systemic circulation (the rest of the body) : The circulation is in between heart and remaining parts of the body.

Note : The circulation of blood in the blood vessels present in walls of heart is known as Coronary circulation.

Functions of the Circulatory System:

(i)  Transport of nutrients to the tissues for their utilization.

(ii) Transport of respiratory gases (O₂ and CO₂) to the cells and from the cells.

(iii) Collection of metabolic wastes from different tissues and their transportation to excretory organs for their removal.

(iv) Transport of hormones from endocrine glands to target organs.

(v) Protection of body by destroying pathogens.

(vi) Uniform distribution of heat in the body.

(vii) Formation of clots to prevent blood loss.

3.0Blood

It is the special connective tissue consisting of a fluid matrix, plasma, formed elements. It is also called 'River of life'.

Composition of Blood 

Red Blood Cells/Erythrocytes

Red blood cells, also called RBCs, have iron containing red coloured pigment known as haemoglobin that combines with oxygen and transports it to all parts of the body. Each RBC is circular, biconcave and enucleated (devoid of nucleus) in most of the mammals. 

The presence of haemoglobin gives red colour to blood. On an average, there are 5 millions to 5.5 millions of RBCs per cubic mm of blood.

RBCs are formed in the red bone marrow in the adults. A healthy individual has 12–16 gms of haemoglobin in every 100 ml of blood. These molecules play a significant role in transport of respiratory gases. RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs). 

White Blood Cells/Leucocytes 

WBC are colourless due to the lack of haemoglobin. They are relatively lesser in number which averages 6000-8000 mm^–3 of blood. Leucocytes are generally short lived. 

White blood cells are rounded or irregular cells and are nucleated. White blood cells, or WBCs, defend the body against infections.

WBC's are of two main types on the basis of presence or absence of granules in cytoplasm and lobed or non-lobed nucleus.

Granulocytes

Neutrophils: 60-65% of the total WBCs 

Neutrophils are most abundant cells of the total WBCs

Function: Phagocytic in action.

Eosinophils: 2-3%

Function : Produces anti-toxins to fight the effect of histamines in allergic reactions. Destroys certain parasitic worms.

Basophils: 0.5-1.0% (least)

Function: Secrete Histamine, Serotonin and Heparin etc and are involved in inflammatory reactions.

Agranulocytes 

Lymphocytes: 20-25%

Function: B and T lymphocytes are responsible for immune response of the body. B-Cells are responsible for antibody production.

Monocytes: 6-8%

Function: Phagocytic in action (after transforming into wandering macrophages).

4.0Platelets/Thrombocytes

Platelets are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1, 50, 000-3, 50, 000 platelets mm^–3 . 

When a blood vessel is cut, blood comes out immediately. After some time, a dark red clot is formed on the cut, which stops further oozing of blood from the damaged blood vessel. This is formed by cells called platelets in blood. 

When the platelet count decreases, excessive bleeding takes place which may even cause death.

Diagrammatic representation of formed elements in blood

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