Transportation

In unicellular organisms, a single cell carries out all the life processes as the cell itself is an organism. In advanced forms 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

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

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

Blood Circulatory System

(a) Blood

(b) Blood vessels

(c) Heart

Lymphatic System

(a) Lymph

(b) Lymph vessels

(c) Lymph nodes

(d) Lymphatic organs

2.0Blood Circulatory System

The circulatory system is the main transportation system of the human body.

The body has about 5 liters of blood continuously travelling through it by the way of the circulatory system. 

Blood

Blood is an important fluid connective tissue which transports the materials to different body

parts.

Composition of Blood

Blood is thicker and slightly heavier than water.  Its pH ranges from 7.35 to 7.45 (slightly alkaline). It consists of two parts, each with its own job in the body.

(i) Plasma

(ii) Cellular components

Plasma

Plasma is a liquid portion of blood. It transports food, CO₂ and nitrogenous wastes in dissolved form. It contains water (90-92%), protein (7% - albumin, globulin, fibrinogen), others (1% - hormones, wastes, gases, salts, ions).

Red Blood Cells (Erythrocytes)

It transports oxygen from lungs to all tissues of the body and returns CO₂ back to the lungs.

In humans, RBCs are circular, biconcave and enucleated. The red colour of RBC is due to the presence of haemoglobin.

White Blood Cells (Leukocytes)

It protects us against diseases & infections. WBCs are of two main types on the basis of presence or absence of granules in cytoplasm. It shows ameoboid movement. It can change its shape.

Platelets/Thrombocytes

Small plate shaped cells that cluster together to help in forming blood clots when bleeding occurs.

Functions of Blood 

• Transportation of oxygen from lungs to tissues.
• Transportation of carbon dioxide from the tissues to the lungs.
• Transportation of excretory materials from the tissues to the kidneys.
• Transportation of digested food from the small intestine to the tissues. 
• Distribution of hormones and enzymes. 
• Formation of clots to prevent blood loss.
• Distribution of heat and temperature control. Prevention from infections and helps in wound healing.

Blood clotting or Coagulation

When we are injured and start bleeding, the loss of blood from the system has to be minimized. In addition, leakage would lead to a loss of pressure which would reduce the efficiency of the pumping system. To avoid this, the blood has platelets which circulate around the body and plug these leaks by helping to clot the blood at these points of injury. 

Blood Clot

Injured tissue + blood platelets releases thrombokinase.

$Prothrombin\underset{C{a}^{++}}{\overset{\text{thrombokinase}}{\to }}Thrombin$

$Fibrinogen\stackrel{\text{Thrombin}}{\to }fibrin$

Fibrin + Dead RBC, WBC and Platelets → Blood clot

Prothrombin and fibrinogen are plasma protein synthesized in liver and are present in plasma.

Serum is a watery fluid expelled from a contracting blood clot.

Serum = plasma – Clotting factors.

Blood Vessels 

In human, three types of blood vessels are present.

(i) Arteries

These vessels carry blood from heart to various organs of the body. The walls of the arteries are thick and elastic. 

(ii) Veins

They collect the blood from different parts of the body and pour it into the heart. The walls of veins are thin and less elastic. They do not need thick walls because the blood is no longer under pressure instead they have valves that ensure that the blood flows only in one direction.

(iii) Capillaries

These are smallest blood vessels and one-cell thick. The walls of capillaries are very thin. Exchange of materials between the blood and tissue cells take place at capillaries. Capillaries lack muscular wall and is lined by simple squamous epithelium (endothelium).

The major differences between various blood vessels have been given in table.