ICSE Class 8 Biology Oxford Chapter 01 Life Processes

Life Processes: Transport of Food and Minerals

Learning Outcomes

Circulatory system in humans: blood, heart, lymph, and blood vessels

Blood transfusion and blood groups

Pulse

Plants: absorption, conduction, rise of cell sap, and transpiration

Introduction

As we had discussed in class 7, our bodies require energy for carrying out various activities, such as those shown in pictures (a) to (c) above. Cells in our bodies release energy from food in the presence of oxygen through a process called respiration. For respiration to occur, food and oxygen must first be transported to different cells of the body. Do you know how this happens? How do food and oxygen reach, say, the cells in your toes?

A special organ system, called the circulatory system, is responsible for transporting materials such as food and oxygen from one part of the body to another. The circulatory system consists of the following parts:

(a) blood and lymph,

(b) heart, and

(c) pipelines or vessels that carry blood and lymph from place to place.

In this chapter we will learn about different parts of the circulatory system.

Circulatory System

Blood and lymph

Heart

Blood vessels

Arteries

Veins

Capillaries

Blood and Lymph

These are fluids that transport materials to different parts of the body.

Blood

Blood is a complex liquid tissue that transports nutrients, respiratory gases, metabolic wastes, and other substances from one part of the body to another. It is not a homogeneous red liquid, as it appears to the naked eye. It consists of a straw-coloured watery part called the plasma (which makes about 55% of total blood volume) and the blood cells (which make about 45% of total blood volume). Water makes up 92% of the plasma, while the remaining 8% consists of salts, proteins, simple sugars, and fats.

Blood cells are of three types: red blood cells, white blood cells, and platelets (Fig. 1.1).

Red blood cells (RBCs) or erythrocytes are shaped like biconcave discs. Mature RBCs lack nuclei. A cubic millimetre of blood contains about 5 million of them. These cells carry an iron-containing respiratory pigment called haemoglobin that plays an important role in transporting oxygen from the lungs to different cells of the body. RBCs are formed in the bone marrow. Worn-out RBCs are destroyed in the liver and the spleen. The normal life span of RBCs is about 120 days.

White blood cells (WBCs) or leukocytes are of different types. They are larger than red blood cells and have large nuclei that are often lobed. The average WBC count varies from 6000 to 9000 per cubic millimetre. Unlike RBCs, these cells carry no pigment. WBCs are shapeless and are able to move on their own through intercellular spaces. They protect the body from disease-causing organisms. WBCs are produced chiefly in the bone marrow, lymph glands, and the spleen.

Blood platelets or thrombocytes are non-nucleated spherical or irregular bodies that are much smaller than RBCs. They are formed in the bone marrow. If blood vessels are damaged (e.g., when one suffers a cut or a wound), platelets prevent the loss of blood by forming a clot over the wound.

Find Out

With the help of books/internet, collect information on different types of white blood cells. Prepare a report based on your findings. Your report should include the name and function of each kind of white blood cell.

Functions of Blood

Blood is concerned with transportation, defense, and regulation of body functions. Let us examine each of these functions in detail.

Transportation

1. Blood transports respiratory gases: oxygen (from the lungs to the tissues as oxy-haemoglobin) and carbon dioxide (from the tissues back to the lungs in the form of carbonic acid).

2. It absorbs nutrients from the intestine and distributes them to cells in other parts of the body.

3. It drains out waste products from cells and transports them to the excretory organs for their removal from the body.

4. It transports the chemical messengers or hormones from the place where they are produced to the target organ or organs.

Defense mechanisms

1. Certain white blood cells help in preventing infections by destroying disease-causing microorganisms that enter the body.

2. Platelets prevent loss of blood from a wound by forming a clot over it.

Regulatory function

1. Blood maintains water content of the tissues and helps in the regulation of the fluid content in different tissues and organs of the body.

2. Blood regulates the acid-base balance of the body.

3. Water has favourable heat absorbing and cooling properties. Blood regulates body temperature because of its high water content.

Functions of Blood Cells

Red blood cells

Carry oxygen from lungs to tissues

Carry carbon dioxide from tissues to lungs

White blood cells

Protect the body by destroying disease-causing microorganisms

Platelets

Prevent loss of blood by forming a clot over wounds

Write the function of the blood cells shown in the picture given alongside

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.

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Blood Transfusion and Blood Groups

In certain cases it becomes necessary to inject blood taken from a healthy person into the body of another person (e.g., in case of severe accidents, during surgery, or in certain rare diseases). Transfer of blood from one person to another is termed blood transfusion. The person giving the blood is called the donor and the one receiving it is called the recipient.

Before a blood transfusion is done, it is checked whether the blood of a particular donor can be given to a particular recipient. This is usually done by matching their blood groups. Blood groups are of four major types: A, B, AB, and O. The following criteria are used to distinguish between different blood groups.

1. There are substances called antigens attached to a person's RBCs. Members of different blood groups are characterized by the presence or absence of certain specific antigens on their RBCs. Members of the A group have antigen A on their RBCs. Members of the B group have antigen B on their RBCs. Members of the AB group have both A and B antigens present on their RBCs, while members of the O group lack both A and B antigens.

2. There are substances called antibodies in the plasma. Members of the A and B blood groups have specific antibodies in their blood plasma. The antibodies in A group plasma are called 'anti-B' because they cause B group RBCs to clump or stick to each other. Similarly, 'anti-A' antibodies is present in the B group plasma, which cause A-group red cells to clump. Members of the AB group have no antibodies in their plasma, whereas those of the O group have both anti-A and anti-B antibodies (Table 1.1).

Based on the reaction they cause when mixed, two blood groups are said to be compatible (no clumping of RBCs when mixed) or incompatible (clumping of RBCs when mixed). Before a blood transfusion is done, it is important to ascertain that the blood groups of the donor and the recipient are compatible. Otherwise, the donor's RBCs may clump and block the narrow blood vessels (capillaries) of the recipient, which could be fatal to the recipient.

Blood group compatibility is summarized in Table 1.2. From this table, it is clear that blood of A-group can only be given to a person with A and AB groups. Similarly, blood of B-group can only be given to a person with B and AB groups. Blood of AB-group can only be given to AB-group. However, a person with AB blood group can receive blood of any group and, hence, is referred to as a universal recipient. Blood of O group can be given to all the groups and, hence, a person with O-group is referred to as a universal donor.

Lymph

As blood flows through the capillaries, water and dissolved substances, along with a few white blood cells (but no red blood cells), pass through the capillary walls into the intercellular spaces (i.e., the spaces between cells). This fluid is called tissue fluid. Some tissue fluid enters tiny vessels called lymph capillaries, which go on to form lymph vessels. Lymph vessels, together with small sac-like organs called lymph nodes, form the lymphatic system. The colourless fluid that flows within the lymphatic system is called lymph. Lymph contains more fatty substances and white blood cells than tissue fluid. Lymph vessels from different parts of the body join to form large tubes or ducts. The main lymph duct is called the thoracic duct. It carries lymph to the shoulder region where it is poured into the left shoulder vein, which brings it back to the blood.

Functions: White blood cells in the lymphatic system protect the body against diseases. The lymphatic system also carries excess tissue fluid back to the blood.

Note: + means compatible; - means incompatible

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