ICSE Class 10 Biology Chapter 07 The Circulatory System

The Circulatory System

Syllabus

Circulatory System: Main features; the structure and working of the heart, blood vessels, structure and functions of blood and circulation of blood (only names of the main blood vessels entering and leaving the heart, liver and kidney will be required).

Scope of Syllabus

Composition of blood (Structure and functions of RBC, WBC and platelets). Brief idea of tissue fluid and lymph. Increase in efficiency of mammalian red blood cells due to absence of certain organelles should be explained with reasons. A brief idea of blood coagulation. Structure of vein, artery and capillary should be explained with the help of diagrams to bring out clearly the relationship between their structure and function. ABO blood group system, Rh factor; concept of double circulation; concept of systole and diastole; blood pressure. Reference to portal system should be made. Working of the heart along with names of the main blood vessels entering and leaving the heart, the liver and the kidney must be taught. Examination of a blood smear under a microscope.

Introduction to Body Fluids

Almost all organisms, including humans, have some kinds of fluids circulating in their bodies. Such fluids constitute the distributing system (to supply substances) as well as a collecting system (to pick up substances) to and from the various parts of the body including the remotest cell. In this chapter, we will study about the circulating fluids in our body, its composition and function.

7A. Body Fluids

7.1 Need For Transport Inside The Body

Every organ in our body requires the involvement of the circulating body fluids. For example:

The digestive system digests and absorbs nutrients which are needed to be transported to every body cell.

The respiratory system draws in air, and the oxygen picked up from it in the lungs has to be transported to all parts of the body. Similarly, the CO2 collected from the entire body has to be carried to the lungs for giving out.

All the extra water, excess salts, and the nitrogenous wastes such as urea have to be removed from different parts and have to be sent to the excretory system to be thrown out of the body.

Hormones secreted by the endocrine system have to be carried throughout the body by the circulating blood to act wherever required, and so on.

All such functions that need transport are performed by the two circulating fluids - the blood and the lymph. Besides transport, these fluids have some other functions also.

7.2 Fluids In Our Body

There are three principal fluids in our body:

Blood, contained in the heart and in the blood vessels (arteries, veins and capillaries) of the circulatory system.

Tissue fluid, occupying spaces between cells in the organs, and

Lymph, which is contained within lymph vessels and lymphatic organs such as the spleen and the tonsils.

Fig. 7.1 shows a diagrammatic representation of the relationship between blood, tissue fluid and lymph as they circulate in their respective vessels or spaces between the cells in different organs. It is clear from the diagram that blood in our body circulates in a closed manner i.e. all the time through blood vessels.

Such a type of blood circulation is called a closed vascular system. As against this, in certain animals such as insects, the blood mostly flows through open spaces, called an open blood circulatory system. Their blood flows from the heart to body tissue spaces without vessels.

There are also some other fluids located in particular organs such as synovial fluid filled in the cavities of skeletal joints, vitreous humour in the eye, etc. They do not circulate.

7.3 The Blood

Never Stationary. Blood is always in motion from the heart to the arteries and back through the veins.

Colour. The blood is a somewhat thick fluid, bright red when taken from an artery or dark red when taken from a vein. Conventionally, in the diagram, we show the veins in blue - this is just for contrast; the blood inside the veins is not blue, it is actually dark red.

Volume. By average an adult human contains 5 to 6 litres of blood by volume in his body.

Taste - saltish. Perhaps we have all "tasted" our blood as in the case when there is a cut in the tongue or bleeding from the gums. The blood is slightly alkaline with a pH of 7.3 to 7.45 (7 is neutral, neither acidic nor alkaline).

7.4 Functions Of Blood

The main functions of the blood in our body can be treated under two broad headings: (A) Transport and (B) Protection.

A. Transport By Blood

Transport of digested food from the alimentary canal to the tissues. These substances are simple sugars like glucose, amino acids, vitamins, mineral salts, etc.

Transport of oxygen from the lungs to the tissues. It occurs by means of red blood cells in combination with haemoglobin in the form of an unstable compound oxyhemoglobin, which on reaching the tissues breaks up to deliver oxygen.

\[\text{Hb} + \text{O}_2 \rightleftharpoons \text{HbO}_2\] oxyhemoglobin

Transport of carbon dioxide from the tissues to the lungs. It occurs partly in combination with hemoglobin and partly as solution in blood plasma.

\[\text{Hb} + \text{CO}_2 \rightleftharpoons \text{HbCO}_2\] carbaminohemoglobin

Transport of excretory material from the tissues to the liver, kidney or the skin for elimination or to render them harmless.

Distribution of hormones secreted by special glands (endocrine glands) directly into the blood.

Distribution of heat. The blood helps in keeping the temperature of the body uniform by distributing heat.

B. Protection By Blood

Blood forms a clot wherever there is a cut in a blood vessel. The clot serves to prevent (i) further loss of blood and (ii) the entry of disease-causing germs.

Its white blood corpuscles protect the body from diseases by engulfing bacteria which may have entered the body.

It produces antitoxins and antibodies which neutralise the poisonous substances or kill the germs which enter the body.

Progress Check

Name the two fluids that circulate in the body.

In the diagrams we conventionally show veins in blue. What is the actual colour of the blood flowing through the following? (i) In arteries (ii) In veins (iii) In capillaries

In a coloured diagram why do we generally show the pulmonary artery in blue and pulmonary vein in red colour?

Name any four substances transported by blood.

7.5 Composition Of Blood

The blood consists of:

Plasma - fluid part, constitutes 55-60 per cent of blood.

Cellular elements - red and white cells, and platelets, 40-45 per cent of blood.

7.5.1 Plasma - The Liquid Portion Of Blood

The plasma is a light-yellow coloured, alkaline liquid. It mainly consists of:

The inorganic salts include mainly sodium chloride and sodium bicarbonate. Among other substances contained in the plasma are glucose, amino acids, fibrinogen, hormones, urea, etc.

The plasma from which the protein fibrinogen has been removed is called serum.

7.5.2 Cellular Elements

The formed or cellular elements (i.e. shaped structures visible under magnification) of the blood are of three categories:

Red blood cells (erythrocytes),

White blood cells (leukocytes),

Blood platelets (thrombocytes)

Red Blood Cells (Rbcs) - The Oxygen Carriers

Red blood cells are also called erythrocytes (erythros: red).

These are minute biconcave disc-like structures, flat in the centre and thick and rounded at the periphery.

These are very small - about 7 micron in diameter (1 micron = one-thousandth of a millimetre and is represented by the symbol "μ")

The small size plus the concavities on either side provide a large surface area which makes them very efficient in absorbing oxygen.

The small size enables the red blood cells to travel through very fine capillaries in the body where they have to travel in a single file.

An adult human male has about 5 million RBCs per cubic millimetre of blood and an adult female has slightly less about 4.5 million.

Haemoglobin - The Effective Chemical Constituent Of Rbcs

The red blood cells have a colourless spongy body or stroma which contains a respiratory pigment haemoglobin (Hb).

The haemoglobin is formed of an iron-containing part (haemin) and a protein (globin).

It has the ability to combine readily with oxygen to form oxyhemoglobin, an unstable compound which readily gives up oxygen to the needy tissues.

Haemoglobin can carry a very small quantity of carbon dioxide in the form of carbaminohemoglobin.

Is It Correct To Say That Veins Carry CO2 And No Oxygen?

NO!

Veins too carry oxygen but only a little less than the arteries!

Inside the lungs the haemoglobin (Hb) becomes 97-99 per cent saturated with oxygen. Inside the respiring tissues Hb releases only about 23% of its oxygen and the blood returning in veins still contains about 75% oxygen.

Carbon Monoxide Poisoning

Haemoglobin has very strong affinity for carbon monoxide forming a stable compound carboxyhemoglobin (HbCO). This cuts down the capacity of the blood of transporting oxygen, sometimes resulting in death. For example, carbon monoxide poisoning may cause death due to furnaces burning in small rooms without ventilation.

Life And Death Of Rbcs

In adults, the RBCs are produced in the marrow of long bones, especially in the ribs, breast bone and ilium of hip girdle.

In an embryo they are produced in the liver and spleen.

In children, the RBCs are produced in bone marrow of all bones until 5 years of age.

The mature red blood cells have no nuclei, but when they are being produced, they have one. As they mature, the nuclei are lost, i.e., they become enucleated.

The average life of an RBC is about 120 days.

The old and weak red cells are destroyed in the spleen, liver and bone marrow; their iron part is retained in the liver while the rest is excreted as a bile pigment (bilirubin).

In a normal adult, approximately 20,000,000 RBCs are destroyed every minute. In other words, about 1 per cent of the total erythrocytes in the body are destroyed everyday.

Deficient, But More Efficient!

Mammalian red blood cells

Mammalian red blood cells when mature circulate in the blood system and are devoid of certain organelles. They have:

- no nucleus

- no mitochondria

- no endoplasmic reticulum

Thus, though deficient of the above organelles, the mature red blood cells, in reality, are more efficient in carrying out their task of picking up and delivering oxygen. The factors making them more efficient in this work are as follows:

Loss of nucleus, makes the red cells biconcave, thus increasing their surface area volume ratio for absorbing more oxygen.

- Space in between increased

- More RBCs can be accommodated in the same space

Loss of mitochondria means that the red cells cannot use oxygen for themselves (cellular respiration). Thus all the oxygen, absorbed from the lungs, is transported and delivered to the tissues unconsumed. Secondly, loss of mitochondria means full transport of glucose in blood plasma, unused by the RBCs.

No endoplasmic reticulum means increased flexibility of RBCs for their movement through narrow capillaries.

More About The Number Of Rbcs

New born infants have a larger number of RBCs - about 6-7 million per cubic millimetre (1 mm3).

RBC count is lower by 5% during sleep.

RBC count is higher during physical activity, pregnancy and emotional upsets.

People living at a height of 4,200 m and above, increase their RBCs by nearly 30%.

Abnormally increased number of RBCs is called Polycythaemia, and their abnormally decreased number is known as Erythropenia.

White Blood Cells (Wbcs) - White Blood Cells, Or Leukocytes

White blood cells, or leukocytes (leuko: white), differ from red blood cells in having a nucleus and do not contain haemoglobin. Their number is much less, usually about 4000-8000 per mm3 of blood. Most WBCs are amoeboid and can produce pseudopodia with which they can squeeze through the walls of the capillaries into the tissues (diapedesis dia: across, pedesis: oozing out) (Fig. 7.3).

White Blood Cell Or White Cells?

Better term is "white cells" or Leucocytes (leukos: white) because they spend most of their time (90%) in tissue fluid or in the lymph and very little time (only about 10%) in the blood.

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