13 The blood

Learning objectives

After reading this chapter you should understand:

• the composition of blood

• the formation of blood cells

• the formation of red blood cells

• the process of blood clotting

• the functions of blood

• blood groups and the Rhesus (Rh) factor

The circulatory system is the transport system of the body by which food, oxygen, water and all other essentials are carried to the tissue cells and their waste products are carried away. It consists of three parts:

1. the blood – the fluid in which materials are carried to and from the tissues

2. the heart – the driving force, which propels the blood

3. the blood vessels – the routes by which the blood travels to and through the tissues, and back to the heart.

The blood is a thick red fluid; it is bright red in the arteries, where it is oxygenated, and a dark purplish-red in the veins, where it is deoxygenated, having given up some of its oxygen to the tissues – the cause of the colour change – and received waste products taken in from them. It is slightly alkaline; the pH varies very little during life as the cells of the body can live only if the pH is normal. It forms about 5% of the body weight, hence the average volume is 3–4 L.

Composition of the blood

Although apparently fluid, blood actually consists of a fluid and a solid part. When examined under the microscope, large numbers of small round bodies, known as the blood corpuscles or cells, can be seen. These form the solid part, while the liquid in which they float is the fluid part or plasma. The cells form 45% and the plasma 55% of the total volume.

Plasma

The plasma (the fluid part of the blood) is a clear, straw-coloured, watery fluid similar to the fluid found in an ordinary blister. It is composed of the following:

• water, which forms over 90% of the whole

• mineral salts – these include chlorides, phosphates and carbonates of sodium, potassium and calcium. The chief salt present is sodium chloride. The correct balance of the various salts is necessary for the normal functioning of the body tissues, and there is a total of 0.9% inorganic substances

• plasma proteins – albumin, globulin, fibrinogen, prothrombin and heparin

• foodstuffs in their simplest forms – glucose, amino acids, fatty acids and glycerol, and vitamins

• gases in solution – oxygen, carbon dioxide and nitrogen

• waste products from the tissues – urea, uric acid and creatinine

• antibodies and antitoxins – these protect the body against bacterial infection

• hormones – from the ductless glands

• enzymes.

The water in plasma provides fresh water to supply the fluid that bathes all the body cells and renews the water within the cells; 60% of body weight is water and in an average person weighing 70 kg this would be approximately 46 L. Of these 46 L, approximately 29 L are within the cells (intracellular fluid) and 17 L are outside the cells (extracellular fluid). The extracellular fluid is divided between the blood vessels (3 L) and the fluid bathing the cells, called the interstitial fluid (14 L).

The salts in the plasma are necessary for the building of protoplasm and they act as buffer substances neutralizing acids or alkali in the body and maintaining the correct pH of the blood. Blood is always slightly alkaline in health and has a pH of 7.4. In plasma there are approximately 155 mmol/L of positively charged ions, chiefly sodium, balanced by 155 mmol/L of negatively charged ions, mainly chloride and bicarbonate. This is referred to as the electrolyte balance and is similar in the interstitial fluid. In the intracellular fluid, potassium replaces sodium as the positively charged ions, and phosphate ions and proteins replace chloride as the negatively charged ions.

The proteins that plasma contains give the blood the sticky consistency, called viscosity, which is necessary to prevent too much fluid passing through the capillary walls into the tissues. If there is a deficiency of protein, as in kidney disease, when protein is constantly lost as albumin in the urine, the osmotic pressure of the plasma is lowered and excess fluid escapes into the tissues. This excess fluid is called oedema. The viscosity of the blood also assists in the maintenance of the blood pressure. Albumin is made in the liver and globulin is derived from the group of white blood cells called lymphocytes. Fibrinogen and prothrombin are produced in the liver and are both necessary for the mechanism of the clotting of blood. Plasma without fibrinogen is called serum; this can be seen as the yellow fluid that oozes from a cut after a clot has formed. Heparin is also formed in the liver and prevents blood clotting in the vessels.

Foodstuffs, in the form of glucose, amino acids, fatty acids and glycerol, are absorbed from the alimentary tract into the blood. They are the end products of carbohydrate, protein and fat metabolism.

Urea, uric acid and creatinine are the waste products from protein metabolism. They are made in the liver and are carried by the blood for excretion by the kidneys.

Antibodies and antitoxins are complex protein substances providing protection against infection and neutralizing the poisonous bacterial toxins.

Enzymes, which are all protein molecules, produce chemical changes in other substances without themselves entering into the reaction.

The blood cells

The cells are of three types: red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes).

The formation of blood cells

The formation of blood cells takes place in the bone marrow and the mature products are released into the blood stream. Eight different cells are formed and all are formed from one type of pluripotent stem cell, which gives rise to five different lines of cells (Fig. 13.1). The myeloblast line gives rise to three types of granulocyte cells and the monoblast and lymphoblast lines give rise to the agranulocyte cells. The erythrocytes (red cells) and the platelets are formed from their own specific cell lines.

Fig. 13.1 Summary of the formation of blood cells.

Red cells

The red cells (erythrocytes) are disc-shaped bodies, concave on both sides (Fig. 13.2). They are very numerous, numbering about 5 000 000 per cubic millimetre (mm³) of blood. They have a diameter of 7.2 micrometres (1 micrometre = 1/1000 mm, abbreviated to μm). They have no nucleus but contain a special protein known as haemoglobin. This is a yellow pigment but the massed effect of these numerous yellow bodies is to make the blood appear red. Haemoglobin contains a little iron, which is essential to normal health, though the total amount in the whole body is said to be only sufficient to make a 5-cm (2-in) nail. Haemoglobin has a great affinity for oxygen. As the red cells pass though the lungs the haemoglobin combines with oxygen from the air (forming oxyhaemoglobin) and becomes bright in colour – this makes the oxygenated blood bright red. As the red cells pass through the tissues oxygen is given off from the blood and the haemoglobin becomes a dull colour (reduced haemoglobin), making the blood a dark purplish-red. The haemoglobin is measured in grams per 100 mL; the normal figure is 14–16 g/100 mL.

Fig. 13.2 Red cells. (a) In rouleaux, as seen in shed blood under the microscope. (b)–(d) Three views of a single cell.

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