Blood is a highly specialised fluid connective tissue (see Ch. 2), consisting of several types of cell suspended in a liquid medium called plasma. Blood makes up about 7% of the total body weight and has a pH of about 7.4. (7.35–7.45).

Functions

Blood has many functions within the body but they can be broadly divided into two groups: transport and regulation.

Transport

• Gases in solution – blood carries oxygenated blood from the lungs and delivers the oxygen to the tissues where it is used. It then collects deoxygenated blood containing carbon dioxide produced by the tissues during their metabolic processes, and carries it back to the lungs, where the carbon dioxide is exchanged for oxygen in the inspired air.

• Nutrients – blood transports nutrients, e.g. amino acids, fatty acids and glucose, which result from the process of digestion, from the digestive system to the liver and to the tissues where they are needed.

• Waste products – blood collects the waste products resulting from metabolism in the tissues and transports them to the kidney and liver where they are excreted from the body.

• Hormones and enzymes – blood transports enzymes and hormones from the endocrine glands to their target tissues.

Regulation

Blood plays a vital role in homeostasis by regulating:

• Volume and constituents of the body fluids – blood carries water in the form of plasma to the tissues and is responsible for maintaining the osmotic balance of the fluids and the cells. The presence of plasma proteins, particularly albumin in the blood, controls the flow of fluid between the fluid compartments and is responsible for maintaining blood volume and blood pressure.

• Body temperature – blood conducts heat around the body to the body surface where if necessary it is lost by peripheral vasodilation.

• Acid–base balance – blood maintains a constant internal pH in the body by the presence of buffers, e.g. bicarbonate, which are able to absorb H⁺ ions when the blood is acid (low pH) and give out H⁺ ions when the blood is alkaline (high pH). In this way, all the processes of the body are able to function effectively.

• Defence against infection – blood helps to prevent infection through the action of the white blood cells, which are part of the body’s immune system. It also carries antibodies and antitoxins produced by the immune system around the body.

• Blood clotting – the clotting mechanism prevents excessive blood loss from wounds and other injuries and prevents the entry of infection.

Oedema is an abnormal accumulation of fluid in the cavities and intercellular spaces of the body. When oedematous fluid accumulates in the peritoneal cavity it is described as ascites. Oedema occurs when there is an imbalance between osmotic pressure and hydrostatic pressure. It can be caused by a number of factors, e.g. anything that increases osmotic pressure outside the blood vessels, such as inflammation, or reduces osmotic pressure in the blood, such as hypoproteinaemia. Increased hydrostatic pressure inside blood vessels resulting from heart failure may also cause oedema.

Composition of blood

Blood is a red fluid that is carried by the blood vessels of the circulatory system. It is composed of a fluid part, the plasma, and a solid part, the blood cells (Fig. 7.1). Plasma forms part of extracellular fluid (ECF) (see Ch. 1). Each constituent of the blood plays a specific part in the overall function of blood.

Fig. 7.1 The composition of blood.

Plasma

Plasma is the liquid part of the blood that separates out when a blood sample is spun in a centrifuge. The main constituent is water (about 90%) in which are a number of dissolved substances being transported from one part of the body to another. These include carbon dioxide in solution, nutrients such as amino acids, glucose and fatty acids, waste materials such as urea, hormones, enzymes, antibodies and antigens.

In addition to these, plasma contains:

• Mineral salts – the main mineral salts found in extracellular fluid are sodium and chloride, but potassium, calcium, magnesium and bicarbonate are also present. The functions of these mineral salts include maintaining osmotic balance and maintaining pH by acting as buffers. Calcium has a number of essential roles in the body, e.g. in blood clotting, muscle contraction and nerve function.

• Plasma proteins – these help to maintain the osmotic pressure of the blood because they are too large to pass out of the circulation. This has the effect of retaining fluid in the blood by osmosis, i.e. it prevents too much water from ‘leaking’ out into the extracellular spaces. If this did occur then the volume of the blood would decrease and the blood pressure would fall with serious consequences. The most important proteins are:

Albumin: helps to maintain the osmotic concentration of the blood, i.e. holds the water in the blood

Fibrinogen and prothrombin: involved in the clotting mechanism of the blood

Immunoglobulins: these are the antibodies produced by the immune system of the body.

Albumin, fibrinogen and prothrombin are produced by the liver, but the immunoglobulins are produced by the cells of the immune system.

Blood cells

The blood cells (Fig. 7.1) make up the solid component of blood and can be divided into three types:

• Erythrocytes – the red blood cells

• Leucocytes – the white blood cells

• Thrombocytes – the platelets, which are cell fragments.

Before studying the different types of blood cell it is useful to understand a number of terms.

Erythrocytes (or red blood cells)

Erythrocytes are the most numerous blood cell – there are about 6–8 million per cubic millilitre of blood (Fig. 7.2). Their function is to transport oxygen and a small proportion of carbon dioxide around the body (most carbon dioxide is carried in solution in the plasma).

Fig. 7.2 The cellular components of blood.

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 197.)

Mature erythrocytes are biconcave circular discs about 7 μm in diameter. Erythrocytes contain a red pigment called haemoglobin, which is a complex protein containing iron. They are the only cells in the body without a nucleus, which allows a greater amount of haemoglobin to be packed into a relatively small cell. Erythrocytes are surrounded by a thin, flexible cell membrane, which enables them to squeeze through capillaries. Their shape and thin cell membrane gives them a large surface area for gaseous exchange and allows oxygen to diffuse across into the cell, where it combines with the haemoglobin to form oxyhaemoglobin.

Erythrocytes are formed from undifferentiated stem cells within the bone marrow by a process known as erythropoiesis. The stem cells change into erythroblasts, which have a nucleus. The cell begins to acquire haemoglobin and its nucleus shrinks – it is now known as a normoblast. As the cell develops further it becomes a reticulocyte, at which point the nucleus consists only of fine threads in the cytoplasm known as Howell–Joly bodies. Eventually, the nucleus disappears and the mature erythrocyte is released into the circulation. This process takes 4–7 days.

If there is a shortage of erythrocytes, e.g. acute haemorrhage or iron-deficiency anaemia, reticulocytes are also released into the circulation to help make up the deficit. These can be seen on a blood smear stained with methylene blue, which is a specific stain for reticulocytes.

A circulating erythrocyte has a lifespan of about 120 days, after which it is destroyed in the spleen or lymph nodes. The iron from the haemoglobin is recycled back to the bone marrow and the remainder is converted by the liver into the bile pigment bilirubin and excreted in bile.

The production of red blood cells is controlled by a hormone called erythropoietin (see Ch. 6), which is released by cells in the kidney in response to low oxygen levels in the tissues. Erythropoietin stimulates the stem cells in the bone marrow to produce more erythrocytes.

Leucocytes (or white blood cells)

Leucocytes are much less numerous than red blood cells and the cells contain nuclei. Leucocytes can be classified as either granulocytes or agranulocytes depending upon whether or not they have visible granules in their cytoplasm when stained and viewed under a microscope (Fig. 7.2). The function of leucocytes is to defend the body against infection.

Granulocytes

This type of leucocyte is produced within the bone marrow and they make up approximately 70% of all leucocytes. They have granules within their cytoplasm and have a segmented or lobed nucleus which can vary in shape. They are referred to as polymorphonucleocytes or PMNs (meaning many-shaped nuclei). They can be further classified according to the type of stain they take up, i.e. neutral, basic or acidic. There are three types of granulocyte:

• Neutrophils – take up neutral dyes and the granules stain purple (Fig. 7.2). Immature neutrophils have a nucleus that looks like a curved band and are known as band cells. Neutrophils are the most abundant of the leucocytes, forming about 90% of all granulocytes. They are able to move through the endothelial lining of the blood vessels into the surrounding tissues and engulf invading bacteria and cell debris by phagocytosis, thus helping to fight disease. A neutrophilia or raised numbers of neutrophils indicates the presence of an infective process while a neutropenia or lack of white cells may be characteristic of certain viral infections.

• Eosinophils – these take up acidic dye and the granules in their cytoplasm stain red (Fig. 7.2). They are involved in the regulation of the allergic and inflammatory processes and secrete enzymes that inactivate histamine. Eosinophils play a major role in controlling parasitic infestation. An eosinophilia or raised numbers of eosinophils occurs in response to parasitic infestation.

• Basophils – these take up basic or alkaline dyes and the granules in the cytoplasm stain blue (Fig. 7.2). Basophils secrete histamine, which increases inflammation, and heparin, which is a natural anticoagulant preventing the formation of unnecessary blood clots. Basophils are present in very small numbers in normal blood.

Agranulocytes

Agranulocytes have a clear cytoplasm. There are two types:

• Lymphocytes – this is the second most common type of white blood cell, forming 80% of all agranulocytes (Fig. 7.2). Lymphocytes are the main cell type of the immune system and are formed in lymphoid tissue although they originate from stem cells in the bone marrow. Lymphocytes are responsible for the specific immune response, and there are two different types: the B lymphocytes, which produce antibodies and are involved in humoral immunity, and the T lymphocytes, which are involved in the cellular immune response.

• Monocytes – these have a horseshoe-shaped nucleus and are the largest of the leucocytes, although they are only present in small numbers (Fig. 7.2). They are phagocytic cells and when they migrate to the tissues they mature and become known as macrophages.