Chapter 7 Blood vascular system
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).
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.
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.
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.
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 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.
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:
The ability to form a blood clot is one of the most important defence mechanisms in the body. It means that injured blood vessels can be sealed and excessive blood loss can be prevented. Blood clotting is essential for wound healing and also prevents the entry of pathogenic microorganisms into the wound. The formation of a blood clot is complicated and involves a number of different chemical factors in the blood. It is described as a cascade mechanism because one step leads on to another in a similar way to a cascade of water.