Page 729 - The Veterinary Care of the Horse

THE COMPOSITION AND FUNCTION OF BLOOD

VetBooks.ir Blood is composed of red blood cells, white blood cells and platelets suspended in fluid

called plasma. It is estimated that an average-sized Thoroughbred horse has over 50 litres (88
pt) of blood in its body. It is pumped by the heart through the tissues in a network of arteries,

capillaries and veins. Its many functions include the transport of oxygen and nutrients to all
the tissues, defending the body against infection and carrying hormones and other chemical

messengers between the organs.

Red blood cells

Red blood cells are also known as erythrocytes or red blood corpuscles. They are very

small cells that are unusual in having no nucleus. They are made up of water (60%) and a
pigment called haemoglobin (33%) within a cell membrane. They have a flattened shape

which is concave on both sides to increase the surface area for gas exchange. In the lungs, the
haemoglobin combines with oxygen to form oxyhaemoglobin which is then circulated to the

tissues. The tissues use oxygen and produce carbon dioxide as a waste product of
metabolism. This is collected by the haemoglobin and taken back to the lungs for excretion.

The red blood cells are made in the bone marrow and have a lifespan of approximately
120 days. When they die, they are removed from the circulation by the liver and the spleen.

Approximately 30% of the red blood cells of a resting horse are stored in the spleen. These
are released into the circulation if the horse is exercised or excited.

White blood cells

White blood cells or leucocytes defend the body against infection and help to clear up any
damaged tissue or foreign matter in a wound. There are 5 cell types.

1 Neutrophils migrate from the bloodstream into the tissues and engulf and digest
invading bacteria.

2 Eosinophils are involved in allergic reactions and parasitic disease. Relatively small

numbers circulate in the blood, most migrate into the skin, the gut and the mucosal lining
of the bronchi.

3 Basophils are only present in very small numbers. Their numbers increase in allergic and
parasitic conditions.

4 Lymphocytes are important for recognizing antigens such as viruses, bacteria and fungi.
B lymphocytes produce antibodies which help to protect the horse from infectious