Blood which is high in carbon dioxide is pumped from the right ventricle along the pulmonary artery to the lungs where the carbon dioxide is exchanged for oxygen in the alveoli. The blood, which is now carrying oxygen, leaves the lungs and returns to the left atrium via the pulmonary veins.

      Filtration: the action or process of filtering something.   Leucocytes: a colourless cell which circulates in the blood and body fluids and is involved in counteracting foreign substances and disease; a white (blood) cell. There are several types, all amoeboid cells with a nucleus, including lymphocytes, granulocytes, and monocytes.

Blood which is high in oxygen is pumped from the left atrium into the left ventricle and pumped out along the aorta to supply the body tissues and organs via a network of arteries and arterioles.

After passing through the networks of capillaries where nutrients are exchanged for waste products from the tissue cells, the blood returns along the veins. These unite to form the cranial and caudal vena cava that empty into the right atrium.


Blood from the spleen, intestines, pancreas and stomach is carried in vessels that unite to form the portal vein. This enters the liver and divides into many fine capillaries. The capillaries re-unite to form the hepatic vein that empties directly into the caudal vena cava. This system allows nutrients to be exchanged in the liver and for harmful substances to be removed. A good example of the nutrient exchange is the conversion of glucose into glycogen and also the reverse process.


At the capillary networks, fluid leaves the blood vessels and surrounds the tissue cells. Not much of this fluid re-enters the blood vessels, most of it being carried away by the lymphatic vessels. Once inside the vessels, this fluid is called lymph. Lymph vessels unite to empty into the vena cava, and so into the heart, mixed with the blood from the veins. While passing along the lymph route, the lymph passes through several lymph nodes for filtration. Harmful material is filtered out, and this is attacked by leucocytes in the nodes. During this process, the nodes swell, and can be seen in the swellings in the groin and under the armpit if you have an infection.


This organ has two functions. It destroys worn out red blood cells, and forms leucocytes in the spleen. In other words, it is involved in the defense mechanisms of the body. The spleen is not an essential organ in the adult as its functions can be carried out by other organs.


Blood is a tissue consisting of cells that move around the body in fluid called plasma.


The red blood cells are called erythrocytes, and there are 5 million in a single millilitre of blood. They are dish-shaped discs whose primary task is to transport oxygen. Oxygen is carried in the blood bound to a chemical called haemoglobin which also gives blood its characteristic red colour. Red blood cells are produced in the marrow of bones and they have a life of 3 – 4 months. After that they disintegrate, and the pigments produced by their destruction are excreted in bile.

Figure 1: A Red Blood Cell

Figure 2: Arterial and Venous System of the Horse

Source: 2.bp.blogspot


These are small oval discs which are formed in the marrow of bones, and which play an important role in the clotting of blood and the prevention of blood-loss from a wound. They do this by sticking to each other and to the walls of blood vessels at the place of an injury. They also release a substance which causes the blood vessels in the area to constrict and produce a drop in blood pressure.


These are called leucocytes and there are between 4000 and 11000 per ml of blood. There are various types of leucocytes of different shapes and sizes. They play an extremely important part in the defence of the body’s health. They can form barriers against disease and engulf harmful material such as bacteria. They play a role in the formation of antibodies and the immunity of the body. They are formed in the bone marrow and in the lymph tissues, spleen, tonsils and lymph nodes.


This is a straw-coloured fluid containing 90% water and 10% solids. The solids are:

  • Proteins: Serum albumin, Fibrinogin which is concerned with the clotting of the blood. Globulin which deals with immunity from disease.
  • Hormones
  • Lipids or Fats
  • Cholesterol
  • Enzymes
  • Inorganic chemicals: These are the ions of salts and acids, some of which are essential in cell metabolism and others which act as buffers, reducing strong acids and alkalis to weaker levels and neutral salts.

Nitrogenous compounds such as amino acids, urea, uric acid and ammonium salts.


The main functions of the blood are:

  • To carry nutrients from the digestive tract to the body tissues and organs;
  • To carry oxygen from the lungs to the tissues, and carbon dioxide from the tissues back to the lungs;
  • To carry waste products from the tissues to the kidneys;
  • To carry hormones from the endocrine glands;
  • To regulate the body temperature by transporting heat from the deeper organs in the body up to the surface of the skin;
  • To maintain the water balance of the body;
  • To maintain the pH (acidity/alkalinity) balance of tissues and organs;
  • Blood has the ability to clot, and so can prevent too much loss of blood through injury; and
  • Blood plays an important part in the body’s defense against disease.

When a blood vessel is injured, a substance called thromboplastin is released, and this is converted into active thrombin. Thrombin reacts with the fibrinogen in blood plasma, and a substance called fibrin is formed. Fibrin has fine, thread-like filaments which wrap around the red blood cells, the white blood cells and the platelets to form a clot which stops further bleeding.


Immunity is the most important form of defense the body has. It develops when the body is exposed to the invasion of any foreign protein or protein-like substance. Such foreign proteins are known as antigens, and when they get inside the body they eventually end up in the bloodstream where they produce poisons called toxins. Bacteria have proteins on their surfaces and so they act as antigens. The presence of antigens in the blood stimulates the production of antibodies which kill the invading antigens, and sometimes destroy the toxins which they have produced. These antibodies are very specific, acting only against one particular antigen, so that each disease stimulates the production of its own antibodies. This is the principle on which vaccination and immunisation work, both of which are discussed in more detail in the Animal Health Course.