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Circulatory System

(16) First, study Fig.1-1, then read the following while still referring to the figure. The circulatory system consists of the heart which is a four chamber suction and pressure pump that moves blood through two separate systems, one to and from the lungs and the other around the body. The blood returns to the heart from the body through a series of veins, which terminate in two large veins called the anterior and posterior vena cava. Blood returns from the lungs through the pulmonary veins. The top two chambers or auricles receive the blood from the veins and pass it into the strong muscular bottom chambers called the ventricles. Oxygen depleted blood from the body enters the right auricle, where it is then pumped into the right ventricle leaving by two pulmonary arteries that deliver the still un-oxygenated blood to the lungs. Oxygenated blood from the lungs is then returned through the pulmonary veins to the left auricle, where it is pumped to the left ventricle and finally out through the main artery, the aorta, to be transported around the body. If the lungs are damaged by disease such as pneumonia, they cannot oxygenate the blood efficiently, the tissues become starved of oxygen and cannot function properly.

When the pig walks or runs its skin may then become blue and it has difficulty breathing. Chronic pneumonia may also hold back the blood supply causing congestion and heart problems.

Arteries are the muscular tubes that carry the blood away from the heart. These branch off into smaller arteries like the branch of a tree eventually becoming very fine arterioles. The arterioles branch further into microscopic tubes called capillaries which exchange fluid through their walls. This enables the cells of the body to receive both oxygen and nutrients and eliminate carbon dioxide. The capillaries then combine to form first small veins, which in turn lead to larger ones. The blood now contains carbon dioxide and reduced levels of oxygen and returns to the heart via the anterior and posterior vena cava to recommence its circulation around the lung.

There is an important subsidiary circulatory system called the hepatic (i.e. liver) portal system. You will see in Fig.1-1 that two arteries provide oxygen to the stomach and intestines (and also the pancreas and spleen). They keep branching until they form capillaries which then join together to form the portal vein which carries the blood to the liver. There the portal vein breaks up into another capillary-type network, where the blood comes into direct contact with the liver cells. The vessels then join together again to form the hepatic veins which discharge the blood into the posterior vena cava. The blood from the intestines carries nutrients from the food eaten and also sometimes harmful substances (toxins). The liver cells are able to modify some of the nutrients for use elsewhere and also to store some. They also detoxify harmful substances. The liver is supplied with oxygen via a separate artery, the hepatic artery.

The internal linings of the heart are covered by a smooth shiny tissue called the endocardium. The rate of contraction is known as the pulse rate. This can be felt either at the base of the ear or under the tail and varies from 200 beats per minute in the young piglet to 70 in the adult.

The blood consists of two main parts, a fluid called plasma and cells. Nutrients such as proteins, sugars and fats are circulated throughout the body in the plasma and waste products are collected to be detoxified in the liver and excreted via the kidneys. The plasma also carries hormones which are produced in one part of the body and act on another. It also carries antibodies to combat infection. The plasma also supports red blood cells (erythrocytes) which contain the substance haemoglobin whose main function is to transport oxygen around the body and bring back carbon dioxide to be expelled from the lungs. The next largest group in the plasma are the white cells (leucocytes) which are the first line of defence against infectious agents. The third type of cells are blood platelets. These are really small fragments of cells which are associated with the clotting mechanisms of blood. When blood clots the liquid that remains outside the clot is serum and this contains the antibodies. Serum may be used to inject into pigs to provide an immediate source of immunity.

Failure of blood to clot and subsequent loss of red cells into the tissues is not uncommon in pigs and occurs in thrombocytopaenic purpura - a clotting defect disease - and warfarin poisoning.

Albumin - The most abundant protein in the blood.
Anaemia - Any reduction in the number of red cells or in the haemoglobin they contain is described as anaemia and the extent of this is measured either by determining the number of red cells or the level of haemoglobin in the blood. The causes of anaemia include:

  • Bowel haemorrhage (proliferative haemorrhagic enteropathy, fungal toxins, acute bowel infection associated with E. coli infection of piglets, salmonella infections or swine dysentery).
  • Damage to bone marrow.
  • Eperythrozoonosis suis. This is a blood borne bacterium that can destroy red blood cells.
  • Gastric ulcers and bleeding - or any other cause of haemorrhage.
  • Heavy parasite burdens.
  • Iron, copper or vitamin deficiencies.
Anoxia - Lack of oxygen. Tissues begin to die after a few minutes.
Antibody - The protective proteins produced in response to the antigenic stimulation. They fight infections.
Antigen - This is the foreign protein contained in viruses, bacteria, fungi or toxins. The body responds by producing an antibody.
Antiserum - This is serum containing higher than normal amounts of antibody against a specific antigen. It is used by injection to give an immediate temporary immunity.
Blood count - A laboratory test that determines the numbers of red and white cells and platelets in the blood.
Blood volume - Approximately 8% of body weight expressed as litres .
Blood platelets (thrombocytes) - These are cell fragments involved in blood clotting.
Blood poisoning - A common term used to describe large numbers of pathogenic bacteria in the blood.
Capillaries - Very tiny tubes about the diameter of a red cell. These allow water oxygen and nutrients to diffuse out to the tissues.
Cyanosis - Blueing of the skin and extremities due either to anoxia, toxaemia (toxins in the blood) or septicaemia (pathogenic bacteria in the blood)..
Endocardium - This is the surface tissue lining the inside of the heart. Endocarditis is the end result of the invasion of this tissue by bacteria, in particular erysipelothrix (which causes erysipelas) and streptococci. Both organisms often cause growths on the heart valves called valvular endocarditis. This makes the valves leaky and less effective.
Erythrocytes - These are the red blood cells. In the normal pig there are approximately 7 million per mm3.
Globulins - The proteins that make up the antibodies. They are called gamma globulins.
Granulocytes - These consist of specialised cells called neutrophils, eosinophils and basophils that engulf and destroy bacteria and viruses. They are also called macrophages.
Haematuria - Blood in the urine often seen in cystitis - inflammation of the bladder.
Haemoglobin - This is the chemical substance in the red cells that is involved in the transport of oxygen.
Haemoglobinuria - Free haemoglobulin in the urine resulting from the breakdown of blood cells.
Haemolysis - This is the process by which haemoglobin is released from the red cells when the cell envelope is damaged.
Hydropericardium - Excess fluid around the heart. It is often seen in bacterial infections and shock reactions.
Hypoglycaemia - A low level of sugar in the blood. Common in newborn piglets.
Leucocytes - These are the white blood cells of which there are two types, granulocytes and agranulocytes. The granulocytes contain granules in the cell and depending on how they stain they are called neutrophils, eosinophils and basophils. Neutrophils engulf bacteria (phagocytosis), eosinophils increase in chronic disease particularly parasitic disease. Basophils produce a substance called histamine during allergic reactions. Agranulocytes consist of monocytes and lymphocytes.
Lymph - Excessive tissue fluid drained by the lymphatic system. It is similar to plasma.
Lymphatics - A drainage system that removes fluids from tissues and the lymph nodes.
Lymph nodes - These act as filters for lymph and are one of the body's first defences against infection.
Lymphocytes - These are important cells of the immune system producing immunoglobulins. They are of two types, T and B. The total leucocytes in a normal pig are approximately 15,000 per mm3 and numbers increase markedly with bacterial infections. However in some viral diseases their numbers can be significantly reduced.
Macrophages - These take in and usually destroy foreign materials including bacteria and viruses. See granulocytes and monocytes.
Monocytes - These cells engulf bacteria. When they migrate into tissues they become localised tissue macrophages.
Myocardium - Heart muscle.
Myocarditis - Inflammation of the heart muscle. Any scientific term ending with the term "itis" implies inflammation. Inflammation is the body's response to tissue damage and is associated with swelling, poor circulation, reddening, pressure and pain.
Diseases causing myocarditis include streptococcal infections, certain virus infections and deficiencies of Vitamin E or iron. Poisons such as selenium and monensin and the porcine stress syndrome can also cause marked changes to heart muscle.
Oedema - Swelling of tissues due to excess fluid. Common in the udder of the newly farrowed sow.
Oxyhaemoglobin - This is haemoglobin combined with oxygen. It is the vehicle by which oxygen is carried around the body.
Pericarditis - The pericardium is the clear sac-like membrane that encloses the heart. Pericarditis occurs as a result of infectious agents which cause respiratory diseases. These include pasteurella, mycoplasma, haemophilus, actinobacillus, streptococci and salmonella bacteria and viruses such as flu and porcine respiratory reproductive virus.
Plasma - Unclotted blood without the blood cells.
Septicaemia - Pathogenic bacteria in the blood stream.
Serum - The liquid left after the blood has clotted. It contains large quantities of antibodies which can be used in the laboratory to test for evidence of exposure to diseases or in the field to provide temporary quick protection.
Thrombocyte (blood platelet) - This is responsible for blood clotting.
Thrombosis - The formation of a blood clot in an artery or a vein.
Toxaemia - Toxins in the blood stream
Spleen - This organ acts as a reservoir for blood.
Vasiculitis - This describes inflammation of either veins or arteries and it is often a consequence of diseases such as swine fever, erysipelas, Actinobacillus pleuropneumoniae, Haemophilus parasuis and salmonellosis.
Viraemia - Viruses in the blood stream.

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