The circulatory system is responsible for the transport of the various essential compounds and other factors around the body, as well as the removal of the metabolic wastes that accumulate in the tissues from body activities to places of elimination. The compounds and other factors transported around the body are blood, nutrients, medications and antibodies to fight infection, the residue of worn out cells and the wastes of metabolism. There are times when undesirable compounds and factors are found in the system as well - compounds such as poisons or toxins and disease causing organisms.
The circulatory system
The circulatory system consists of a number of organs and an associated transport system. These include the heart, the blood vessels, the spleen, the bone marrow, the blood and the lymph vessels. The blood and vascular system develops very early in the life of the embryo - the nutrition of the rapidly developing embryo is urgent and a transport system is required to transport the nutrients in the egg to where they are needed. Evidence of the system can be seen within about an hour and the system being clearly defined and operating within 2 days. By day three it is possible to see the beat of the embryonic heart with the naked eye.
Fluids that act as a carrier of the various compounds and factors form part of the system. Each of these body fluids has it’s own function. Only circulatory fluids are discussed here. There are two types of body fluids:
1. Circulatory fluids (these move around):
o blood
o lymph
2. Static body fluids (remain stationary):
o intracellular fluid or cell sap.
o intercellular, interstitial or tissue fluid.
o cerebrospinal fluid of the central nervous system.
o synovial fluid in the joints of the skeleton.
o aqueous and vitreous humor in the eye (fluid).
o endolymph and perilymph in the ear (cochlea).
The circulatory system consists of a number of organs that produce the blood cells and the organs that pump it around the body - the heart, and the blood vessels such as the arteries, veins, capillaries and lymph vessels that carry it.
After the arteries leave the heart they divide to ever smaller and smaller arteries to provide the many branches required to service the many systems, organs, tissues and cells. When they reach the tissues they divide into capillaries that are very small and very thin walled (one cell thick). This allows the nutrients and factors being delivered to move out of the capillary into the tissues and the material to be collected to enter. When they leave the tissue the capillaries rejoin to form veins until when near the heart the blood is moving through either the vena cava or the pulmonary veins.
· The lymphatic system is a special fluid collecting system that collects the fluid left behind in the tissues by the capillaries and transports it to the region of the heart where it enters the vena cava.
In broad terms the heart acts as the pump that pumps in two directions:
i. To the lungs where the carbon dioxide in the blood is removed and the oxygen replaced
ii. To the rest of the body to deliver the nutrients and oxygen to the cells and to collect wastes and carbon dioxide
The blood leaves the heart via arteries - the aorta to the body, the pulmonary to the lungs. The blood always enters the heart via veins - the vena cava from the body and the pulmonary from the lungs.
This organ is located in the thoracic cavity between the two lobes of the liver and mainly in front of that organ. It is relatively large and is enclosed in a thin membrane called the pericardium. The avian heart has four chambers - two atria and two ventricles as in mammals. In general shape, it is typically conical with its apex or pointed end directed to the rear and slightly left of middle. A shallow groove, visibly externally, indicates the division between the relatively small atria in the cranial end and the quite large ventricles making up the largest part of the organ in the caudal end.
Atria and ventricles and fibers
The walls of the atria are thin while those of the ventricles are quite thick - the atria have only to move the blood from the atria to the ventricle while the ventricles are responsible for the pumping of it around the body. Microscopically the muscle of the heart is similar to that of mammals with the fibres being nucleated, striated and forming a syncytium (a mass of cytoplasm with numerous nuclei). Numerous fibres called Purkinje fibres are found in the heart under the endocardium and in the myocardium adjacent to the larger arteries.
The large veins draining the body - the two cranial vena cava and the caudal vena cava enter the right atrium. The two pulmonary veins from the lungs join before entering the left atrium. The opening from the right atrium into the right ventricle is crescent in outline shape. A strong muscular plate replaces the membranous tricuspid valve found in mammalian hearts. The opening between the left atrium and left ventricle is circular in shape with membranous flaps corresponding to the bicuspid valves of mammals. The septum (wall) between the two ventricles is thin with even a thinner area - the fossa ovalis - located near the centre.
The external wall of the right ventricle is much thinner than that of the left. At its base is the opening for the pulmonary artery with its three pocket like semi-lunar valves the open mouths of which face away from the heart. The wall of the left ventricle is very thick except at the apex (pointed end). This is to provide the structural strength required for the heavy workload of that segment as it pumps blood around the body through the aorta. This leaves the heart from the left ventricle through an opening with semi-lunar valves similar that of the pulmonary artery. Tendinous cords called the chordae tendonae prevent too much movement of the valve segments from the force of the pumping heart i.e. from turning the valves inside out.
The heart has it’s own blood supply via the two coronary arteries which divide from the aorta shortly after that artery leaves the left ventricle. The autonomous nervous system controls the pumping action of the heart.
Schematic diagram of the arterial system
The pulmonary system starts as the single pulmonary artery leaving the right ventricle and shortly after divides into the right and left branches - each entering the lungs and dividing to ultimately be involved in the gaseous exchange of respiration that takes place in the lungs. The aorta leaves the heart via the left ventricle and very shortly after divides to provide the two coronary arteries that supply blood to the heart itself. Shortly after that the first of many divisions occur. Blood from the aorta is supplied to the head, the thoracic region including the wings, the abdominal region including the legs and tail. Figure 1 is a diagrammatic representation of the arterial system.
New or oxygenated blood is bright red while old or deoxygenated blood is a very dark colour. In the pulmonary system, the arteries carry old blood while the veins carry new blood. In the main system, the arteries carry new blood and the veins carry old. As the arteries divide they become smaller and smaller - the very small arteries are often referred to as arterioles. When they enter the target organ they divide to become very thin walled capillaries and it is from these that the oxygen, nutrients, wastes and other factors either leave or enter the system.
Schematic diagram of the venous system
The capillaries in the various tissues and organs rejoin to form the veins that carry the blood back to the heart. Those draining the body other than the lungs enter the right atrium as three veins called:
i. Caudal vena cava
ii. Right cranial vena cava
iii. Left cranial vena cava
The caudal vena cava drains the abdominal region while the two cranial vena cava drain the head via the juggler veins and the upper body including the wings and thoracic region excluding the lungs.
Kidney, small intestine and liver and lungs
The kidney system is very complex because of their function. They remove excess water and the wastes of metabolism from the blood stream. In addition they need their own supply for the maintenance of their own tissues and cells. As in mammals, blood containing nutrients absorbed from the small intestine, is carried to the liver by the portal vein. This is separate to the hepatic system that is responsible for the maintenance of the liver as an organ. The portal and hepatic systems join together in the liver to form the hepatic veins that enter the caudal vena cava near the heart. The capillaries in the lungs formed from the pulmonary arteries re-join to form the pulmonary veins that enter the heart via the left atrium for pumping to the rest of the body.
Blood is the transport vehicle for the body. It is a very complex compound and consists of:
i. Liquid intercellular substance or plasma
ii. Suspended formed elements
The formed elements include the red corpuscles or erythrocytes, the white corpuscles or leucocytes and the thrombocytes that correspond to the mammalian platelets. Blood also carries a great number and variety of other substances and factors. These include:
· Products from metabolism (wastes).
· Hormones.
· Enzymes.
· Antibodies.
· Effete products of tissues and organs (effete = worn out).
· Many other organic and inorganic compounds.
These are large, oval, flat, nucleated cells (mammalian erythrocytes are non-nucleated). Occasionally erytheroid structures without a nucleus are observed but these are normal avian erythrocytes from which the nucleus has been lost. The nucleus is centrally located and tends to be elongated in shape with a length of about 12 microns and a width of about 7 microns. The red colouring is caused by the presence of haemoglobin - an iron compound that carries oxygen. The function of the erythrocytes is to transport the oxygen from the lungs to the tissues and the carbon dioxide from the tissues to the lungs. The erythrocytes are formed in the red bone marrow.
In the adult fowl there are 2.5 million erythrocytes in each millilitre of blood. The cell volume may be measured by taking a sample of the blood and centrifuging it at 3000 rpm for 15-20 minutes. The cells and the plasma separate and the volume of the cellular portion can be measured. This volume is called the haemocrit and, while it is mainly erythrocytes, it contains the leucocytes as well as a small quantity of plasma that has been trapped. The error as a result of the trapped plasma is approximately 5%.
These are nucleated, amoeboid cells with a colourless cytoplasm. Some have fine granules in their cytoplasm, others carry coarse granules and others are non-granular. Because of the presence of granules the leucocytes are often called granulocytes. Most are phagocytic. The shape is therefore variable, but under normal conditions they are round (spherical). They are mono-nuclear although at times they appear to be multi-nuclear because of the shape of the nucleus - it is often multi-lobed.
The leucocytes are formed in the spleen, lymphoid tissue and in special cells in the bone marrow. The average count is about 30,000 per millilitre in adult birds and 10,000 per millilitre in day old chickens. Leucocytes may be classified by the character of their cytoplasm and nucleus. Protein granules may be acidic, basic or have neutral pH and this determines, to some extent, the type of stain they take up. (Microscopic bodies such as cells and bacteria are usually stained in the laboratory to aid in their identification. Without staining, individually they are mainly colourless and it is very difficult to see their characteristics clearly even with powerful microscopes). Those identified in this way are:
1. Acidic granules that stain with a basic (alkaline) stain and are called basophils.
2. Basic (alkaline) granules that stain with an acidic stain and are called acidophils, oxyphils or eosinophils.
3. Neutral granules may be stained with a neutral stain and are called neutrophils.
Using this system there are five main groups of leucocytes or granulocytes:
i.Polymorphonuclear heterophilic granulocytes
ii.Polymorphonuclear eosinophilic granulocytes
iii.Polymorphonuclear basophilic granulocytes
iv.Lymphocytes
v.Monocytes
Polymorphonuclear heterophilic granulocytes (heterophils)
These are known as heterophils. They are round cells with a diameter of 10-15 nanometre (nm). The nucleus is distinctly lobulated, with the lobes joined by thick strands of nucleoplasm (nucleus material). These cells play an important part in the body’s defence against bacterial invasion - they have a bactericidal function and ingest protein.
Polymorphonuclear eosinophilic granulocytes (eosinophils)
These are of similar size to the heterophils i.e. 10-15 nm and the nucleus is frequently bi-lobed. The eosinophilic granules in the nucleus are round and are not so numerous as the granules in the heterophils. The function of the eosinophils is not definitely known but it is believed that they increase in number in cases of internal parasite infestations. It has been suggested also that they have a de-toxifying function.
Polymorphonuclear basophilic granulocytes (basophils)
These are also about the same size as the heterophils and eosinophils. The nucleus is generally not lobulated but appears to be oval in shape. There tends to be fewer of these cells and their function is unknown. It has been suggested that they are a stage in the degeneration of eosinophils.
These are the most numerous of all of the leucocytes. Their size varies significantly, the older cells being somewhat smaller than the newer because of the loss of cytoplasm from the older cells. The nucleus size is the same in all cells but may be kidney shaped in some cases. The mobility of these cells is greatly reduced and their phagocytic action is probably nil. Their function is thought to be that of regulating toxic materials. They release enzymes that assist in the synthesis of nucleoplasm as well. In their degeneration they release special compounds called gamma globulins that have antibody properties.
Monocytes are the largest of all of the leucocytes. They are difficult to separate from the large lymphocytes because of a strong similarity in appearance. The nucleus may be round, oval or kidney shaped. They are very mobile and very phagocytic in their activity. Their function is to remove by ingestion bacteria, protozoan and tissue debris.
The thrombocytes are similar to the blood platelets of mammals. However, they are less involved with blood clotting. They are quite numerous at 25000 per millilitre. They are the smallest of all of the blood cells and group together as small clumps. Thrombocytes are formed in the bone marrow in a way similar to the red blood cells.
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