Circulatory System

Introduction

In the circulatory system of vertebrates there are two systems of elaborately branching tubes which ramify throughout the body and carry fluids to the tissues, they are a blood vascular system and a lymphatic system; when the term circulatory system is used, it refers only to the blood vascular system. The blood vascular system is a closed system in vertebrates, it has a contractile heart and continuous tubes called vessels. The lymphatic system is an open system with lymph spaces. The blood vessels which carry blood away from the heart are arteries, the arteries divide into thinner arterioles which branch into extremely thin and small capillaries.

The capillaries form a network in all body tissues except cartilage and epithelium. From the capillaries the blood passes into thin venules which combine to form veins.

circulatory system

Blood Vessels

In a vertebrate embryo small clusters of messenchyme cells appear and are called blood islands. The blood islands are at first solid but soon become hollow to form small endothelial tubes, the blood island cells secrete plasma within the tubes. Some loose mesenchyme cells floating in the plasma become corpuscles. The endothelial tubes grow and join each other to form a network of blood vessels. But some blood vessels appear as clefts in the mesenchyme.

An artery is made of three coats, an outermost adventitia or tunica externa of connective tissue having longitudinal elastic and collagen fibres; a thick middle tunica media made of circular smooth muscles and elastic fibres; and an innermost intima or tunica interna made of endothelial cells and an elastic membrane. A vein is also made of the same three coats, its diameter is larger than that of a corresponding artery but its wall is much thinner because the tunica media is extremely thin with no muscles, tunica externa makes up the wall largely. The larger arteries and veins have their own small blood vessels in their coats called vasa-vasorum which supply nourishment and oxygen to these blood vessels.

Blood

The blood contains a fluid plasma  in which blood cells called corpuscles are suspended unattached to one another. Plasma is about 55 percent and corpuscles are 45 percent. Plasma contains a liquid called serum and a coagulable substance known as fibrinogen, it also has an anti-coagulant antiprothrombin or heparin. Plasma is composed of water and inorganic and organic substances in solution. inorganic substances are chlorides, bicarbonates, sulphates, and phosphates of Na, K, Ca, Fe and Mg. Organic substances are plasma protines,glucose,amino-acids,hormones,enzymes,and waste products.

Red blood corpuscles or erythrocytes contain iron,water,various inorganic salts,and haemoglobin,they carry oxygen and help in removing carbon dioxide. Haemoglobin ,the respiratory pigment,is a complex made of 95% globin, and 5%haematin. In man there are about five million erythrocytes per cu.mm.,and 4.5 million per cu.mm.in women.

There are two main types of white blood corpuscles or leucocytes, according to the structure of nuclei and presence or absence of granules in their cytoplasm they are granulocytes and non-granular leucocytes. Granulocytes or granular leucocytes have numarous granules in the cytoplasm and the nucleus is segmented into two or more lobes,they are of three types, they are Basophils,Eosinophils and Neutrophils.

Non-granular leucocytes may or may not have smalls granules of neutral nature but the nucleus is never segmented into lobes, they are of two types,they are lymphocytes and monocytes.

The tissues,which from the bloodcorpuscles of vertebrates are called haemopoietic tissues,and the prosses of the formation of corpuscles is haemopoiesis. When the liver is formed it takes over haemopoiesis,this is followed by the spleen and lymphatic organs. In adults vertebrates several organs are haemopoietic :kidneys,liver,spleen,yellow bone marrow,red bone marrow.

Functions of Blood

The blood and blood vessels form a transport system of an animal by which substances are taken to and from the tissue cells.The tissue cells are not in direct contact with blood,they are surrounded by a tissue fluid which acts as an intermediary between the tissue cells and the blood capillaries.Plasma carries digested food and water to all body cells; the haemoglobin of red blood corpuscles carries oxygen from respiratory surfaces to tissue cells; plasma and erythrocytes remove carbon dioxide from tissue cells to the respiratory organs from where it passes out.

Blood regulates and equalizes body temperature by carrying heat away from regions where it is produced, phagocytic leucocytes combat bacteria and foreign bodies,thus prevent diseases,the plasma contains antitoxins and antibodies, which neutralize the harmful effects of bacteria and toxins.Blood plasma prevents loss of much blood from an injury by forming a clot from its fibrinogen ,blood platelets and thrombocytes also help in this, while antithrombin present in plasma prevents clotting of blood inside the body.

Heart

Development:The heart is an unpaired organ but its origin is bilateral.In an embryo the mesenchyme forms a group of endocardial cells below the pharynx,these cells become arranged to form a pair of thin endothelial tubes.The two endothelial tubes soon fuse to      form a single endocardial tube lying longitudinally below the pharynx.In mammals the endocardial tube is formed from a single endothelial tube.This two-layered tube will form the heart in which the splanchnic mesoderm thickens to form a myocardium or muscular wall of the heart and an outer thin epicardium or visceral pericardium. The endocardial tube becomes the lining of the heart known as endocardium. Folds of splanchnic mesoderm meet above to form a dorsal mesocardium which suspends the heart in the coelom. Soon a transverse septum is formed behind the heart which divides the coelom into two chambers, an anterior pericardial cavity enclosing the heart and a posterior abdominal cavity.

Coronary circulation: In addition to the blood inside the heart, there is a coronary circulation in the tissues of the heart which supplies nourishment and oxygen, and removes metabolic wastes.

Heart beat: Beating of the heart is under the influence of nervous impules, but there are contraction centres centres or pacemakers in the heart itself which cause waves of contractions.