Structure and Function
The blood is kept in circulation through the body in a system of specialised tubes called blood vessels. The two main types of blood vessels are arteries and veins. All blood vessels leading from the heart are called the arteries whether they contain oxygenated blood as in aorta or deoxygenated blood as in pulmonary artery. The blood vessels which bring blood towards the heart are the vein. Except pulmonary veins, all other veins bring deoxygenated blood to the heart. The pulmonary veins bring oxygenated blood from the lungs.
Transverse Sections of Artery and Vein
Structure of Arteries
Arteries have thick elastic muscular walls, which have three layers, viz. Tunica externa, Tunica media and Tunica interna. The tunica externa, also called tunica adventitia is the outermost, and is made up of loosely arranged elastic collagen fibres. The tunica media is formed of a thick layer of unstriped muscles and elastic connective tissue fibres, arranged in a circular fashion. Tunica interna, the innermost layer consists of flat thin cells and a thin elastic membrane. It is also called the endothelium.
Structure of Veins
Basically, the veins resemble arteries in structure, but are very thin and inelastic. The muscles and connective tissues (tunica media layer) is poorly developed. In many veins, there are valves (semilunar valves) which allow the blood to flow only towards the heart. These valves are numerous in the veins of limbs.
Longitudinal Section of Vein
The arteries branch into smaller vessels called arterioles and end in networks of still smaller vessels called capillaries. The capillaries lead into small veins called venules which form small veins and finally larger veins. In this system of circulation the blood is always confined to blood vessels, flowing from the heart to the arteries, then to the arterioles and capillaries, and through the venous capillaries to venules, veins and finally returns back to the heart. Such a circulatory system is called a closed circulatory system, which is present in all vertebrates, and in some invertebrates such as earthworms and octopus.
In many invertebrates like prawn, cockroach and some molluscs, the blood pumped by the heart comes out from the blood vessels into open tissue spaces during circulation. These irregular tissue spaces, called lacunae, carry the blood to wider spaces called sinuses. From the sinuses the blood ultimately returns to the heart. Such a system of circulation is called the open system of circulation.
Major Arteries and Veins
The aorta is the main distributing artery of the body. Its first branches are the coronary arteries which supply blood to the heart muscles. The aorta then curves upwards and backwards and lies along the backbone.
The veins collecting blood from the legs unite to form a major vein called the inferior vena cava. The inferior vena cava in its course towards the heart receives veins from the abdomen, gonads, kidneys and liver. The blood returning from the digestive tract is first brought to the liver by a portal vein. The venous blood from the liver is poured into the inferior vena cava through the hepatic veins. A number of smaller veins collect the blood from the upper parts of the body. These smaller veins unite to form the paired innominate veins. These in turn unite to form the large superior vena cava which opens into the right auricle. The coronary arteries supply blood to the chambers of the heart, opens into the right auricle. The coronary vein returns the blood collected from the walls of the heart.
The Human Arterial System
The Human Circulatory System
Different Types of Hearts
In human beings and other mammals as well as in birds, the heart is completely partitioned into right and left halves. The right half receives and pumps deoxygenated blood. The left half on the other hand receives and pumps oxygenated blood. This system is known as the double-circuit system.
In lower vertebrates like fishes the heart is more or less a simple tube without any partitions or septa. It receives and pumps deoxygenated blood, which flows through arteries to the gills for oxygenation. In the gills the exchange of respiratory gases takes place and the oxygenated blood is carried through another set of arteries from the gills to the different tissues and organs of the body. From the tissues and organs the deoxygenated blood is returned to the heart through veins. This circulation is called the single-circuit system.
In amphibian (frog) and reptiles (lizards and snakes) the heart is incompletely partitioned and receives both oxygenated and deoxygenate bloods which get mixed inside the heart. In this type of circulation the heart pumps out mixed blood through the arteries. This is called the intermediate type. However in birds and mammals it is four chambered.