Comprehensive Essay on Human Blood Vascular System

The human blood vascular system is an example of closed type with muscular heart, blood (blood plasma and blood corpuscles) and blood vessols (arteries and veins).

Human Heart :

Human heart is located between the lungs in the thoracic cavity. An average adult heart is about 12 cm. Its weight varies in males from 280-340 g (average 300 g) and in females from 230-280 g (average 250 g). Weight of the heart is said to be about 0.45% of body weight in males and 0.40% in females. Adult weight is achieved between 17-20 years.

Pericardium is a two layered sac consisting of outer parietal pericardium and inner visceral pericardium. In between the two layers, a space, the paricardial cavity is present which is filled with a pericardial fluid. The pericardium protects the heart form shocks and mechanical injuries and also allows free movements of the heart.

Structure of Human Heart External Structure:

As stated ealier, there are left and right atria which have thin walls. The left atrium is smaller than the right atrium. The superior vena cava carries from the body’s upper region. The inferior vena cava is larger than the superior and carries blood from the lower body’s region. The coronary sinus carries the majority of blood from the heart itself. The coronary veins open into the coronary sinus. The right atrium receives deoxygenated blood.

The left atrium is less in volume than that of right atrium but it was thicker walls, the left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins. There are present left and right ventricles with thick walls. The wall of the right ventricle is thinner than that of the left ventricle. The walls of the left ventricle are about three times thicker than the right ventricle. The pulmonary trunk arises from the right ventricle.

It divides into left and right pulmonary arteries that carry deoxygenated blood to the lungs. The aorta arises from the left ventricle. It is divisible into the ascending aorta, arch of aorta and descending aorta. The right and left coronary arteries arise from the ascending aorta. The arch of the aorta (also called aortic arch) gives rise to the brachiocephalic artery (in nominate artery), left common carotid artery and left subclavian artery. The descending aorta runs through the thorax and abdomen and hence it is divisible into thoracic and abdominal parts.

The pulmonary trunk is connected with the aorta by the ligamentum arteriosum that represents the remnant of an embryonic connection between the pulmonary trunk and aorta. In the embryo, the ligamentum arteriosum is called ductus arteriosus.

Internal Structure:

The internal structure of the heart can be better studied by dissecting it from the ventral side. The right atrium receives the openings of superior vena cava, inferior vena cava and coronary sinus.

The opening of inferior vena cava is guarded by eustachian valve. The opening of the coronary sinus has coronary or Thebasian valve. In the right atrium adjoining the interatrial septum, an oval depression, the fossaovalis is present. It marks the position of an opening, the foramen ovale, between the two atria in the foetus, but in the adult it persists only as a depression.

The left atrium receives four openings of pulmonary veins. The artrioventricular opening between the left atrium and the left ventricle is guarded by the bicuspid valve, also called mitral valve (having two flaps).

The right atrioventricular opening is guarded by the tricuspid valve, as it has three flaps. Attached to the flaps of the bicuspid and tricuspid valves are special fibrous cords, the chorded tendineae, which are joined to the other ends with the special muscles of the ventricular wall, the papillary muscles. The chordate tendineae prevent the bicuspid and tricuspid valves from collapsing back into the atria during powerful ventricular contractions.

The chordate tendineae can be seen extending from the waives to the columnae carneae, which are the muscular ridges or projections on the walls of the ventricles. The columnae corneae divide the cavity of the ventricles into smaller spaces, known as fissures. The walls of the ventricles are thicker than the atria. The thickest portion of the human heart is the wall of the left ventricle. The pulmonary trunk and aorta arise from the right and left ventricles respectively. At the base of the pulmonary trunk and aorta are located three half- moon shaped pockets known as semi lunar valves. These valves allow the free and forward flow of blood, but prevent any backward flow.

Histologically, the heart has an outermost smooth coelomic epithlium-the visceral pericardium, the middle thick muscular layer-the myocardium, composed of cardiac muscle fibres and the innermost layer, the endothelium consisting of simple squamous epithelial cells.

Working of Human Heart :

A contraction of the heart is called a systole and its relaxation is diastole. The contractions and relaxations of atria and ventricles take place in a sequential order. The atria and ventricles contract alternately. The contraction of heart (systole) and the relaxation of heart (diastole) constitute the heart beat.

The right atrium receives de-oxygenated blood from different parts of the body and the muscular walls of the hart itself through the openings of superior vena cava, inferior vena cava and coronary sinus respectively. The left atrium receives oxygenated bloods from the lungs through the openings of the pulmonary veins. The de-oxygenated and oxygenated bloods are forced into their respective ventricles through atrioventricular openings by the contraction of the atria.

The contraction of atria is initiated and activated by the sinoatrial node (SA Node-pace maker) which spreads waves of contraction across the walls of the atria via muscle fibres at regular intervals. When the wave contraction originating from the sinoatrial node reaches the atrioventricular node (AV Node- pace setter), the latter is stimulated and excitatory impulses are rapidly transmitted from it to all parts of the ventricles via bundle of His and Purkinje’s fibres. These impulses stimulate the ventricles to contract simultaneously. The ventricles force blood through long system of arteries and have must exert great pressure on the blood.