Human Circulatory System - Human Heart

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Human Circulatory System

• Human circulatory system constitutes blood vascular system and lymphatic system . Both are of closed type.
• The blood vascular system comprises heart, blood and blood vessels.
• The lymphatic system consists of lymph, lymphatic capillaries, lymphatic vessels, lymphatic nodes and lymphatic ducts.
• Harvey discovered circulatory system.

Human Heart

• Location, Size and Shape. 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. The heart is a hollow, fibromuscular organ of a somewhat conical or pyramidal form with upper broad part, the base and lower narrow, the apex.
• The apex is slightly directed to the left. Pericardium (Protective Covering).
• It is a 2 - layered sac consisting of outer parietal pericardium and inner visceral pericardium attached to the heart.
• In between the two layers, a space, the pericardial cavity is present which is filled with a pericardial fluid.
• The pericardium protects the heart from shocks and mechanical injuries and also allows free movements of the heart.

Structure of Human Heart

External Structure

• Human heart is four chambered , consisting of two atria and two Ventricles.
• Grooves (Sulci). The left and right atria are separated externally by a shallow vertical interatrial groove.
• The atria are demarcated externally from the ventricles by an oblique groove called atrioventricular sulcus.
• There are present coronary sulcus, anterior inter ventricular sulcus and posterior interventricular sulcus.
• These have coronary arteries , through which the heart receives blood

• Atria ( sing . atrium ). As stated ealier , there are left and right atria which have thin walls . The left atrium is smaller than the right atrium.
• The right atrium is a roughly quadran gular chamber. Each atrium has an appendage called an auricle (L auris - car), so named because its shape resembles a dog's ear.
• The auricle increases the atrium's surface area. The superior vena cava, inferior vena cava and coronary sinus open into the right atrium.

• The superior vena cava carries blood 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 has thicker walls. The left atrium is roughly cuboidal . It projects as the left auricle towards the left side.
• The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins.

• Ventricles. As mentioned earlier, 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 left ventricle is longer and narrower than the right ventricle.
• The walls of the left ventricle are about three times thicker than the right ventricle.

• Pulmonary Trunk and Aorta. 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 (Innominate 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 embryo the ligamentum arteriosum is called ductus arteriosus.
• Coronary arteries arise from the ascending aorta and supply blood to the heart.

Internal Structure.

• The internal structure of the heart can be better studied by dissect ing it from the ventral side.
• Atria. The two thin walled atria are separated from each other by the interatrial septum.
• 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 adjoin ing the interatrial septum, an oval depression, the fossa ovalis 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 artrium receives four openings of pulmonary veins.

• Bicuspid and Tricuspid Valves. 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 atrio - ventricular opening is guarded by the tricuspid valve, as it has three flaps.

• Ventricles. Attached to the flaps of the bicuspid and tricuspid valves are special fibrous cords, the chordae tendineae, which are joined to the other ends with the special muscles of the ventricular wall , the papillary muscles.
• The chordae tendineae prevent the bicuspid and tricuspid valves from collapsing back into the atria during powerful ventricular contractions.
• The chordae tendineae can be seen extending from the valves to the columnae carneae, which are the muscular ridges or projections on the walls of the ventricles.
• The columnae carneae 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.

• Semilunar valves. As stated in the external structure, 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 pulmonary semilunar valves and aortic semilunar valves respectively.
• These valves allow the free and forward flow of blood , but prevent any backward flow.
• Thus the valves of the heart are bicuspid valve, tricuspid valve, aortic semilunar valves and, pulmonary semilunar valves.

• 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.
• The heart collects blood through both the atria and then distributes it through the ventricles.
• The action of heart includes contractions and relaxations of the atria and ventricles.
• A contraction of the heart is called a systole and its relaxation a diastole.
• The atria and ventricles contract alternately. The contraction of heart (systole) and the relaxation of heart (diastole) constitute the heart beat. The contraction of atria is initiated and activated by the sinoatrial node
• which spreads waves of contraction across the walls of the atria via muscle fibres at regular intervals.
• When the wave of contraction originating from the sinoatrial node reaches the atrio ventricular 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 hence must exert great pressure on the blood.

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