Human Circulatory System

THE HUMAN HEART

STRUCTURE OF THE HUMAN HEART

The human heart is a hollow, conical and muscular organ that is placed obliquely between the two lungs in front of the 5^th, 6^th, 7^th and 8^th thoracic vertebrae, just above the diaphragm. In adults it weighs about 300 g in males and 250 g in females; the size of the heart is almost the same as the size of a closed fist. The heart is enclosed in a protective, fibrous sac called the pericardium. The pericardium is differentiated as parietal pericardium and visceral pericardium (epicardium). Between the two layers is a space called the pericardial space filled with pericardial fluid. The pericardium protects the heart from shocks and injuries and also keeps the heart moist and prevents friction during functioning.

The human heart has four chambers – the upper right and left thin-walled auricles (atria) and the lower right thick-walled ventricles. The auricles are the receiving chambers while the ventricles are referred to as pumping chambers. The auricles are separated by a vertical wall called the inter-auricular septum. Two large veins called the superior and inferior vena cava convey deoxygenated blood to the right auricle. The oxygenated blood from the lungs enters the left auricle by way of the pulmonary veins, two from each lung.

The ventricles are separated by a vertical wall called the inter-ventricular septum. The inner walls of the ventricles have irregular projections called trabaculae formed by the papillary muscles of the ventricular wall. The right ventricle communicated with the right auricle by an opening called the right aurico-ventricular opening which is guarded by a valve having three cusps (flaps) called the tricuspid valve. It allows the blood to flow only from the right auricle to the right ventricle. The left ventricle communicates with the left auricle by an opening called the left aurico-ventricular opening which is guarded by a valve called the bicuspid valve or mitral valve. It allows the blood to flow only from the left auricle to the left ventricle. These two valves are connected to the trabaculae by thin, elastic cords called chordate tendinae. These cords regulated the opening and closing of the two valves.

The right ventricle opens into a large blood vessel called the pulmonary artery, which bifurcates; each branch supplies a lung. The opening of the right ventricle into the pulmonary artery is guarded by a semi-lunar valve called the pulmonary valve. The left ventricle leads into a large blood vessel called the aorta which bends or arches to the left side of the heart (in man and all mammals) to form to form the left aortic arch. The opening of the left ventricle into the aorta is guarded by another semi-lunar valve called the aortic valve. These two valves regulate the flow of blood from the ventricles to their respective vessels.

The wall of the heart consists of three layers namely

(i) The inner endocardium – made of the endothelium which lines the chambers, covers the valve surfaces and continues lining the blood vessels that enter and leave the heart.

(ii) The middle myocardium - a thick layer consisting of the cardiac muscles. It is thinner on the atrial walls and thicker on the ventricular walls. It forms the irregular trabaculae on the inner surfaces of the ventricles.

(iii) The out epicardium or visceral pericardium is a thin and transparent layer composed of fibrous tissue with an outer covering of mesothelium. Above the visceral pericardium is the parietal pericardium and in between these layers is a space called the pericardial cavity which is filled with pericardial fluid.

THE HEART AS A DOUBLE PUMP

The hearts of all mammals including human beings are called double pumps. Since, during one complete circulation, one side of the heat pumps deoxygenated blood and the other side of the heart pups oxygenated blood. This type of circulation is referred to as double circulation.

Double circulation involves two separate pathways, the pulmonary circulation and the systemic circulation. Each pathway is constituted by both arterial and venous systems. The arterial system consists of arteries, arterioles and capillaries while the venous system consists of capillaries, venules and veins. The arterial system carries blood away from the heart while the venous system brings blood back to the heart.

The arteries have thick, elastic, muscular walls with narrow lumen; veins have thin, non-elastic fibrous walls with wide lumens. Arteries don’t have valves, while veins have valves to prevent backflow of blood. Arteries are deep seated (located) while veins are superficial. In arteries, blood flows with jerks under high pressure, while in veins the blood flows smoothly under low pressure. All arteries convey oxygenated blood except the pulmonary artery while all veins convey deoxygenated blood except pulmonary veins.

Pulmonary Circulation

It is the pathway of the blood taken between the heart and the lungs, hence it is referred to as pulmonary circulation.

Deoxygenated blood from the heart goes to the lungs via the pulmonary artery. After gaseous exchange, oxygenated blood returns to the heart via the pulmonary veins.

Pulmonary Circuit

The pulmonary artery arises from the conus arteriosus (front upper portion of the right ventricle) of the right ventricle. It lies in front of the ascending aorta and then passes to its left and under the aortic arch, where it divides as the left and right pulmonary arteries. The right pulmonary artery is larger and longer than the left pulmonary artery. The right pulmonary artery passes horizontally in front of the right bronchus to the root of the right lung, where it divides into two branches – the upper and the lower branch.

The upper branch is smaller than the lower branch and supplies the upper lobe of the right lung. The lower branch divides further and supplies the middle and lower lobes of the right lung.

The left pulmonary artery passes horizontally in front of the descending aorta and the left bronchus to the root of the left lung where it divides into two branches, each one supplying the upper lobe and lower lobe of the left lung.

The arterioles of the pulmonary arteries open into the capillary beds in the lungs. These capillary beds are found surrounding the alveoli. The capillaries join the vennules which in turn lead into the pulmonary veins. Four pulmonary veins, two from each lung enters the posterior upper side of the left auricle. These vessels are without valves.

Systemic Circulation
It is the pathway of the blood taken between the heart and the different parts of the body.

Oxygenated blood from the heart goes to different parts of the body via the aorta and its branches. After gaseous exchange at the tissue, the deoxygenated blood returns to the heart via the superior and inferior vena cava.

Systemic Circuit

The starting point of the arterial system of the systemic circuit is the large aorta (25 mm in diameter). It is the largest artery in the body which supplies oxygenated blood to all the arteries of the systemic circuit.

The aorta which originates from the aortic valves extends upwards as the ascending aorta, then arches or bends to the left side to form the aortic arch and passes in front of the root of the left lung and runs down as the descending aorta along the left side of the thoracic vertebrae. The descending aorta is differentiated into the thoracic aorta and the abdominal aorta.

The ascending aorta is found within the pericardial sac. It gives off two branches, namely the left and the right coronary arteries, each of which profusely branch to supply the muscles of the heart (myocardium) and brings back deoxygenated blood by the coronary veins. This part of the systemic circulation constitutes the coronary circulation.

The right and left coronary arteries are the only branches of the ascending aorta. They originate from bulb-like swellings called the aortic sinuses, located behind the cusps of the aortic valve; hence blood can enter these arteries only when the left ventricle is relaxed. The right coronary artery arises from the right aortic sinus, passes between the right auricle and the conus arteriosus into the coronary sulcus (a groove between the right atrium and ventricle) and turns around the inferior margin of the heart and divides into a right posterior inter-ventricular descending artery, a right marginal artery and a transverse artery. The left coronary artery, which is larger than the right one, divides into a left anterior inter-ventricular descending artery and a circumflex artery.

The branches of the left and right coronary arteries completely encircle the heart forming an upside-down crown around it, hence called coronary arteries.

The left coronary artery supplies the anterior part of the inter-ventricular septum and the adjacent part of the right ventricle as well as the anterior surface of the left ventricle, a small part of the inferior surface of the left ventricle and the left margin of the left ventricle.

The right coronary artery supplies a major portion of the right ventricle, the posterior part of the inter-ventricular septum and many parts of the left ventricle.

The venous system of the coronary circuit is represented by the large coronary sinus, lying in the coronary sulcus on the posterior side of the heart.

The tributaries of the coronary sinus are the greater cardiac vein, the small cardiac vein and the middle cardiac vein. All these veins enter the coronary sinus which in turn empties deoxygenated blood into the right atrium (except the anterior cardiac vein which directly empties into the right auricle).

The greater cardiac vein runs with the left anterior inter-ventricular and circumflex branches of the left coronary artery. The small cardiac vein runs with the marginal branch of the right coronary artery. The middle cardiac vein runs with the posterior inter-ventricular branch of the right coronary artery.