Cardiovascular System – The Heart

In large, multi cellular organisms, rapid transport of molecules over the long distances between internal cells and the body’s surface, and between the various, specialized tissues and organs is achieved by the circulatory system which includes a pump (the heart) a set of interconnected tubes (blood vessels or vascular system) and a mixture of extracellular fluid and cells that fill the tubes (the blood).
This body-wide transport system is often termed the cardiovascular system.

Human Heart

• The heart is a muscular organ responsible for pumping blood through the blood vessels by repeated, rhythmic contractions.
• In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left (although sometimes it is on the right, underneath the breastbone).
• The heart is located in the mediastinum. It is the space between the two lungs, i.e., the central subdivision of thoracic cavity. It extends from the sternum in front to the vertebral column behind and is flanked on either side by the right and left pulmonary cavities, which house the lungs.

Structure of Heart

• The human 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.
• The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue; and a double membrane structure (parietal and visceral pericardium) containing a serous fluid to reduce friction during heart contractions. You can find the human heart picture below.
• Human heart anatomy describes that it is 4 chambered, consisting of two atria and two ventricles.
• The left and right atria are separated externally by a shallow vertical interatrial groove and internally by interatrial septum.
• The atria are demarcated externally from the ventricles by an oblique groove called atrioventricular sulcus. There are also present coronary sulcus, anterior interventricular sulcus and posterior interventricular sulcus. These have coronary arteries, through which the heart receives blood.
• The right atrium receives the openings of superior vena cava, inferior vena cava and coronary sinus. • The superior vena cava, carries blood from upper body and the inferior vena cava carries blood from the lower body region.
• Coronary sinus carries blood from heart itself. The right atrium receives deoxygenated blood.
• The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins. An oval depression known as fossa ovalis is present in the right atrium near inter atrial septum. It marks the position of an opening between two atria in the foetus i.e. foramen ovale but in the adult it persists as a depression.
• Ventricles are thick walled. The left ventricle is longer and narrower than the right ventricle. Its walls are about three times thicker than the right ventricle. The inner surface of the ventricles is raised into a network of low, muscular ridge called the columnae carneae, or trabeculae carneae, and a few large, conical, muscular elevations termed the musculi papillares, or papillary muscles.
• The two ventricles are separated from each other by a thick, curved partition, the interventricular septum. A prominent muscular trabeculum, called the moderator band, extends from the interventricular septum to the anterior papillary muscle in 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 (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. The coronary arteries exit behind the aortic valve cusps and lead to a branching network of small arteries, arterioles, capillaries, venules, and veins similar to those in other organs.
• Most of the cardiac veins drain into a single large vein, the coronary sinus, which empties into the right atrium.

Heart Valves
• There are no valves at the entrances of the superior and inferior venae cavae (plural of vena cava) into the right atrium, and of the pulmonary veins into the left atrium.
• However, atrial contraction pumps very little blood back into the veins because atrial contraction, constricts their sites of entry into the atria, greatly increasing the resistance to backflow.
• Actually, a little blood is ejected back into the veins, and this accounts for the venous pulse that can often be seen in the neck veins when the atria are contracting. Name Location Function
1 Bicuspid valve or Mitral valve (has two flaps) Between left atrium and ventricle in the heart Allows the blood to move from left atrium to left ventricle in the heart.
2 Tricuspid valve (has three flaps) Between right atrium and ventricle Allows the blood to move from right atrium to right ventricle.
3 Aortic semilunar valve Between aorta and left ventricle Allows unidirectional flow of oxygenated blood from left ventricle to aorta. Prevents back flow.
4 Pulmonary semilunar valve Between pulmonary artery and right ventricle in the heart Allows unidirectional flow of deoxygenated blood from right ventricle to pulmonary artery. Prevents back flow.
5 Eustachian valve Right atrium Guards the opening of inferior vena cava (functional only in foetus).
6 Thebasian valve (Coronary valve) Right atrium Guards the opening of coronary sinus in the heart.

Functioning of valves
• The opening and closing of the AV valves are passive processes resulting from pressure differences across the heart.
• Location and functions of heart valves are summarized in the table given below.
the valves. When the blood pressure in an atrium is greater than in the corresponding ventricle, the valves is pushed open and blood flows from atrium to ventricle in the heart.
• In contrast, when a contracting ventricle achieves an internal pressure greater than that in its connected atrium, the AV valve between them is forced closed. Therefore, blood does not normally move back into the atria and is forced into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle. The below picture represents the valves in the heart.
• To prevent the AV valves from being pushed up into the atria when the ventricles are contracting (a condition called prolapse), the valves are fastened to muscular projections (papillary muscles) of the ventricular walls by fibrous strands chordae tendineae.
• The papillary muscles do not open or close the valves. They act only to limit the valves’ movement and prevent the backward flow of blood in the heart.

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