ANATOMY OF THE HEART
The heart is in left thoracic cavity. The base of the heart is cephalic and the apex is caudal. The heart is a muscular organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contraction. The term cardiac means “related to heart” and comes from Greek “kardia” for the heart. The heart is surrounded by fluid-filled sac called the pericardium.
The size of the heart can vary depending upon the age, size, or condition of the heart. A normal, healthy adult heart most often in size of an average clenched adult fist. Some diseases of the heart can cause it to become larger.
The heart of a vertebrate is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. The average human heart, beating at 72 beats/min will beat approx. 2.5 billion times without over passing during a life time (About 66 years) (Figs 1.1 and 1.2).
LOCATION OF THE HEART
The human heart is a four chambered muscular organ, shaped and sized roughly like a men's clenched fist, located under the ribcage between right and left lung with two-thirds of mass to the left of midline.
The heart enclosed in pericardial sac is lined with parietal layers of serous membrane. The visceral layer of serous membranes forms the epicardium (Fig. 1.3).
LAYERS OF THE HEART WALL
Three layers of tissues form the heart wall. The outer layer wall is the epicardium, the middle layer is the myocardium and the inner layer is the endocardium.
CHAMBERS OF THE HEART
The internal cavity of the heart is divided into four chambers:
- Right atrium
- Right ventricle
- Left atrium
- Left ventricle.
The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are due to variation in amount of myocardium present, which reflects the amount of force each chamber is required to generate.
The right atrium receives deoxygenated blood from systemic veins: the left atrium receives oxygenated blood from the pulmonary veins.
VALVES OF THE HEART
Pumps need a set of valves to keep the fluid flowing in one direction and the heart is no exception. The heart has two types of valves that keep the blood flowing in correct direction. The valves between the atria and ventricles are called atrioventricular valves (also called cuspid valves), while those at the bases of large blood vessels leaving the ventricles are called semilunar valves.
The right atrioventricular valve is the tricuspid valve. The left atrioventricular valve is the bicuspid valve or the mitral valve. The valve between right ventricle and pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and aorta is the aortic semilunar valve (Fig. 1.4).
When the ventricles contract, AV valves close to prevent blood from flowing back into the atria. When ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.
PATHWAY OF BLOOD THROUGH THE HEART
It is important to realize that both atria contract at the same time and both ventricles contract at the same time. The heart works as two pumps, one on right side and one on the left, working simultaneously. Pathway of blood through the heart is illustrated in Flow chart 1.1.
THE PULMONARY AND SYSTEMIC CIRCUITS AND THE BLOOD SUPPLY TO THE HEART
The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. The heart is able to pump blood to both regions to efficiently, because there are really two separate circulatory circuits with the heart as the common link. Some authors even refer to the heart as two separate hearts—a right in the pulmonary circuit and left heart in the systemic circuit.
In the pulmonary circuits blood leaves the heart through pulmonary arteries, goes to the lungs and returns to the heart through the pulmonary veins.
In the systemic circuit, blood leaves the heart through the aorta, goes to all organs of the body through the systemic arteries, and then returns to the heart through the systemic veins. Thus, there are two circuits—pulmonary and systemic. Arteries always carry blood away from the heart and veins always carry blood towards the heart. Most of the time, arteries carry oxygenated blood and veins carry deoxygenated blood. But there are exceptions: the pulmonary artery leaving the right ventricle for the lungs carry deoxygenated blood and pulmonary veins carry oxygenated blood. The blood does not have to travel as far when going from heart to lungs as it does from heart to toes, thus making left side heart larger than the right side (Fig. 1.5).
BLOOD SUPPLY TO MYOCARDIUM
The myocardium of heart wall is a working muscle that needs a continuous supply of oxygen and nutrients to function with efficiency. For this reason, cardiac muscle has an extensive network of blood vessels to bring oxygen to the contracting cells and to remove waste products.
The right and left coronary arteries, branches of ascending aorta supply blood to the walls of the myocardium. After blood passes through the capillaries in the myocardium, it enters a system of cardiac (coronary) veins. Most of cardiac veins drain into coronary sinus, which opens into the right atrium.