System overview: heart, blood vessels and blood constitute the cardiovascular system
The cardiovascular system comprises three principal components namely the heart, blood vessels, and blood transports materials to and from various parts of the body. Among the materials transported by the cardiovascular system are oxygen, carbon dioxide, nutrients (products of digestion) and hormones.
The heart receives blood from the whole body through two major blood vessels called superior vena cava and inferior vena cava respectively; these major blood vessels empty deoxygenated blood into the heart.
The heart pumps oxygenated blood into the aorta, which carries the blood to the whole body. Generally, arteries carry oxygenated blood whereas veins carry deoxygenated blood: the only exception being pulmonary artery and pulmonary veins. With pulmonary veins returning oxygenated blood to the heart and are the only veins that carry oxygenated blood. Pulmonary artery carry deoxygenated blood away from the heart and is the only artery in the body that carries deoxygenated blood.
Heart is a muscle pump
The heart, a hollow organ muscular organ about the size of a fist is made up of specialized muscle cells called cardiac muscles. The heart of an average healthy adult at rest beats 72 times per minute. With each beat, the heart pumps about 5 L of blood into the vessels. Interestingly, the heart must continue to beat and pump blood in order to sustain life.
If the heart ceases to pump (as occurs during cardiac failure and heart attacks) at any time, the body cells fail to get nutrient and excrete waste and may result in death if prompt and immediate medical attention is unavailable.
Functional anatomy of the heart
The heart lies in the center of the thoracic cavity, above the diaphragm, behind the sternum and between the lungs. Actually, the heart has the shape of an inverted cone with a broader base that lies behind the sternum and a pointed apex that angles down to the left side of the body ultimately resting on the diaphragm.
The walls of the heart constitute three layers:
- An outer fibrous sac called pericardium
- Middle myocardium
- Inner endocardium
Outer pericardium: The pericardium, located in the thorax, forms a fibrous sac that encloses the heart. Another fibrous membrane lines the heart and closely affixes itself to the myocardium. This fibrous layer called epicardium plays a significant role in normal heart function. There is an extremely narrow but functionally significant space between the pericardium and epicardium. This space contains a thin layer of clear fluid called pericardial fluid; pericardial fluid acts as a lubricant that prevents unnecessary friction as the heart moves within the sac as the heart contracts and relaxes.
In abnormal situations where the pericardial fluid is grossly inadequate, the lubrication diminishes making the heart to rub against the pericardium. In extreme cases where the fluid is almost absent, the friction between the heart and pericardium rises to the point that creates sound called friction rub. The pericardial covering of the heart is a thin membranous layer.
Middle myocardium: The myocardium constitutes cardiac muscle cells. Myocardium forms the bulk of the thickness of the heart wall. Cardiac muscle cells (also called cardiomyocytes) are the cells that make up the myocardium. The myocardium forms the main functional layer of heart wall. Cardiac muscles are responsible for heart contractions, which pump blood to the body.
Inner endocardium: Endocardium is the thin layer of cells that form the innermost lining of heart wall. Cells that form the endocardium, called endothelial cells separate the myocardium from the heart chambers. In addition to forming the innermost lining of the heart chambers, endothelial cells also form the inner wall of all blood vessels in the body.
Human heart has four chambers
The heart is a hollow organ that has four chambers, separated by valves and a septum. A septum runs from the base of the heart to the apex separating the heart into right and left halves. Because of the septum, blood on left side does not mix with blood on the right side and vice versa. During contraction, the two atria contract together, before the ventricles contract together.
The four heart chambers constitute two atria and two ventricles. Atrio-ventricular valves further divides each halve of the heart into right atria and right ventricles: and left atria and left ventricle.
The right atrium receives blood from all parts of the body except from the lungs through the superior and inferior vena cavae, and pumps the blood into the right ventricle. Left atrium receives blood from the lungs through the pulmonary veins and pumps the blood into the left ventricles. Therefore, the two atria mainly receive blood from veins and deliver the blood to the ventricles through atrio-ventricular valves. The valves ensure that blood flows in one direction only, preventing turbulent flow.
The right ventricle pumps blood into the pulmonary artery, which takes the blood to the lungs. The left ventricle, on the other hand pumps blood into the largest artery in the body, the systemic aorta, which takes the blood to all tissues of the body. When contracting, the ventricles contract from bottom up, functionally; this squeezes blood out of the top into the arterial trunks.
Four fibrous connective tissue rings surround the atrio-ventricular junction and act as an insulator blocking the transmission of electrical signals from atria to the ventricles. Recall that connective tissues are unable to conduct electrical impulses since they are non-excitable tissues. Because of this arrangement, electrical signals from the atria pass to the ventricles through specialized cardiac muscle fibers that form the conductive system of between the atria and the ventricles. The specialized fibers that form the conduction system conduct electrical impulses from the atria to the apex of the heart before conducting the impulses upward along the free walls of the ventricles. This nature of conduction ensures the bottom-to-top contraction of the heart.
Morphologically, the free wall of the left ventricle and its septum are significantly thicker than the wall of the right ventricle. This is functionally significant since the left ventricle has to pump blood to the systemic circulation where the pressure is obviously larger than that of the pulmonary circulation, which arises from the right ventricles.
Two sets of heart valves ensure one-way flow of blood within the heart
Two sets of heart valves make it possible for blood to flow in one direction within the heart. These valves are present between
- the atria and the ventricles (atrio-ventricular valves)
- the ventricles and the pulmonary and aortic arteries (semilunar valves)
Perhaps, the two sets of heart valves have different structures; they serve the same basic function, which is to prevent backward flow of blood.
One set of heart valves called atrio-ventricular valves guard the opening between each atrium and its ventricle. The other set called semilunar valves guard the opening between the ventricles and the arteries.
There are two atrio-ventricular: bicuspid valve and the tricuspid valve. These valves guard the opening between each atrium and its ventricle. AV valves form from thin flaps of tissues connected at the base to the connective tissue ring present at the AV junction.
Morphologically, the two AV valves are not identical; AV valve that separates left atrium from left ventricle has only two flaps giving it the name bicuspid valve. Bicuspid valve is also called mitral valve. The AV valve that separates the right atrium from the right ventricle has three flaps, which gives it the name tricuspid valve.
Semilunar valves separate the ventricles from the major arterial trunks: the pulmonary artery and the aorta. The aortic valve lies between the left ventricle and the aorta whereas, the pulmonary valve lies between the right ventricle and the pulmonary artery. Unlike the atrio-ventricular valves, which have different structures, both sets of semilunar valves have the same structure; they have three cup-like flaps that snap closed when blood attempting to flow back into the ventricles presses on them.