The human heart is a muscular pump, which is located in between the lungs, and slightly to the left side of the sternum or breastbone. The heart of an average adult female weighs around nine ounces; and in an average adult male, the organ weighs around 10.5 ounces. Its length is around six inches, and the width is roughly four inches. An average human heart beats approximately 72 times per minute, and pumps four to five liters of blood (per minute) at rest.
Location and Structure
The heart is located in the middle of the chest - anterior to the spine and posterior to the sternum or breastbone (a long flat bone in the center of the chest). The heart lies slightly to the left, from the center of the thorax (region between the head and the abdomen). Hence, the left lung is smaller, when compared to the right lung.
The heart is divided into two cavities (left cavity and right cavity) by a muscle wall called the septum. Each cavity consists of two chambers. Upper chambers are called atria, and the lower ones are called ventricles. The right cavity receives de-oxygenated blood from various parts of the body (except the lungs), and pumps it to the lungs; whereas the left cavity receives oxygenated blood from the lungs, which is pumped throughout the body. Let us discuss the anatomy of this amazing organ in detail.
- Outer Covering - Pericardium: The heart and the roots of its major blood vessels are surrounded and enclosed by a sac-like structure called pericardium. It comprises two parts - the outer fibrous pericardium, made of dense fibrous connective tissue and an inner double-layered membrane. The fibrous pericardium is attached to the spinal column, diaphragm, and other parts of the body, by ligaments. The double-layered membrane consists of an inner layer called visceral pericardium, an outer layer called parietal pericardium (fused to fibrous pericardium), and a pericardial cavity (lies between the two layers), which contains serous fluid, called pericardial fluid. This fluid helps in reducing the friction caused by the contractions of the heart.
- Heart Wall: The wall of the heart is made up of three layers of tissues - outer epicardium, middle myocardium, and the inner endocardium. The outer epicardium functions as a protective outer layer, which includes blood capillaries, lymph capillaries, and nerve fibers. It is similar to the visceral pericardium, and consists of connective tissues covered by epithelium (membranous tissue covering internal organs and other internal surfaces of the body). The inner layer called myocardium, which forms the major part of the heart wall, consists of cardiac muscle tissues. These tissues are responsible for the contractions of the heart, which facilitates pumping of blood. Here, the muscle fibers are separated with connective tissues that are richly supplied with blood capillaries and nerve fibers. The inner layer called endocardium, is formed of epithelial and connective tissues that contain many elastic and collagenous fibers (collagen is the main protein of connective tissues). These connective tissues contain blood vessels and specialized cardiac muscle fibers called Purkinje fibers. This layer lines the chambers of the heart and covers heart valves. It is similar to the inner lining of blood vessels called endothelium.
- Chambers of the Heart: As discussed earlier, the human heart has four chambers, the upper chambers known as the left and right atria, and the lower chambers called the left and right ventricle. Two blood vessels called the superior vena cava and the inferior vena cava, bring deoxygenated blood to the right atrium from the upper half and the lower half of the body, respectively. The right atrium pumps this blood to the right ventricle through the tricuspid valve. The right ventricle pumps this blood through pulmonary valve to the pulmonary artery, which carries it to the lungs (to get re-oxygenated). The left atrium receives oxygenated blood from the lungs through the pulmonary veins, and pumps it to the left ventricle through the bicuspid or mitral valve. The left ventricle pumps this blood through the aortic valve to various parts of the body via aorta, which is the largest blood vessel in the body. The heart muscles are also supplied with oxygenated blood through coronary arteries. The atria are thin-walled, as compared to the ventricles. The left ventricle is the largest of the four chambers of the heart, and its walls have a thickness of half an inch.
- Valves of the Heart: Basically, the valves in the heart can be classified into two types - atrioventricular or cuspid valves and semilunar valves. Cuspid valves are located between the atria and ventricles, whereas the semilunar valves are located at the openings of the aorta and the pulmonary arteries. Tricuspid and bicuspid (mitral) valves are atrioventricular valves, and pulmonary and aortic valve are semilunar valves. These valves allow the blood to flow only in one direction and prevents reverse flow. The human heart pumps around five liters of blood per minute.