The Circulatory System

Forwards and backwards to the right and are at the same level of the fifth to eight dorsal vertebrae. The apex of the heart points downwards and forwards to the left and corresponds to the space between the fifth and sixth ribs. However, in thin people, the hearts apex may be pointing more downwards than to the left. Its atrial border corresponds to a line drawn across the sternum on a level with the upper border of the third costal cartilage. Its apex corresponds to a line drawn across the lower end of the same bone. Its upper surface is rounded and convex, directed upwards and forwards, which is formed mainly by the right ventricle and a part of the left ventricle. The back surface of the heart is flattened and rests upon the diaphragm. Of its two borders, the right is the longest and thinnest, the left is shorter but thicker and round. The muscles that make up the heart are known as cardiac muscles. Cardiac muscle only exists in the heart, not like skeletel muscle which is found in many parts of the body. Cardiac muscle fibers possess striations that are typical of skeletel muscle. However, they only respond to the autonomic nervous system and electrical commands that are generated from the heart. Skeletel muscle may have many nuclei, but cardiac muscle only has one nucleus. As well, cardiac muscle is very small compared to the larger skeletel muscle. As fitting with its duty, cardiac muscle has many mitochondria to convert food into energy faster than other muscles. Cardiac muscles communicate between junctions that are laid down between the muscles. They are called intercalated disks. Along certain points of the disks, cell membranes fuse together. The electrical current required to cause the muscles to contract pass through the cells easily and the adjoining cells will respond as well due to the intercalated disks. The cardiac muscle is really a large number of cells working together that function to act as a single cell.
There are many proteins that give cardiac, as well as other muscles, to contract. Thin bundles of protein called myofibrils run the length of each fiber. Within the myofibrils are filaments (tiny threads of protein) that are arranged in a repeating pattern called a sarcomere. The filaments in each sacromere are made up of the proteins actin and myosin. Two clusters of actin are set in each end of the sacromere stretch towards the centre but do not touch. There are continuos threads of myosin located at the end of the sarcomere. The contraction can occur because of the region where the actin and myosin over lap each other. Small hooks on the myosin binds to the actin filaments and pull towards the centre of the sarcomere. This happens through the rapid ratchet-like actions of the myosin and actin pulling together. When the sarcomere pulls together, the fiber contracts and so does the muscle. In order for this to occur again, the sarcomere must be stretched out, which is caused by the blood re-entering the heart, expanding it.
In an adult, the heart measures about five inches in length, three and a half inches in the broadest part of its horizontal diameter, and two and a half inches in its posterior. The average weight in the males is from ten to twelve ounces. In the female, the average weight is eight to ten ounces. The heart will continue to grow in size up to old age. This growth is more obvious in men than in women.

The heart is subdivided by a muscle called the septum into two halves, which are named right and left according to their position. A muscle divides each half into two cavities. The upper cavity on each side is called the atria or auricle, and the lower side is called the ventricle. The right atrium and ventricle form the venous side of the heart. Dark venous blood is pumped into the right atrium from the entire body by the superior vena cava(SVC) and inferior vena cava (IVC), and the coronary sinus. From the right atrium, the blood passes into the right ventricle and from the right ventricle, through the pulmonary artery into the lÜ¥e #À  ñ Ð " ߝ ,º l