Respiration, Blood, heart, and circulation

Respiration
The air we breathe in goes through the nose, or sometimes through the mouth, via the throat (pharynx) and the larynx (voice box) into the wind-pipe (trachea), and thence into the lungs. The trachea divides into two branches, called bronchi, which lead to the left and right lungs. These bronchi keep on dividing into smaller bronchi, like the branches of a tree. The terminal ends of these bronchi are called bronchioli. The brochioli end up in tiny cavities called alveoli (like the twigs of a tree, which end up in the leaves). The air is drawn into each alveolus during inspiration (breathing in). The oxygen in the air enters the capillaries (the smallest blood vessels) in the alveoli, and the excess of carbon dioxide is. discharged into the alveoli from the same capillaries. During expiration (breathing out) the carbon dioxide is expelled into the air.The chest cavity expands during inspiration by the outdrawing of the muscles of the chest and the downward movement of the diaphragm, which is the thick muscular wall that separates the chest cavity from the abdominal cavity.

Blood, heart, and circulation
Blood is essential for life. Blood is circulated to all parts of the body, by the action of the heart, through a network of blood vessels, which are referred to as arteries and veins. The heart is about the size of the fist (closed hand). It is enclosed within a sac called the pericardium, and is situated inside the chest on the left side. It pumps the blood to the periphery through the arteries, and the veins bring back the blood from the periphery to the heart. The terminal ends of arteries which are microscopic in size are referred to as capillaries. At this point the capillaries are referred to as the arterial capillaries.The arterial capillaries gradually become venous capillaries, which ends up as veins. At the arterial end of the capillary, Oxygen as well as the fluid comes out of the blood and bathes the cells, thus supplying them with Oxygen and nutrients. At the venous end of the capillary, fluid and carbon dioxide (which the body needs to get rid of) flow back into the capillary, and thence into veins. The veins take back this blood to the heart, which pumps the blood into the lungs. If we cut a finger. It would cut through some of the small blood vessels (capillaries) which cannot seen by the naked eye. That is why blood comes out through a wound.

The heart can be best described as an organ consisting of two pumps, which are joined together and exist side by side. The left side of the heart (left pump) consists of two chambers. The upper and smaller chamber, the left atrium – receives oxygenated blood from the lungs. This oxygenated blood flows (from the left atrium) through a valvular opening called the mitral valve, to a bigger and stronger chamber called the left ventricle. The left ventricle pumps this blood into a large artery called the aorta through a valve called the aortic valve. From the aorta the blood is transported through its many arterial branches which ends up in tiny vessels called capillaries, into all parts of the body into the space between cells. After delivering Oxygen and other substances the blood is brought back to the right side of the heart through a network of veins. The blood in the veins contain less oxygenated blood but large amount of Carbon dioxide when compared to the blood in the arteries. The right side of the heart also consist of two chambers. The smaller chamber, the right atrium which is situated above, receives the venous blood from all parts of the body. This venous blood flows from the right atrium through a valve called the tricuspid valve to the right ventricle. The right ventricle pumps the venous blood through a valve called the pulmonary valve into the lungs. In the lungs the blood gives up a large amount of carbon dioxide, and takes in oxygen. It is then returned to the left atrium.