The human blood circulatory system, like other vertebrates, consists of a heart, a system of blood vessels and blood. Study it in your class using a model of human heart.
Structure and Action of Heart (Human Heart)
The human heart like other vertebrates id a muscular, contractile structure. The cardiac muscles of the heart are capable of strong contraction which brings about the circulation of blood through out of the body.
The mammalian heart (including that of man) is two pumps in one. The mammalian heart has four chambers-two atria and two ventricles which in turn pumps blood into the pulmonary arteries. From lungs the oxygenated blood is carried to left atrium through pulmonary vein. From left atrium the blood is pumped in to left ventricle which in turn pumps blood into the aorta- the systemic circulation (this is pump two). Thus there are two pumps working simultaneously in the single pumping organ-the heart.
The heart bag- The Pericardium
Heart is enclosed in a tough, fibrous bag. This bag is the pericardium. Between the pericardium and the heart is a fluid called pericardial fluid which acts as shock absorber and protects the heart from the external shocks. The pericardium is not flexible or extensible. It also probably limits the over-extension of the heart in certain extreme conditions.
The heart consists of four chambers. The upper half of the heart consist of two independent, thin walled chambers, the atria or auricles separated from each other by a partition or septum called inter atrial or inter auricular septum. The right atrium receives the deoxygenated or i.e., oxygen deficient blood from all tissues of the body whereas the left receives the oxygenated or oxygen-rich blood from the lungs. The lower half of the heart consists of two thick walled chambers completely separated from each other by an inter-ventricles are much larger then the atria and are involved in pumping the blood out of the heart.
The inter atrial and inter ventricular septa (singular septum) prevent the mixing of deoxygenated and oxygenated blood. Each atrium pushes the blood into the ventricle aperture. The atrio-ventricles apertures are guarded by valve, the atrio-ventricular valves. These valves are made of almost transparent, thin but very tough flaps of tissue. The right atrio ventricular valve consists of three cup like flaps and is called bicuspid valve. The atrio-ventricular valves allow the flow of blood from the atris to the ventricles but at the time of ventricular contraction, they prevent the backward flow of blood from the ventricles to their respective atria.
The two atria contract simultaneously to push blood into ventricle. On the other hand, the ventricles have to contract with a great force as they have to push the blood to the lungs and to the rest of the body. The left ventricle contracts more forcefully than the right one, as it has to push the blood all around the body, while the right one has to push the blood to the lungs which are fairly close to the heart.