The heart is a muscular pumping organ. It is thought that it has evolved from the highly muscular region of an artery. It is divided into four chambers: the left and right sides do not Communicate. The upper chambers, the atria, which are relatively thin-walled, receive blood from the veins Oxygenated blood from the lungs enters the left atrium Via the pulmonary veins and deoxygenated blood from the body enters the right atrium from the venae cavae. Relaxation of the ventricular muscle allows the ventricles to expand and fill with blood which flows in from the atria and veins.

Simultaneous contraction of both atria forces the blood they contain into the corresponding ventricles and, about 0-1 sec. later, the ventricles contract simultaneously, expelling their blood into the arteries and round the body. Both ventricles have thick muscular walls but those of the left are thicker, having to pump blood all round the entire body via the aorta.

The right ventricle pumps blood to the lungs through the pulmonary arteries. When the ventricles contract, blood is prevented from returning to the atria and veins by the closure of parachute-like valves between the atria and ventricles. Powerful contraction of the ventricles forces blood into the aorta and pulmonary arteries. When the ventricles relax, the pocket like semilunar valves in these two arteries are closed and prevent the return of blood to the ventricles. The heart contracts about 70 times a minute when an adult person is at rest, but this rate increases to 100 or more during activity or excitement. In a sparrow the rate is nearly 500 a minute. The heart's rhythmnic muscular contraction is basically automatic and needs no nervous stimulation to bring it about. If kept in the right solution of salts a frog's heart will continue to beat for some hours after removal from the body, and the same is true of a mammalian heart if an artificial circulation to the heart muscle is maintained. Nervous stimulation is, however, superimposed on the heart's natural rhythm and helps to maintain and control its rate. An increased heartbeat increases the speed with which the blood is supplied to the tissues and so allows a greater rate of activity. The coronary arteries shown in carry oxygenated blood to the ventricular muscle whose constant activity demands an unceasing supply of food and Oxygen.

Blood pressure. To force blood through a capillary system and to overcome atmospheric pressure, which tends to flatten the vessels, a fairly high pressure must be developed by the heart, This pressure varies according to the part of the body considered and the age of the individual, but an average pressure produced in the ventricle when it contracts is equal to 130 mm of mercury 

Exchange between capillaries, cells and lymphatics

At the arterial end of the capillary bed blood pressure is high and forces plasma out through the thin capillary walls. The fluid so expelled has a composition similar to plasma, containing dissolved glucose, amino acids and salts but has a much lower concentration of plasma proteins. This exuded fluid permeates the spaces between the cells of all living tissues and is called tissue fluid. From it the cells extract the glucose, oxygen, amino acids, etc. which they need for their living processes and into it they excrete their carbon dioxide and nitrogenous waste.

The narrow capillaries offer considerable resistance to the flow of blood. This slows down the movement of blood, so facilitating the exchange of substances by diffusion between the plasma and the tissue fluid. The capillary resistance also results in a drop of pressure so that at the venous end of a capillary bed the blood pressure is less than that of the tissue fluid and the latter passes back into the capillaries. The fact that the plasma contains more proteins than the tissue fluid gives the blood a low osmotic potential which tends to cause water to pass from the tissue fluid into the capillary. 

At the arterial end of the capillary network, the blood pressure is greater than this osmotic pressure, so forcing water out, but at the venous end water from the tissue fluid enters the capillary by osmosis.

Lymphatic system. The capillaries are not the only route by which the tissue fluid returns to the circulation. Some of it returns via the lymphatic system. The proteins in the tissue fluid are unable to re enter the capillaries but can drain into blindly-ending, thin-walled vessels which are found between the cells. These lymphatics join up to form larger vessels which eventually unite into two main ducts and empty their contents into the large veins entering the right atrium.

The fluid in the lymphatic vessels is called lymnph. Its composition is similar to plasma but it contains less proteins. It also contains a certain type of white cell, lymphocyte, which is made in the lymph nodes.

The larger of the two lymphatic ducts is the thoracic duct which collects lymph from the intestine and the lower half of the body. The lacteals from the small intestine open into the lymphatic system. After a meal containing fats the lymph is a milky-white colour due to the fat droplets absorbed in the lacteals.

At various points along the lymph vessels are lymph nodes. In these nodes antibodies and new white cells are produced. Stationary white cells in the nodes ingest any bacteria which have gained access to the lymph.

The lymph flow takes place in only one direction, from the tissues to the heart, and there is no specialized pump. The flow is brought about partly by the pressure of the lymph that accumulates in the tissues, but one of the most important factors in the circulation of Iymph is muscular exercise. Some of the lymphatics have valves in them, pressure from the contracting muscles around them forcing the lymph along the vessels in one direction.