CIRCULATORY SYSTEM

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CIRCULATORY SYSTEM

Blood as we have seen elsewhere, is partly fluid (plasma) and partly solid (red and white corpuscles). The plasma is the fluid that is carried to each individual cell of every part of the body-to give oxygen, salts and food material to the cells and to take from cells, their waste or metabolic products. When blood clots, the straw-coloured transparent fluid that remains is called serum. The fluid which continually exudes from blood-vessels into the tissues is called lymph. Plasma is the fluid part of blood, serum is the fluid left after clotting of blood; and lymph is the exudation from blood-vessels into tissues. The proportion of salts is greater in lymph than in plasma.

If we take a vessel and partition it off by an animal membrane into two non-leaky compartments; if then we put into one compartment: (a) say two ounces of water containing ten grains of table-salt to the ounce and into the other compartment (b) two ounces plain (unsalted) water; we will find that, after a time, both sides contain the same amount of salt per ounce—i.e., 10 gm. salt from compartment (a) have travelled, via the solid animal membrane, into compartment (b) while nothing has travelled from (b) to (a). This diffusion of salt through animal membrane, till the solution on both sides is equal physically, is osmosis. This process of osmosis is very constantly going on inside our body. Blood laden with food matter and freshly charged with oxygen is sent out from the heart by arteries, via the aorta, to all parts of the body. The arteries become smaller (arterioles) and smaller till they become minute tubes with walls made up of a single layer of cells (capillaries). It is in this condition (capillaries) that blood-vessels surrounded cells to facilitate osmosis between the fluid inside or surrounding each individual cell and the fluid slowly flowing through capillaries. It is thus that cells give to the plasma in the capillaries (via the lymph surrounding the cells) whatever effete material they have to give up; and it is lymph oozing out of the capillaries that carry oxygen and food matter to cells. Circulation then is the interchange of waste products on the one hand and food material and oxygen on the other, between blood and cells.

Turning to the solid constituents of blood, we find two kinds of corpuscles—red and white. There are approximately 5,000,000 red blood corpuscles (R.B.C.) and 600 white blood corpuscles (W.B.C.) to each cubic millimeter of blood.

Red corpuscles—These give to blood its scarlet colour. In man, they are non-nucleated, elliptical discs. They contain a substance called haemoglobin, which greedily combines with oxygen. Red corpuscles are being constantly destroyed in the body and changed into pigments for colouring bile faeces, urine. They are being regenerated from bone marrow, spleen etc.

White corpuscles—These are irregularly sized, nucleated bodies that are capable of wandering about (amoeboid movement) and extruding out of the vessel-walls (diapedesis). They serve as the scavengers of the body—in that they can absorb in their body and thus remove dead tissue. They are also the soldiers of the body, because they have the power of eating up (called phagocytosis) and absorbing intruding foreign germs.

A word here with reference to inflammation will not be amiss. A combination of localized heat, redness, swelling and pain goes by name of inflammation. What happens when a part is inflamed? Or, rather when a foreign body enters any part of our body what occurs? As soon as a foreign body enters any part of our body, the blood vessels there dilate (vaso-dilatation) producing what is called congestion or determination of blood to the part, i.e., an extra amount of blood is supplied there. There being an extra amount of blood, the number of leucocytes or rather phagocytes (foreign-body eaters) also increase tremendously. What happens is that an extra amount of lymph it exuded from the capilary walls to bathe the part and an army of phagocytes run pell-mell towards the intruder to swallow it up and to remove it. If the foreign body can be quickly removed, the congestion slowly subsides as soon as the intruder is disposed of. But if the struggle is keen and prolonged, as many phagocytes accumulate locally, that it looks white -pus is formed in other words. Pus then is dead leucocytes, massed together and floating in lymph. We can now understand why the part looks red (because there is too much blood supply) why it is swelled (due to the same cause), why it pains (because the extra amount of blood presses on the nerves), and what pus is.

Reviewing the circulatory process, we find that

  1. The Heart—is the central pump, into which fluid from all parts of the body flows; and from which fluid is thrown into all parts of the body.
  2. The arterial system—conveys pure (i.e., oxygenated and food-laden) blood from heart to all parts of the body. The arteries become smaller and smaller till they merge into—
  3. The Capillaries—which exude lymph so as to bathe every cell in the body and which are reinforced in this work by lymph-vessels, that carry to and from distant overflow-areas all the available lymph. And finally we have—
  4. The Venous system—that picks up all waste ridden blood back into the right side of the heart to be oxygenated in the lungs.

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