Popular physiology V: Air and water
The ‘internal ocean’ must be churned constantly; it has to be replenished, filtered, cleaned and kept in pristine condition. The earliest vertebrates, fish, achieve this by passing the blood through a set of ‘gills’ (folds in the throat) which saturate the blood with oxygen.
This oxygen is, of course, dissolved in water and gills are specialised to draw out oxygen from water. However, in land animals, gills are useless, and then animals resort to lungs (an outgrowth from the gastro-intestinal tract), which can draw oxygen from air. (Fish have a rudimentary lung as well, which is just an empty sac called the ‘swim bladder’ which serves the same purpose as ballast tanks in a submarine).
Heart and blood vessels
To make the blood move, veretebrates have followed a mostly linear design; one particular blood vessel thickens, gains some muscle, and becomes the heart, and goes on pumping blood. Deoxygenated (‘used up’) blood must return to the heart, and then be pumped to the lungs to get a fresh shot of oxygen. The ‘oxygenated’ blood then must return to the heart. In early vetebrates till reptiles, the ‘deoxygenated’ and ‘oxygenated’ blood get mixed in the heart, and thus reptiles are not very agile creatures. Birds and mammals, however, have hearts that completely separate the oxygenated and deoxygenated blood, allowing us to attain very high oxygen saturation in blood.
Blood returns to the right side of the heart via veins which are thin-walled, spacious vessels; when one is lying down, there is no difficulty bringing blood back to the heart (gravity does the job). However, when standing, the muscles of the legs must do the job of pumping the venous blood upwards (reason people faint if left standing without movement — for a while). The heart then pumps the blood to the lungs through pulmonary arteries, which the get ‘oxygenated’ and come back to the left side of the heart via ‘pulmonary veins’. A substantial thrust from the left side of the heart then pushes this blood to the large arteries, and then to smaller arterioles, until each capillary and each cell is perfused. The network of arteies and arterioles offer considerable ‘resistance’ to the flow of blood (try pushing in some water in a tight rubber tube and you can feel the resistance). Plus of course, we live under kilometers of air above us. The thrust from the left side of the heart is — thus — quite a heave; it can be felt in every artery — small and large (check your wrist, neck, or even over the angle of your mouth; you can feel an artery ticking) — and is known as ‘blood pressure’.
Blood pressure must — obviously — overcome atmospheric pressure (which is already 760 mm of mercury at sea level), as well as peripheral resistance from arteries, and still keep pushing blood. The normal blood pressure oscillates between 760 + 120 and 760 + 80 mm of mercury, with each contraction and relaxation of the heart.
This ‘cardiac cycle’ completes in less than a second (0.8 second, approximately), and the heart might beat 60–100 times a minute without overtly exerting itself. The motion is fully automated; however, the heart may be sped up or slowed down by the brain, through the autonomic nervous system. A sudden fright pushes the heart rate up (the thoracic sympathetic nerves), whereas the parasympathetic nerve (vagus, the tenth cranial nerve) slows the heart down. Like any linear system, the amount of blood leaving the heart and going to lungs must be same as the amount coming back; thus deep breathing — which draws in more blood to the chest from the limbs — typically increases heart rate, and the heart slows down during exhaling.
With repeated acts of anxiety, stress and fright (i.e. any given day in modern life), more is demanded of the heart, more it thickens, more the arteries thicken, and before you know you got ‘essential (as if a necessity) hypertension’ and what have you. Or, floating garbage in the blood (from your last taco) might just end up blocking an artery and you get an ischemic attack (and there’s no saying where that might happen: heart (a ‘heart attack’), brain (a ‘stroke’) or legs (‘intermittent claudication’)).
Respiration
Like heartbeat, the act of breathing is automatic as well, although there is a greater degree of voluntary control (we can stop breathing at will, at least for a while). The acts of breathing and rate of heartbeat are both controlled by the brainstem, the most primitive part of the brain. Whereas heartbeat will go on even without the brainstem (the heart contracts on its own even when taken out of the body), respiration will cease in case brainstem is damaged.
What drives the respiratory effort? It seems the presence of carbon-dioxide in blood is the chief stimuli for the respiratory center. For this reason, blood is never entirely free of carbon-dioxide. Even arterial — oxygenated — blood is rich in carbon dioxide as well. This is because carbon dioxide serves a vital function — making bicarbonate (HCO3-) ions, which go on to buffer acids produced during cellular metabolism (more on this later).
With each inspiration (i.e. taking the air in), the ‘diaphragm’ (the partition between the chest and the abdomen) moves down, thus creating a vacuum inside the chest, which draws air in. Of course, there can be no exchange of gases in the ‘trachea’ (the thick tube you can feel in front of your neck) or the airways (‘bronchi’). The exchange of carbon dioxide and oxygen happens at the very end of the airways, within microscopic sacks (‘alveoli’) surrounded by blood vessels.
The air now must be exhaled back through the trachea. At the very top of the trachea there is a little box of cartilage (‘larynx’), which houses a kind of a valve (‘vocal cord’). This valve is under voluntary control. You can bring the two leaves of the valve close — at will — so that the exhaled air faces some resistance, creates a turbulence, and ‘voice’ is produced. Or you can leave it open, which is the normal state of things. However, the valve will close reflexively whenever you swallow something (i.e. to prevent food getting into the wrong tube). This reflex might fail every once in a while, and getting even tiny morsel of food stuck in your larynx/ trachea is one of the worst feelings one can have. The trachea and bronchi react quite violently, with a fit of cough, to get the thing out.
All considered, the heart and lung might be considered a single, tightly coupled organ system, which keeps blood moving and fresh at all times. This is no small feat. The blood going into lungs per second must be exactly the amount leaving it, or you develop life threatening conditions such as pulmonary edema. The blood coming into the heart must be fully pumped out, else the blood will accumulate in the legs, and then internal organs, and before long the person is in heart failure. That the whole system works so well for so long is no short of a miracle.
