MEDICAL NOTES.

Auckland Star, Volume LXIII, Issue 263, 5 November 1932, Page 9 (Supplement)

MEDICAL NOTES.

HEAD OF PRESSURE. HOW IT IS MAINTAINED.

ARTERIAL STRESS.

(By PERITUS.)

A lady correspondent has written to me asking me '"not to tell my readers too muclj." What I am always fearing is that I may tell them too much of what they know already. The human body is very interesting. I have written about it, and its tricks and vagaries, for nearly twenty years, and weekly articles for that. time amount to over one thousand columns, and if my correspondent has read them all she perhaps has reason to cry "halt." I have written (tiwce) of blood pressure, both before and after it became an obsession with nervous people, and before I knew myself how far afield one might have to look for its cause when abnormal and dangerous. A Complicated Mechanism. Normally the internal he»d of pressure is maintained by a somewhat complicated mechanism, yet very easily understood by any mechanic. If you look at the compression chamber attached to a simple pump you will see that the head of pressure may be altered in two ways, viz. (1) increasing the inflow by speeding up the pump, or giving thfe piston a greater length of stroke; or (2) diminishing the outflow. Pressure may be lowered by allowing a freer flow, or slowing the action of the pump. If the outflow is much reduced, a very small inflow will maintain the head .of pressure. Nature uses both these means to regulate the head of pressure in the human body. The pumping strokes of the heart may be, increased or decreased in both number and volume of output, and a system of valves in arteries and veins assist to regulate the pressure to the needs of the living machine. The heart is a muscular pump. All the muscles of the body when in action require about eight times as much oxygen fuel as when in a state of rest. When the body is in action the heart has to pump faster. When a man is at rest his heart pumps about five pints of blood into his arteries every minute; 70 beats of one and a half ounces per stroke of pump; but if he is walking, say, four miles an hour the output of blood increases four times," and if he runs upstairs seven times. The heart works upon an average total of eight and a half pints of blood; thus»four times a minute the living pump has forced the whole of the "blood through the arteries and begun a further round of circulation; this during violent exercise (Prof. A. V. Hill). The arteries themselves are surrounded by a wall of spiral muscular fibres which can compress or release tne arteries. These are controlled by the nerve centres in the spinal cord, and are not under voluntary control. When the muscles of the body are used in such work as digging the blood supply to the stomach is reduced, whilst the pressure from' the veins speeds up the action of the heart to maintain the head of pressure to meet the needs of the muscls. The Valvular System. The valvular system through which the blood circulation works is automatic in action. It is not easy to do brain work after a full meal, for then the blood supply is switched on to the digestive organs.

On rising in the morning the feet and legs, if governed by gravity alone, would drain away most of the blood from the head and body were it not for the series of valves which respond to the unconscious messages sent along the nerves. Not only is the whole internal pressure regulated, but the pressure in each part of the body in relation to position ai»d work. If this valvular system of the blood vessels breaks down from any cause there is trouble, as when blood is drained from the brain, and collapse and unconsciousness result. In cases of anger, fright, or strong exertion, a nerve message reaches glandular bodies about the kidneys, the adrenals. In response to the message the . glands secrete a fluid which circulates' in and with the blood. This fluid has the effect of tightening up the valves in the path of the blood, and so places more blood at the disposal of the muscles. At the same time this secretion causes the liver to throw a form of sugar into the circulation, and this sugar is the chief fuel or food of "the muscles. A feeling of tiredness follows the excretion of adrenalin, even when not called on for muscular effort, which is why there are signs of exhaustion after a great fright or an attack of temper. The head of pressure is also raised by a fluid sent out in minute quantities by another gland, the pituitary, attached to the base of the brain. You will see, therefore, that: if the head of pressure is above normal in a. man at rest —mentally and physically at rest—the fault may not be arterial or venous, or due to a growth, but there may be messengers in the blood conveying unwanted orders. That is, the glandular secretions may be causing disorder. To return to human mechanics. There is in a motor cycle something in the rough to represent the heart and its system. It has, I think", a circulation, j Muscle fibres carry a " combustion mixture," oxygen and blood-sugar brought to them by arteries and the waste pro- ! ducts carried away by the veins, the heart pumping oixt the fluid. In the motor cycle there is the circulation of a combustion mixture by means of piston and cylinder (pump or heart), the petrolair entering by inlet pipe (artery) and leaving by outlet pipe (vein). There is a suction and exhaust stroke; for two strokes in each cycle the engine acts as a heart. v This is a primitive condition, the one engine acting as both heart and muscle. In the body, circulating forcesand driving power are carried on by separate mechanisms, a higher point in engine evolution.