The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

The effect upon the distribution of blood throughout the body of squeezing the viscera is experienced after taking a deep breath and contracting the muscles of the abdomen. The contents of the abdomen are compressed between the depressed diaphragm and its muscular wall.

Certain other forces co-operate with the beat of the heart in causing blood to circulate. Two such factors are especially deserving of attention. In the first place, the movement of blood in veins is largely dependent upon external pressure. The veins are valved at frequent intervals, the folds in their interior being of course directed towards the heart. Any external force which empties a section of a vein drives blood forward. A “good stretch” brings the lateral pressure of contracting muscles to bear upon the walls of the veins which lie between them, or beneath. More blood is delivered to the heart. The exercise of standing erect in the attitude of attention, and then slowly raising the arms until the thumbs meet above the head, and slowly lowering them again, has a remarkable effect in quickening the circulation—increasing the blood-supply of the brain. Changes of posture, by relieving pressure on subcutaneous veins, removes an impediment to the flow of blood.

The second of the factors to which we have referred as adjuvant to the heart’s action is the negative pressure of inspiration. In explaining the effect of this force upon the circulation, the relation of the lungs to the thorax must be taken into account. The box in which the lungs are enclosed is too big for them; nevertheless, being extensible and elastic, they always fill it. They follow its movements when in inspiration the muscles between the ribs enlarge it, and when in expiration it diminishes again. No air or fluid, save the moisture which lubricates the surface of the pleura, reducing friction, occupies the (potential) space between the lungs and the chest. But the moment the chest is punctured the lungs collapse. Air is sucked into the pleural cavity. The lungs fill the chest only so long as there is neither air nor fluid between it and them. Lung-tissue is extremely delicate. Each air-cell is a cup of thin membrane holding together a basket-work of capillary vessels. So long as the chest-wall is stationary the negative pressure in the pleural cavity has no effect upon these slender tubes. But when the chest expands, the capillaries are between two minus pressures, the pull of the chest-wall and the resistance offered to the entrance of air into the lungs by the passages through which it has to pass. The calibre of the lung-capillaries is increased, just as it would be increased were they hanging in an air-pump while the piston was drawn out. More blood passes to the left heart through the wider capillaries. Ejected into the aorta, it raises the pressure in the arterial system. A record of the pressure in an artery shows a rhythmic rise for each heart-beat. It shows also a rise with inspiration and a fall with expiration. These larger undulations correspond with the movements of the chest, although they are necessarily somewhat late on respiration, for the first effect of the dilatation of the capillaries is to cause them to hold more blood and to deliver less. The first effect of expiration, on the other hand, is to urge on the blood which the dilated vessels contain. In any case a single beat is needed to throw into the aorta the blood which has been received by the right auricle.

The expansion of the chest influences the flow of blood in yet another way. The heart and the great vessels which join and leave it are themselves enclosed within the chest, subject to the negative pressure produced within that cavity by the elasticity of the lungs. The lungs pull upon the pericardium, the membranous covering of the heart. When this pull is increased owing to the forcible expansion of the chest, blood is sucked into the great veins, just as air is sucked into the windpipe. The thick-walled aorta, containing blood at high pressure, does not feel the effect of slight variations in the pressure round it. The soft-walled veins are expanded during inspiration to a not inconsiderable degree. What relief a deep yawn gives by hastening a languid circulation! Leaning over an account-book late in the afternoon, every condition is unfavourable to the flow of blood. It accumulates in the legs and in the abdomen. The head is thrown back and the mouth opened wide, while the chest expands in a long deep inspiration. Down on the liver, stomach, and intestines presses the flattened diaphragm, squeezing their blood towards the heart. The negative pressure within the chest sucks this up, and draws down the blood contained in the great veins of the neck. The capillaries of the lungs are widened, allowing blood to pass more quickly from the right side to the left side of the heart. The heart responds to the call upon it, throwing all that it receives into the aorta. Only a great effort of the will had kept the pale brain at work; in the attic it suffers more than organs on the lower storeys from insufficient pressure. For a short time after the yawn it finds itself nourished with an adequate supply of blood.

The negative pressure in the thorax is considerable at all times. If a manometer—a U-shaped tube with mercury in its loop—be connected with a cannula passed through the wall of the chest, the difference of level of the mercury in the two limbs of the U is a measure of the force with which the lungs are endeavouring to shrink away from the chest-wall. Even at the end of expiration the mercury in the limb next the chest stands about 6 millimetres higher than the mercury in the outer limb. During a deep inspiration the pressure in the chest falls 30 millimetres below the atmospheric pressure. Hence a problem is presented of which no completely satisfactory solution has yet been given. How comes it that lymph is not sucked into the pleural cavity? In health there is no more pleural fluid than just suffices to keep the membrane moist. The endothelial cells which cover the surface of the pleura resist further exudation. Valves in the lymphatic vessels prevent backward flow. Yet in disease, when the pleura is inflamed, lymph pours out quickly, often to be reabsorbed with equal rapidity when the pleurisy subsides. This flow uphill, from a lower to a higher pressure, can be explained only as a phenomenon due to the “secretory” capacity of endothelium. As an answer to the hydrostatic problem this is hardly satisfactory.

The circulation of the blood is the result of the difference between the pressure in the vessels through which it leaves the heart, and that in the vessels through which it is returned. The pressure in the aorta amounts to about 200 millimetres of mercury. In the venæ cavæ it is nil, or, owing to the aspiration of the thorax, less than nil.

The Heart.—Inspection of the liver, the spleen, or the kidney helps but little to the comprehension of the mechanism of these organs. It is quite otherwise in the case of the heart. Its mechanics being comparatively simple, physiology is concerned with measurements, with the conditions under which it can and cannot work, and with the action upon it of the nervous system and of drugs. The heart of any mammal will suffice for anatomical study. A sheep’s heart is about the same size as that of a man, and exactly similar, save in minute particulars, which do not appreciably affect its mode of working.

The heart is a hollow muscle, composed of minute contractile cells. Each cell is a cylinder, about twice as long as it is broad, with an oval nucleus in its centre. There is no impropriety in speaking of the heart as a single muscle. Muscles which we can move at will, “voluntary muscles,” consist of fibres, each from 1 inch to 2 inches long, and of about the thickness of a piece of thread ([Fig. 16]). Every fibre is surrounded by a membranous sheath, its sarcolemma, which completely isolates it from the others. Each has its separate nerve-supply. A voluntary muscle-fibre is a cell-complex. The single embryonic cell which grew into the fibre underwent nuclear division until hundreds of nuclei were formed, but its cell-substance was not divided into territories appertaining to the several nuclei. In heart-muscle, on the other hand, nuclear division has been followed by cell division; but minute protoplasmic bridges are left between the cells. The whole of the heart-substance is thus in structural continuity. The cells are not invested with sarcolemma. As the result of this arrangement, an impulse started in one part of the heart spreads over the whole, with certain limitations as to the directions in which it is able to travel, whereas in voluntary muscle a separate impulse must be delivered to each fibre. The wave of contraction commences in the great veins, the venæ cavæ and pulmonary veins, near their junction with the heart, spreads from cell to cell throughout the auricles, and onwards down the ventricles to the apex of the heart. The substance of the heart has not, however, a homogeneous appearance. Its cells are collected into fascicles, which lie in various planes and cross the axis of the heart at various angles. In a boiled sheep’s heart it is easy to separate one fascicle from another, and to distinguish the sheets into which the fascicles are collected. The four valves of the heart lie in almost the same plane. They are supported by a fibrous plate divided into four rings ([Fig. 11]). Most of the fascicles are attached to this plate, though some which encircle the auricles are independent of it. With one or with both ends attached to the plate, fascicles loop over the auricles. They run down the ventricles with a twist from right to left. Those on the surface turn in at the apex of the heart, and run up the inner surface of the ventricles. Some of them go to form the free columns which are found on the inner surface of the ventricles, pointing towards the valves—musculi papillares. The fibrous plate which supports the valves cuts off almost all of the muscle which makes the walls of the auricles from that which constitutes the ventricular walls; but a thin sheet is continued from the inner surface of the auricles down the interventricular septum. To a considerable extent the walls of the two auricles and of the two ventricles are respectively continuous, insuring synchronous contraction.

The arrangement of the fascicles accounts for the changes in form which the heart undergoes when it contracts. Systole commences in the cardiac ends of the venæ cavæ and pulmonary veins. They empty the last of their blood into the auricles, and close to prevent regurgitation, their mouths not being valved. Then the auricles quickly shrink in all dimensions, and as soon as their contraction is at its height the ventricles contract, while the auricles relax. The ventricular wave runs from base to apex too rapidly to be followed with the eye, and ends, owing to the involution of the fascicles, in the musculi papillares. As soon as ventricular systole has commenced, the auricles relax. After emptying their contents into the aorta and pulmonary artery, the ventricles relax, their contraction giving way first at the apex, and being longest held at the base. Then follows a pause (diastole), during which both auricles and ventricles are flaccid. If the pericardium is open, the heart is seen to become round instead of oval in transverse outline during systole. It shortens. Its apex twists a little to the right, and projects forward. But if it is within its pericardium the shortening is not accompanied with any displacement of the apex. Instead of the apex mounting, the base descends. The front of the right ventricle, at some little distance from the apex, presses the chest-wall forwards in the fifth intercostal space, about an inch to the inner side of a line falling vertically through the nipple. This pressing forwards is felt as the “impulse of the heart.”