Popular Science Monthly/Volume 19/September 1881/The Blood and its Circulation II
|THE BLOOD AND ITS CIRCULATION.|
IN vertebrates alone is there a closed circulation—a complete system of tubes from whence the blood never escapes into the body-cavity. We find an approach to it in the higher mollusks. Indeed, in power and general efficiency, the circulation of the highest mollusks is greatly superior to that of the low vertebrates. Nevertheless, the perfectly closed circulatory system of even the lowest vertebrates is of higher type. Although the circulating system of the vertebrates is perfected in principle, it still admits of very great and curious modifications.
There exist in vertebrates three sets of capillary blood-vessels, which are usually spoken of as three systems, although together they constitute but a single circuit. They are distinguished as the body or systemic circulation, the respiratory or pulmonary circulation, and the liver or portal circulation. Connected with the blood-system by the thoracic duct is the lymphatic circulation.
The lymphatic system, which has previously been mentioned as the second source of blood material, deserves some notice on account of its intimate relation with the blood system of the vertebrates. The lymphatics are minute capillary vessels, found in all parts of the body of vertebrates, excepting, perhaps, the bulb of the eye, the cartilages, and the bones. They unite to form, with the lacteals, the thoracic duct, which was described in the article on digestion, in the September number of the "Monthly."
The office of the lymphatics is to collect the waste matter of the tissues and return it to the blood, to be again used elsewhere, or, if wholly useless, to be excreted from the body. They also collect the blood which may be poured upon the tissues in excess of their needs. The fluid which the lymphatics carry is called lymph. It is colorless, and contains corpuscles resembling the white corpuscles of the blood.
The lacteals, which take the new food from the intestines, are lymphatics modified for a special purpose, and, when they are not busy with the chyle, they also carry lymph.
The lymphatic tubes are provided with valves to keep the lymph flowing toward the larger trunks.
This lymphatic system of the vertebrated animals is, however, expressed in technical language, only the differentiated interstitial sinuses of the lower animals, which has, in the latter, a share in the Fig. 1. Diagram of the Circulation in a Fish. (The portion of the system containing pure blood is black; the part obtaining impure blood is white.) a, auricle, receiving venous blood from the body; v, ventricle; m, bulbus arteriosus; n, branchial artery, carrying venous blood to the gills (b, b) c, aorta, carrying arterialized blood to all parts of the body. venous circulation. Indeed, in the lower vertebrates the lymphatic tubes frequently assume the form of large sinuses, and connect with the veins. They are even found in the birds. In the frog four of these sinuses have muscular walls, and rhythmically contract. These are known as lymphatic hearts.
In various parts of the body the lymphatics form glands, such as the thymus, thyroid gland, and the spleen.
Fishes have a heart resembling that of the mollusks. It is a double force-pump, consisting of a receiving-chamber (auricle), and a propelling chamber (ventricle), with all the valves necessary to prevent a backward flow of the blood. But this heart is respiratory—it sends the blood directly to the breathing organs; consequently, it passes only impure blood. When the blood has traversed the gills and is purified, it passes around the circuit of the body through the systemic and portal capillaries, and back to the heart without any further propulsion.
The low, worm-like fish, lancelet, or amphioxus, has no special heart, but a number of contractile bulbs in the veins. The eel has such an auxiliary heart in its tail, while the hag has the circulation aided by the contractility of the portal vein.
Lepidosiren, one of the mud-fishes, approaches the amphibians in the possession of two auricles; for, in addition to gills, it has true lungs. The vein conveying the purified blood from the lungs joins the left auricle.
Amphibians and reptiles exist under conditions incompatible with a high temperature of the body. In the adult state they are air-breathers, and, if their circulation were complete, they would be "warm-blooded." But the temperature is subdued by imperfect circulation, which results from the arrangement of the heart-chambers. There is but one ventricle for the two auricles, hence the pure blood from the breathing organs and the impure blood from the body are mingled, so that, besides the venous and arterial, they have a mixed blood. The blood which goes to the lungs is never wholly impure, and that which goes to the body is never entirely pure. However, by a complex and beautiful action of the parts and valves of the heart—too complex to be here described—the mingling of venous and arterial blood is not complete.
The change which the amphibians undergo in outward form, from the tadpole or larval state to the frog-like condition, is accompanied
|Fig. 3.||Fig. 4.|
by a remarkable inward change in the circulation. In the larval stage, with respiration by gills, the heart and circulation resemble that of the fishes—a single auricle and ventricle and complete purification of the blood. But, as the gills disappear and the lungs develop, and the blood is diverted from the former to the latter, there is a corresponding change in the carrying capacity of the blood-vessels, resulting in the final disappearance of the vessels connected with the gills. Moreover, while the blood was not returned directly from the gills to the heart, it is returned directly from the lungs, and a second auricle is developed. But the aerial respiration of the frog, with its mixed circulation, is more rapid than the aquatic respiration with the perfect circulation of the tadpole.
In the reptiles circulation is essentially the same as in the amphibians; but the ventricle is more or less divided by a partition into two chambers. This membranous partition is perfect only in the crocodile, where we find a right and left ventricle without communication, and the heart structurally like that of a bird or mammal. But the circulation is still the same as in the lower reptiles, for the pure and impure blood are somewhat mingled by a communication between the two arteries near their point of origin.
Although birds in their general organization are closely allied to reptiles, their circulation is similar to that of the mammals. In these
two highest classes of the animal kingdom, there are always two auricles and two ventricles, and the right and left sides of the heart are entirely distinct. Functionally these are two hearts: a systemic heart, forcing pure blood to the body; and a pulmonary, forcing impure blood to the lungs. The pure and impure blood are never mingled, and all the blood has to pass through the lungs and be oxygenated every time it makes the complete circuit. This perfect circulation, with aerial respiration, produces more rapid chemical changes in the blood and tissues, and consequently the higher temperature of the "warm-blooded" animals. In the embryonic stages of the heart, the septum dividing the auricles is slowly formed, and an aperture exists for a time, called the foramen ovale. Cases rarely occur of human subjects in which the opening persists. Such persons are physiologically reduced to the condition of a reptile. It is stated that human infants have lived several days with a circulation as mixed as that of a frog.
To economize space and muscular effort, these two hearts are formed of the same circular muscles, and are inclosed by a lubricating membrane called the pericardium. In the dugong, however, the two ventricles are quite separate, showing a structural distinction corresponding with the functional difference.
|Fig. 6. Dorsal View of the Heart of the Dugong (Halicore), its Cavities being laid open, showing the separation of the ventricles. R v, right ventricle; L v, left ventricle.|
On account of the structural union, the two hearts contract and dilate in unison, producing the "beating" of the heart. The cause of the first sound in the heart-beat is uncertain, but it occurs at the time of the ventricle contraction. The second sound is produced when the ventricles dilate, by the flapping back of the semilunar valves, those placed at the origin of the arteries to prevent the regurgitation of the blood.
Each half of the heart of birds or mammals is, like the entire heart of the fishes, a double force-pump, with perfection of valves and tubes and surpassing efficiency. The power is enormous. It has been estimated that, while an engine can lift its own weight three thousand feet in an hour, and an active climber can ascend four thousand feet, the human heart performs hourly a labor equal to lifting its own weight twenty thousand feet. Its daily work is also estimated at seventy-five thousand kilogramme-metres. We can otherwise gain an idea of the power of Nature's enginery, by observing what the heart actually performs. The quantity of blood in the human body is at least six quarts. In its course it has to traverse many feet of tubing and two sets of capillaries, and, notwithstanding the friction and loss of power, all the blood completes the circulation in about thirty-two heart-beats. We should further observe that the heart never rests, but is ceaseless from birth to death. Its cessation is death.
The necessity of uninterrupted action of the heart requires that it should be involuntary, and so its action is placed beyond our control. It is said that an individual once lived who could stop for an instant, at will, the beating of his heart. But, it is also stated in connection, that he died as the result of a too successful attempt.
The flow of blood through the arteries by successive impulses is facilitated by their branching at acute angles. Veins, on the contrary, branch at greater angles, which is compatible with a steady and slower flow. As the veins carry in any given time the same amount of blood as the arteries, while the rate of flow is slower, it follows that their diameter or capacity is greater.
The pressure of the blood-current diminishes from the heart. In the carotid artery of man it is probably equal to the weight of one hundred and fifty to two hundred cubic millimetres of mercury. The pressure in the pulmonary artery is only thirty to forty cubic millimetres.
There is much disagreement among writers regarding the velocity of the blood. In the carotid artery of the horse, it probably flows at the rate of about three hundred millimetres per second; in the dog, at the rate of three hundred to five hundred millimetres. The velocity in the large arteries of man can hardly be over twenty inches per second, but varies greatly at different times. The length of the capillaries is about one half of a millimetre, and the blood passes through them in about one second. In the human retina the corpuscles travel at the rate of 75 millimetre per second. The small arteries pulsate within one sixth of a second after the main trunks; but the rate of flow is much slower than the wave-progression.
In vertebrates, the rapidity of the circulation is generally proportionate to the activity of the animal. The pulse of aërial birds is about 150 per minute; of the cat, 115; dog, 95; man, 72; ox, 35. But this generalization does not hold with the invertebrates. Insects, the most active of all creatures, have a very sluggish and imperfect circulation, for in this class the air is so freely admitted into the body as to obviate the necessity of great movement of the blood.
The human pulse is somewhat more rapid in childhood, and again in old age; slightly faster in the evening than in the morning, in summer than in winter, and probably increases with geographical altitude. In fever the circulation is very greatly and mysteriously quickened.
All the blood of a man probably completes the round of the circulation in about thirty-two heart-beats, or in less than half a minute. The blood of a horse, it is estimated, completes the circuit in thirty seconds, that of a dog in fifteen, and that of a rabbit in seven seconds.
The velocity of the blood decreases from the ventricles toward the capillaries, and then increases from the capillaries toward the auricles. The velocity being necessarily the reverse of the carrying capacity, or sectional area. The capillaries have a sectional area several times that of the aorta, the purpose of this being to delay the blood at the time it is brought into most intimate contact with the tissues.
The walls of the capillaries are of extreme tenuity, and easily permeable under the physical action called osmosis. Even the corpuscles can pass outward through the walls.
To what degree the heart is aided by other forces is yet a matter of investigation. Probably there are several forces assisting. The elasticity of the arteries increases their carrying capacity. They are firm, elastic tubes, which expand under the pressure from each heart-contraction, and then by their own elasticity contract and help the onward flow of the blood. In the smaller arteries the flow loses the intermittent character it possesses in the larger arteries, and becomes a steady stream. The elasticity of the arteries serves precisely the same purpose as the air-chamber of any force-pump, that of equalizing the flow, and so increasing the amount delivered. The whole force is derived from the heart; the arteries cause the force to act continuously.
The veins are lax tubes, somewhat larger than the arteries, and capable of holding all the blood of the body. They convey the same amount of blood as the latter, but more slowly. In the larger veins, however, near the auricles, the velocity may be two hundred millimetres per second. They are provided with valves which effectually prevent the blood from flowing backward toward the heart. Any compression, produced by muscular contraction, or otherwise, will therefore assist the forward flow of venous blood. This is one explanation why exercise hastens the circulation. The movement of the chest in breathing probably aids the pulmonary circulation, the blood, as well as the atmosphere, tending to fill the vacuum during inspiration.
Physical capillary force is not generally regarded as an active force in the circulation. But there is an admitted force in the capillaries, resulting from the attraction of the tissues for the arterial blood, containing the required oxygen and nutriment. "The vital condition of the tissue becomes a factor in the maintenance of the circulation." It is this force, primarily, which adapts the amount of blood to the varying needs of any organ; the nervous system regulates the supply by varying the caliber of the vessels.
The force in the capillaries, or some other force, carries the blood, after death, from the arteries, where the heart leaves it, into the veins.
Finding the arteries empty after death gave rise to the idea that they conveyed only air; whence the name. It was this belief which Harvey overthrew in 1620.
In bats the heart is aided by the rhythmic contraction of the veins in the wing. Other accessory hearts of the lower animals have already been mentioned. In some of the lowest creatures, the cause of the circulation may be wholly the movements of the body, as in the jelly-fish and anemone; or by cilia, as in the sponge, where the sea-water answers to blood.
Under the influence of nerve-force, the walls of the arteries and capillaries are usually somewhat contracted. The withdrawal of nerve stimulus allows the tubes to relax, which consequently permits more blood to pass through, or to accumulate, and perhaps add color to the skin. This is the physiology of blushing. Congestion is produced by a permanent expansion of the capillaries. We might call blushing a momentary congestion.
In an emergency the arteries are capable of great expansion. They are connected by branches, or loops; and, in case of stoppage of the circulation in a large artery, either by disease or a surgical operation, another will, after a time, perhaps a few hours, expand to a size requisite to carry sufficient blood. This variation in the carrying capacity of the arteries is the important secondary means of adapting the amount of blood to the wants of any part of the body.
In man there is a greater and more direct supply of blood to the right arm, corresponding to the greater use of that limb. But, in birds, equality of supply is necessary for the equality of strength needed in steady flight.
For protection, the arteries are as deep-seated as possible, lying beneath the muscles, and appearing rarely at the surface. At the joints they form loops, so that the circulation may not be stopped by compression of a single trunk. A fine example of adaptation is seen in the arm of the lion, where the main artery, to be protected by the powerful muscles, passes through a perforation in the bone.
- Concluded from page 468.