Blood Channels In General
The Evolution of Organic Irrigation
Blood channels may be regarded as a device for increasing the inner surface area of an organism with reference to exposure to adequate nutritive and respiratory factors. In most invertebrates blood channels are largely open lacunar, or perivisceral spaces.
The first evolutionary stages in the development of a circulatory apparatus are perhaps to be seen in the porous sponges, whose “blood,” that is, the surrounding water, carries a random load of microscopic food and dissolved air past the loosely organized colonial cells within the sponge body.
In flatworms, and some medusae also, there are neither true blood channels nor any specialized food-carrying medium of blood, since in these lowly creatures the digestive tube itself branches out like the twigs of a tree, extending between the cells of the body in such a way as to effect a direct delivery of needful nutriment without the mediation of a blood stream.
Among vertebrates two general types of channels appear, namely, a haemal system of closed tubes carrying blood, and an auxiliary lymphatic system carrying lymph, which is practically blood without respiratory red corpuscles. In general these channels form an irrigation system of flexible plumbing, consisting of a continuous series of cavities, lined throughout with flat endothelial cells, in which the blood circulates.
The fact that blood is not subject to ebb and flow but is constantly in motion always in one direction, and that it repeatedly during life makes the entire circuit of the blood vessels, was established in 1619 by William Harvey (1578-1657), long before anyone actually saw the blood pass through the smaller connecting channels. It was Malpighi who in 1661 discovered the capillaries by which the out-going and in-coming blood vessels are connected. In 1696 Leeuwenhoek charmingly described the capillary circulation in a bat’s wing as follows: “I perceived in many places an artery and a vein placed close beside each other and of a size large enough to admit the passage of ten or twelve globules at the same time; and in this artery the blood was protruded or driven forward with great swiftness, and flowed back through the vein, which was a most pleasing spectacle to behold.”
Although the circulatory system penetrates to nearly every part of the living organism, there are a few regions of the highly differentiated vertebrate body that are not invaded by blood vessels of any kind, namely, the cornea of the eye, cartilage tissue, and the epidermis together with its derivatives, hair, nails, feathers, horns, claws, and the enamel of teeth.
General Plans of Circulation
Blood vessels are related to each other as shown in Figure 276.
Two longitudinal blood vessels, one dorsal and one ventral, with collateral connections, and joined at either end by capillary networks, make up the main circulatory system of the practically heartless annelids. The blood flows forward through the dorsal vessel and backward through the ventral vessel in the simplest manner (Fig. 277).
The circulatory system of amphioxus is in many respects a preview of the basic vertebrate plan. In the first place the blood flows around in the opposite direction from that of the circuit in the annelid plan. By rotating an annelid 180° on its long axis, thus shifting its dorsal and ventral sides, the course of its general circulation may be brought into agreement with that of amphioxus.
As in the lower vertebrates, a large part of the anterior capillary system is associated with the pharynx region, which makes up about half the length of the digestive tube in amphioxus. Many of the posterior capillaries enwrap the remainder of the digestive tract. In addition there is introduced a new feature consisting of an extra capillary network spreading over the liver diverticulum to interrupt the vessel leading from the intestine to the gills. Thus an hepatic portal system is set off from the rest of the veins (Fig. 277).
At the posterior end of the pharynx the hepatic vein, from the liver, is joined by cardinal veins, from the body wall, to form the ventral aorta, which carries blood to the pharyngeal wall. It is by means of peristaltic waves, which pass anteriorly along the ventral aorta, that the blood is pumped through the various parts of the circulatory system of amphioxus. At least the posterior part of this ventral aorta is probably homologous with the heart, the great pumping organ of vertebrates.
Gill Plan of Fishes
The amphioxus plan becomes further elaborated in fishes (Fig. 278) by the development of a heart, or blood pump, and by the introduction of an additional capillary complex involving the excretory organs, thereby establishing the renal portal system.
The heart is simply a muscular enlargement and modification of a part of the main ventral blood vessel, lying between the hepatic capillaries and the gills, through which the blood flows forward. The development of the vertebrate heart from this ventral vessel has its possible homology in the rotated annelid plan, since in the latter it is the dorsal vessel which becomes muscular enough to pulsate and serve as a pumping organ.
The return of blood from the region of the large propeller-like tail, characteristic of fishes, is effected by means of the rental portal system which carries the blood through a capillary network in the kidneys, whence it joins the main blood stream. In fishes, therefore, besides the capillaries which unite the outward-bound distributing system of blood vessels (arteries) with the inward-bound collecting system (veins), there are two major strainerlike complexes of capillaries within the kidneys and the liver respectively, that interrupt the large vessels and modify the stream of blood returning to the heart.
Lung Plan of Mammals
The general plan of circulation among higher vertebrates, when reduced to the simplest terms, may be represented by a diagram (Fig. 279). The dotted lines, which are connected at only one end with the closed haemal circulatory system, show the relation of the auxiliary lymphatic vessels by means of which lymph is collected from all regions of the body, together with the white blood cells that have escaped by diapedesis from the capillaries, returning them to the venous system just before reaching the heart.
The change from branchial respiration by means of gills to pulmonary respiration through lungs makes necessary the introduction of a double blood circuit, namely, the systemic circulation over the body and the pulmonary circuit to the lungs, with two central clearing houses, or hearts, instead of a single one as in fishes. These two hearts are placed so intimately together, however, that when looked at superficially they have the appearance of a single heart.
With the diminishing importance of the tail upon emergence from the water and the evolution of locomotion on land by means of legs, the renal portal system becomes discontinued. Thus it will be seen that the changing methods of respiration as well as of locomotion have modified the circulatory plan in land animals.
Structure of Blood Vessels
The walls of blood vessels show certain differences that serve to distinguish arteries, veins, capillaries, and lymphatics from each other.
Arteries and veins of the same size externally are not easily confused, since veins have thinner walls and a larger bore inside, and consequently are more liable to collapse than arteries when emptied of blood. The walls of both arteries and veins are made up of three layers of tissue (Fig. 280) known as tunica intima, tunica media, and tunica adventitia. The inside layer, or tunica intima, is invariably composed of a lining of flat endothelial cells, continuous and universal throughout all the blood vessels including the heart itself. This lining, except in the capillaries, is wrapped about by reinforcing connective tissue. The middle layer, or tunica media, is largely composed of smooth muscle cells, arranged mostly in circular fashion and interspersed with connective tissue fibers, while the outer layer, or tunica adventitia, is principally connective tissue, more or less elastic and penetrated by lymphatics as well as vasomotor nerve fibers that control the changing caliber of the blood vessel.
There are certain large veins in man, such as the umbilical, iliac, splenic, renal, and superior mesenteric, which are noteworthy because longitudinal muscle fibers also are found in the outside layer of the wall.
Arteries are thick-walled conduits carrying blood away from the heart, and are characterized by a well-developed elastic tunica media that is thick enough to maintain the shape of the blood vessel without collapse. The tunica adventitia in arteries is relatively thin.
As arteries follow their course throughout the body away from the heart, they gradually decrease in size, at various stages being called arterioles and arterial capillaries, until eventually they become true capillaries with very thin walls and minute bore, making it necessary for blood cells to pass in “Indian file” and even to assume distorted shapes in order to squeeze through.
The true capillaries, which lack both a tunica media and a tunica adventitia, form innumerable anastomoses and networks between the arterial capillaries on the one hand and the venous capillaries on the other. Arterial and venous capillaries, therefore, are transitional in location as well as in size and in thickness of their walls between the capillaries proper and arterioles and venules respectively.
Veins, which always take their rise from capillaries, are relatively thin-walled and collapsible. Since their walls are largely deficient in elastic tissue and muscle cells of the tunica media, the tunica adventitia becomes the most highly developed of the three layers in the walls of veins.
Pocket-like valves that prevent or hinder the back-flow of the blood (Fig. 281) are present in the larger veins but not in arteries, except in the immediate neighborhood of the heart of some gill-breathers. Valves are not present in all veins, being largely absent from the veins of the brain, cord, meninges, bones, and the umbilical vein, as well as most visceral veins, excepting branches of the hepatic portal system. The thin-walled veins are much more likely to anastomose, become varicose, or to enlarge into sinuses, than the thick-walled arteries.
Lymphatics are typically varicose as well as capable of great distension. Ordinarily they do not acquire thick muscular or elastic walls and are very collapsible, although the larger lymphatics nearer the heart develop a definite tunica media with muscle cells that even enable them to pulsate. Lymphatics entwine around the other blood vessels in the most intimate fashion, yet do not communicate with them except at one or two definite openings near the heart where the lymph may be returned to the general blood stream. Lymph capillaries, although never as small as haemal capillaries, have the same sort of thin endothelial walls.
Physiologically, if not morphologically, the large serous cavities, such as the body cavity, and the pericardial and pleural cavities, as well as the synovial spaces around joints, belong to the lymphatic system, although their walls have a somewhat different origin and structure from those of ordinary lymphatic vessels.
Like veins, lymphatics possess valves along their course in the form of crescentic folds of the tunica intima which act like sluice-gates preventing the retreat of the fluid to any great extent in the direction away from the heart. Thus they establish one-way traffic in the lymphatic system.
The Role of the Capillaries
The first blood vessels to form in the embryo and the last to be discovered on account of their size are capillaries (capilla, little hair). Physiologically they are the most important part of the whole intricate system of blood channels in the vertebrate body, because in them the final transfers of the circulatory system are made. If the entire circulatory system be compared to a railroad system, the capillaries correspond to stations where passengers and freight are entrained and detrained, while the more conspicuous arteries and veins are simply lines of track connecting the stations.
Anatomists have always been more concerned with following out and homologizing veins and arteries, which it is possible to trace and describe, than with the nameless capillaries that defy isolation and cataloguing. When one considers Krogh’s estimate that there may be at least 2000 capillaries permeating a cubic millimeter of human muscle, no one of which is over a millimeter in length, and that the total length of all the capillaries of the human body, if untangled and placed end to end, would be as much as ] 00,000 kilometers, that is, equal to two and a half times around the earth at the equator, it is small wonder that anatomists are forced to describe them in the most general terms.
Unlike the twigs of a tree that come to an end, capillaries are continuous and keep right on, forming anastomosing networks which have a larger total carrying capacity than the blood vessels they immediately connect (Fig. 282). The result is that the rate of flow of the blood stream slows down as it goes through the capillary networks, just as a swiftly flowing river that spreads out upon entering a lake loses its momentum. Blood cells in capillaries may be said to “crawl,” but as the size of the blood vessels they are passing through enlarges, they “hustle” more and more.
In capillaries the rate of movement, which varies within wide limits, has been given as one twentieth of an inch per second, while in the highway of the aorta it is three hundred times as rapid.
Capillaries may measures from 0.003 mm. to 0.01 mm. in diameter, while the largest human arteries and veins sometimes attain a diameter of 3 cm. This is a difference of 10,000 diameters or 100,000,000 times in carrying capacity. Single capillaries may be so small that, when undilated, blood cells that can penetrate a fine filter readily are unable to pass through even in single file, or can only squeeze through with difficulty by temporarily distorting their shape (Fig. 283).
Although capillaries generally are intermediaries between veins and arteries, they may sometimes connect veins and veins, when they constitute a “portal system,” or arteries and arteries, as in the “red gland” within the swim bladder of fishes, or in the glomeruli of the kidneys.
Outside the single layer of endothelial cells that form the walls of capillaries some vertebrates, but probably not mammals, have at intervals flat branching involuntary muscle cells (Rouget cells) that control the caliber of these minute vessels (Fig. 284). Resistance to blood flow is furthermore exerted by cufflike circular muscles around the arterioles and arterial capillaries under the control of vasomotor nerves. When the bore of the arteriole is lessened by the contraction of these circular muscles, blood cells pass into the capillaries at a slower rate or are temporarily excluded. The operation of these neuromuscular stopcocks of the arterioles is also influenced by mental states as reflected when a person is “pale with anger” or “flushed with joy.”