capillaries form networks which accommodate themselves, to the
structure of the organs, e.g. longitudinal networks in muscle, loops
in the papillae of the skin,
close-meshed networks round the
alveoli of glands, cells of liver, &c.
In the liver the blood penetrates
into the substance of the liver cells.
As the capillaries join
together to form the vennules,
 |
 |
| From Young and Robinson, Cunningham’s | From Young and Robinson, Cunninghan’s |
| Text-Book of Anatomy. | Text-Book of Anatomy. |
| Fig. 19.—Transverse Section | Fig. 20.—Transverse Section |
| through the Wall of a Large | of the Wall of a Vein. A, |
| Artery. A, tunica intima; | tunica intima; B, tunica |
| B, tunica media; C, tunica | media; C, tunica externa. |
| externa. |
coat the walls of the latter. The veins have a greater capacity than the arteries. Blood vessels, the vasa vasorum, supply the walls of the large vessels with nutrition.
From Young and Robinson, Cunningham’s Text-Book of Anatomy.
Fig. 21.—Structure of Blood Vessels (diagrammatic). A1, capillary-with
simple endothelial walls. A2, larger capillary with connective tissue sheath, “adventitia capillaris”; B, capillary arteriole—showing muscle cells of middle coat, few
and scattered; C, artery-muscular elements of the tunica media forming a continuous layer.
The vaso-motor nerves end in a plexus of fibrils among the muscle fibres. Ganglion cells occupy the larger nodes of the nerve plexus. The ends of a torn artery retract, coil up within the external coat and prevent hemorrhage. The arteries contract when mechanically irritated and remain contracted for a long time after excision. They tend to contract when submitted to increased blood pressure. The capillaries cannot contract of themselves, but their lumen can be widened or narrowed by the varying contractility or turgidity of the tissues in which they run.
The arteries successfully withstand elastic strain of the pulse 70 times a minute throughout the years of a long life. It has proved possible to stitch divided arteries and veins together so perfectly that the circulation can continue through them. A kidney as thus been successfully transplanted from one dog to another, and has continued to function ate normally.
The elastic coefficients of the several layers of the coat of an artery increase from within out, and thus great strength is obtained with the use of a small amount of material. Over-expansion of the arteries is checked by an external coat of inextensible connective tissue. The elasticity of a healthy artery is almost perfect, while the breaking strain is very great and far above that exerted by the blood pressure. The small arteries and arterioles are essentially muscular tubes, and can, under the influence of the central nervous system, vary considerably in diameter.
By the expulsion of the blood at each systole the walls of the aorta are suddenly distended. From the aorta a wave of distension ripples down the walls of the arteries. This wave of distension is called the pulse. As the pulse is distributed over an ever-widening field its energy is expended and it disappears finally in the arterioles. From a wounded The pulse. artery the blood flows out in pulses, from a wounded vein continuously. To stop the hemorrhage the ligature must be applied between the wound and the heart in the case of the artery, and between the peripheral parts and the wound in the case of the vein. The pulse travels about 20 times as fast as the blood flows in the arteries (7–8 metres per second). By feeling the pulse we can tell whether the heart-beat is frequent, quick, strong, regular, &c., and whether the wall of the artery is normal and the pressure in the arteries high or low. Frequency expresses the number per minute, quickness the duration of a single beat. The pulse is a most important guide to the physician. The pulse can be registered graphically by means of a sphygmograph. A lever rests on the radial artery and transmits the pulse to a system of levers which magnifies the movement and records it on a smoked surface moved by clockwork.
In such a record, or sphygmograph, the upstroke corresponds to systolic output of the left ventricle, marking the opening of the aortic valves, and the pouring of the blood into the arteries.
The downstroke represents the time during which the blood is flowing out of the arteries into the capillaries. There are subsidiary waves on the down stroke. The chief of these is called the dicrotic wave, the notch preceding which marks the closure of the semi lunar valves. The dicrotic wave is caused by the jerk back of the blood towards the heart when the outflow ceases, and is most manifest when the systole is short and sharp and the output of blood from the arterioles rapid, in other words when the heartbeat is strong, the systolic pressure high and the diastolic pressure low. A smaller wave, predicrotic, preceding this occurs during the period of output and sometimes is placed on the ascending limb of the pulse curve. This occurs when the peripheral resistance is great, and the pulse is then termed anacrotic.
 |
 |
| Fig. 22.—Anacrotic Pulse. | Fig. 23.—Dicrotic Pulse. |
Figs. 22, 23 and 24 from Allchin's Manual of Medicine, by permission of Macmillan & Co. Ltd.
Fig. 24.—Normal Pulse, and Time Tracing in 110 sec. A, Primary wave. C, Dicrotic wave.
B, Predicrotlc wave. D, Post-dicrotic Wave.
The form of these waves is modified by the pressure of application of the sphygmograph, and by instrumental errors; and we have no scale by which we can measure the blood pressure in sphygmograph tracings. To do this another instrument, the sphygmomanometer, is employed.
The pulse may pass through the arterioles and reach the capillaries when the arterioles are dilated or when the capillaries are only filled at each systole, as may be seen in the pink of the nail when the arm is held above the head, and in cases of aortic regurgitation.
A venous pulse may be recorded in the jugular vein; it exhibits oscillations synchronous with auricular and ventricular systole, and affords us important information in certain cases of heart disease. The normal average pulse rate is 72 per minute, in woman about 80; but individual variations from 40–100 have been observed consistent with health. In the newborn the pulse beats on the average 130-140 times a minute; in a one-year-old child 120–130; three years 100; ten years 90; fifteen years 70–75. Active muscular exercise may increase the pulse rate to 130. Nervous excitement, extreme debility and rise of body temperature also increase it markedly. The pulse is more frequent when one stands than when one sits, or lies down, and this is especially so in states of debility. The taking of food, especially hot food, increases it. By placing tambours on, say, the carotid and radial arteries and recording the two pulses synchronously, it has been found that the pulse occurs later, the further the seat of observation is from the heart. The velocity with which the pulse wave travels down the arteries has been determined thus. It is about 7–8 metres per second. The wave length of the pulse is obtained by multiplying the duration of the inflow of blood into the aorta by the velocity o the pulse wave. It is about 3 metres. As the return of venous blood and pulmonary circulation is favoured during inspiration so that the output of the left ventricle during the first part of inspiration is lessened and subsequently increased,
