in systole and fall to 150 mm. in diastole. Thesmaller the artery the less is this difference and the more uniform the rate of flow.
From Allchin's Manual of Medicine, by permission of Macmillan & Co., Ltd.
Fig. 26.—Diagram showing General Relations of the Velocity of the Blood in the Arteries, Capillaries and Veins.
The flow in the large veins is approximately equal to that in the large arteries. In the jugular vein of a dog the mean velocity was found to be 225 mm. and in the carotid 260 mm. per second. The velocity in the capillaries has been measured by direct observation with the microscope. It is very small, e.g. 0·5–1 mm. per second. The variation of velocity in different parts of the vascular system is explained by the difference in width of bed through which the stream flows. The vascular system may be compared to a stream which on entering a field is led into a multitude of irrigation channels, the sum of the cross sections of all the channels being far greater than that of the stream. The channels unite together again and leave the field as one stream. If the flow proceeds uniformly for any given unit of time, the same volume must- flow through any cross section of the system. Thus the greatest velocity is where the total bed is narrowest, and slowest where the bed widens to the dimensions of a lake.
The blood in leaving the heart may take a short circuit through the coronary system of the heart and so back to the right heart, or it may take a long and devious course to the toes and back, or through the intestinal capillaries, portal system and hepatic capillaries. It is obvious, then, that the time complete any two particles of blood take to complete the circuit The time necessary for a complete circulation. may be widely different. Experiments have been made to determine how rapidly any substance, like a poison, which enters the blood may be distributed over the body. A salt such as potassium ferrocyanide is injected into the jugular vein, and the blood collected in successive samples at seconds of time from the opposite jugular vein. These samples are tested for the presence of the salt, or a strong solution of methylene blue is injected into the jugular vein, and the moment determined with a stopwatch when the blue colour appears in the carotid artery.
The velocity of flow also can be determined in any organ by injecting salt solution into an artery, and observing, with the aid of a Wheatstone's bridge arrangement, the galvanometric change in electrical resistance which occurs in the corresponding vein when the salt solution reaches it. The moment of injection and that of the alteration in resistance are observed with a stop-watch (Stewart).
It has been determined that the blood travelling fastest can complete the circuit in about the time occupied by 25 to 30 heartbeats, say in 20 to 30 seconds; a result which shows how rapidly methods must be taken to prevent the absorption of poisons-for example, snake-poison. The blood travelling fastest in the pulmonary circuit occupies only about one-fifth of the time spent by that in the systemic circuit. That some of the blood takes a very long time to return to the heart is shown by the long time it takes to wash the vascular system free of blood by the injection of salt solution.
That the blood is under different pressure in the various parts of the system has long been known. From a divided artery the blood flows out in forcible spurts, while from a vein it flows out continuously and with little force. It takes very little pressure of the fingers to blanch the capillaries of the skin, but an appreciable amount to obliterate the radial The pressure relation in the vascular system. artery.
Stephen Hales (1733) was the first to measure the blood pressure. He inserted a brass tube into the femoral vein of a horse and connected it to a long glass tube held vertically, using the trachea of a goose as a flexible tube, and found the blood rose to the height of 8 ft., oscillated there with each heart-beat, and rose and fell somewhat with inspiration and expiration. In the vein he found the pressure to be only about 12 in. Poiseuille (1828) adapted to the same purpose the mercurial manometer, a U-shaped tube containing mercury, which, being 13·5 times heavier than blood, allowed the manometer to be brought to a convenient height. The introduction of rubber tubing for the connexions made the method of inquiry comparatively simple. The tubing connecting the arterial cannula and the manometer was filled with a suitable fluid to prevent coagulation of the blood; also to prevent more than a trace of blood entering the connexions. A saturated solution of sodium sulphate, or a 1% solution of sodium citrate, may be employed for this purpose. Ludwig (1847) added a float provided with a writing style to the mercurial manometer, and brought the style to write on a drum covered with smoked paper and driven slowly round by clockwork-a kymograph By this means tracings of the arterial blood pressure are obtained, and the influence upon the blood pressure of various agents recorded and studied. For the veins a manometer filled with salt solution is used, as mercury is too heavy a 'fluid to record the far slighter changes of venous pressure. he manometer may be connected with a recording tambour.
From Howell's Text-Book of Physiology, by permission of B. Saunders Co.
Fig. 27.—Diagram showing Systolic, Mean and Diastolic Pressure.
The arterial blood-pressure record obtained with the mercurial manometer exhibits cardiac and respiratory oscillations as shown in fig. 18. The method gives us a fairly accurate record of the mean pressure, but the mass of the mercury causes such inertia that the instrument is quite unable to faithfully record the systolic and diastolic variations of pressure. To effect this record, delicate spring manometers of rapid action and small inertia have been invented. A mercury manometer provided with maximum and minimum valves has also been employed to indicate the maximal systolic and minimal diastolic pressure. To determine the blood pressure in man, an instrument called the sphygmometer is used. The writer's sphygmometer consists of a rubber bag covered with silk which is filled with air, and connected by a short length of tube to a manometer. This manometer consists of a graduated glass tube, open at one end. A small hole is in the side of the tube- near this end. A meniscus of water is introduced up to the side hole—the zero mark on the scale—by placing the open end of the tube in water. The bag is now connected to the gauge so that the side hole is closed by the rubber tube. Covering the rubber bag with the hand and pressing it on the radial artery until the pulse (felt beyond) is obliterated, one reads the height to which the meniscus rises in the manometer, and this gives us the systolic pressure in the artery. The air above the meniscus acts as a spring, converting the instrument into a spring manometer. It is empirically graduated in mm. Hg.
It is very necessary to remember that the blood pressures, taken in different vessels and postures, vary with the hydrostatic pressure of the column of blood above the point of measurement. Thus in the standing posture the arterial pressure in the arteries of the leg is higher than in the arm by the height of the column of blood that separates the two points of measurement. In the horizontal posture the pressure is practically the same in all the big arteries. The pressure in the ascending aorta is kept about the same in all postures, while that of the leg
Fig. 28.—Hill's Sphygmometer.
arteries varies widely. The effect of gravity is compensated there by active changes in heart force, splanchnic dilatation, &c. (L. Hill). The systolic pressure of young men, taken in the radial artery with the arm at the same level as the heart, may be taken to be about 110 mm. of Hg. In men of 40–60 years the systolic pressure is often about 140 mm., but in some robust men it is no higher than in youth.
The venous pressure in man may be measured by finding the pressure just required to prevent a cutaneous vein refilling after it has been emptied beyond a valve. There is no accurate method
