The Velocity of the Blood Flow.—This depends on the rela tion between the bore of the blood vessel and the amount of blood passing through it in a unit of time. The total area of the cross sections of the arterial system gradually increases from the aorta to the periphery. The increase in area is especially great when the arteries break up into capillaries, the area of which is probably over a thousand times larger than that of the cross section of the aorta. The velocity of the blood flow will obviously be propor tionately reduced the further we go towards the periphery, so that while in the aorta the mean velocity under resting conditions may be about 500 mm. per second, in capillaries it is barely o.5 mm. per second. As the capillaries join to form veins, the area of the vascular bed diminishes and the velocity of the blood flow in creases ; however, it never reaches the velocity which is observed in the aorta since the cross section of the large veins near to the heart is about double that of the aorta.
Intensity of the Circulation.—The amount of blood received by the heart from the veins and passed to the arteries in a unit of time, cannot be measured directly. However, a knowledge of the cardiac output is of considerable impor tance because it controls the amount of oxygen carried to the tissues and the rate of the removal of various products of metabolism from the tissues. The great value of the study of the cardiac output of man and animals has only been realised comparatively recently, and there still remains much to be done; nevertheless methods of sufficient precision have been worked out, and the main factors governing the cardiac output are gradually being disclosed.
Zuntz calculated the output by comparing the difference in the oxygen content of the arterial and venous blood with the amount of oxygen consumed in a given time. For instance in a horse weighing 36o kilo., 2,735 cc. of oxygen were consumed per minute; the arterial blood contained 10.33% more oxygen than the venous.
Since every too cc. of blood that passed through the lungs had taken up 10.33 cc. of oxygen, 2,733 cc. of oxygen had been taken up in the course of one minute by I°° X 2733 = cc. of blood. 10.33 Krogh modified this method by making the animal or man breathe a gas which was harmless and easy to detect, viz., nitrous oxide. He knew how much of the gas could be absorbed by Icc. of blood, and was therefore able to calculate the cardiac output by deter mining the amount of gas taken in during one minute. Recently
other methods have been introduced which are claimed to be more exact. In one of them, ethyl iodide is the gas absorbed, and in another the principle of Zuntz is modified and the output is cal culated by comparing the carbon dioxide content of the arterial and venous blood with the amount of carbon dioxide given off in a unit of time.
Cardiac Output in Man.—In man, under conditions of rest, the cardiac output varies in different individuals between about 31 and 6 litres of blood per minute, that is between 6o and about I 10 cc. for each heart beat. The cardiac output, i.e., the inflow of blood into the heart, as in the heart-lung preparation, is within wide limits independent of the heart rate. It primarily depends on (a) how freely the blood passes from the arterial system through the tissues into the venous system, i.e., on the peripheral resistance; and (b) the power of the heart to pump the inflowing blood into the arteries; and (c) the capacity of the vascular system relative to the amount of blood. When the arterioles dilate, the blood passes quickly through the organs and, since the capacity of the vas cular bed will be changed but little, the blood will reach the heart during the next few beats in a larger amount. The heart will therefore force more blood into the arterial system, and thus within limits prevent a fall of arterial blood pressure, which would have taken place if the arterioles dilated without causing an in crease in the blood flow.
If we turn now to the second way in which the output may be increased, namely the strengthening of the heart beat, it is clear that, if the pumping action of the heart becomes inadequate, blood will stagnate on the venous side, and the output will diminish. If the heart beat is improved by nervous or pharmacological agencies, there will be better emptying of the venous side, and a consequent increase in the circulation rate.
The third factor governing the output is the capacity of the vascular bed relative to the amount of blood. This factor can be appreciated by considering the tremendous capacity of the cap illary system. Krogh has shown that to of a muscle is made up of capillaries, and in the heart there is at least one capillary to every muscle fibre. If all these capillaries lost their tone, the ani mal would literally bleed into them, the arterial and venous sys tem would become empty, and the output of the heart would fall to vanishing point.