In order to form an estimate of the time in which a given quantity of blood may pass through the heart, or of the time in which the whole quantity of blood contained in the body would take to pass through the heart, several data are required which are not yet furnished by accurate experiments. In the first place, we must know the average quantity of blood contained in the body, and, in the next place, the quantity which is evacuated from the heart at each stroke or systole of the ventricles.
With regard to the first of these points, a number of calculations have been made which vary greatly in their results. • Animals have been bled to death by the section of the larger bloodvessels, and the quantity of blood lost has been measured. The quantity of blood lost in this way seems to have varied from 1-10th to 1-30th of the weight of the whole body, and Dr. Moulins, who formed his estimates from experiments of this kind, rated the quantity of blood in the human body at eight or nine lbs. only, or 1-20th of the weight of an average sized man, taken at 150 or 160 lbs. But it is obvious that when one of the larger bloodvessels is opened, from the suddenness of the flow, the animal faints or dies before the whole or even a considerable proportion of the blood has been lost; and it has been ascertained from numerous obser vations, that when the blood flows more gra dually and from small vessels, as occurs in hemorrhages from the nose, stomach, rectum, or uterus, a proportionally much greater quan tity of blood may be lost than occasions death in animals experimented upon by the section of the larger arteries or veins. Instances are on record in which from ten to twenty lbs. and even greater quantities of blood have flowed from the human body within twenty four hours.* We feel inclined on these grounds to coincide with the estimate formed by Haller, that the blood forms about a fifth of the weight of the body, or equals from twenty five to thirty lbs. in a man of the average weight of 150 lbs. It is obvious that this must vary in different individuals from other circumstances besides a difference of stature. In the young, the quantity of blood is con sidered to be greatest. Of the whole of the blood contained in the body, it is estimated by Haller, and probably with accuracy, that four parts are contained in the arterial and nine in the venous system.
In endeavouring to estimate the quantity of blood which passes through the heart in a given time, we must find the capacity of the cavities of the heart, we must ascertain whe ther the cavities on the two sides are of the same size, and, as it is almost impossible to measure the quantity of blood evacuated from the heart at each stroke, we must find to what extent the ventricles empty themselves during their systole. It is obvious that, so long as the circulation is uniform and no local accumu lation of blood takes place, the same quantity of blood must pass out of the ventricles into the larger arteries which enters by the veins, and for the same reasons,,that the quantity of blood passing through the right and left cavi ties of the heart must be exactly equal. The
circumstance that an equal quantity of blood passes out of the right and left cavities of the heart during their systole does not entitle us to conclude that the capacity of the different auricles and ventricles is the same, because any one of them during its systole Inay be more or less completely emptied than the rest, and a regurgitation obviously takes place from some of them, so that the whole blood which they contain is not propelled in its onward course. According to some anatomists the au ricles are larger in capacity than the ventricles, probably in the proportion of three or two and a half to two, and the auricles are by no means completely emptied during their systole. An opinion has very generally prevailed that the cavities on the right side of the heart are some what larger than those on the left. There is no doubt that in making measurements of the rela tive capacity of the two sides after death, it is most frequently found so; but it is obvious that some have very much overrated the difference, and there is much reason to believe that the greater capacity of the right auricle and ven tricle depends in part on the accumulation of blood which generally takes place in most kinds of slow death in the pulmonary arteries, and in part also upon the greater thinness and consequent distensibility of the right ventricle. In men dying suddenly, and in animals killed purposely, in which the pulmonary artery is opened so as to allow of the free egress of the blood from the right side of the heart, the capacity of this ventricle is not greater than that of the left, and the proportions of the capacity of the two sides of the heart usually found after slow death are sometimes reversed when a ligature is placed on the aorta and the pulmonary artery is opened.* Most authors seem to have agreed to follow the estimate of the capacity of the ventricles given by Hales in his Medical Statics. This author esti mates the capacity of the left ventricle at 1i oz. fluid measure, and that of the right at 2 oz. The contemplation of the muscular structure of the left ventricle, and the great diminution in size it undergoes during its sys tole, would induce us to conclude that it must be completely emptied during the contraction, and that there cannot remain any blood even among the columnw carnew. The right ven tricle does not appear from the quantity of its muscular substance to be so well suited to be emptied, but its position round the left must assist considerably in the diminution of its size during its systole. In some cases of sud den death in healthy persons, both ventricles have been found completely empty.