l88 THE DISTRIBUTION OF THE BLOOD. 



THE DISTRIBUTION OF THE BLOOD. 



The metkods employed for determining the quantity of blood contained in 

 individual organs and members must unfortunately as yet be regarded as inade- 

 quate, (i) The quantity of blood contained in the part may be determined after 

 death in frozen cadavers. This method is inaccurate, because after death, par- 

 ticularly through the stimulation of the vasomotor center, the quantity of blood 

 contained in any given part undergoes profound changes in consequence of the 

 fact that different parts of the body die and freeze at different times. (2) A part 

 may be forcibly ligated off from an animal during life, then be at once severed, and 

 the quantity of blood in the tissues be determined while they are still warm. This 

 method is, unfortunately, inapplicable to many internal organs. 



J. Ranke determined in this way the distribution of the blood in the 

 living rabbit at rest. He found one-fourth of the entire quantity of 

 blood in (a) the resting muscles, (b) the liver, (c) the circulatory organs 

 (heart and large arterial trunks), (d} the remaining organs taken to- 

 gether; of the last the lungs contained between 7 and 9 per cent. 



The amount of blood is influenced by: (i) The anatomical distribution of the 

 vessels in general, that is, the number o'f vessels in individual parts of the body; 

 (2) especially the size of the vessels, which is dependent upon physiological causes: 

 (a) the blood-pressure within them; (b) the state of irritability of the vasocon- 

 strictor or vasodilator nerves; (c) the condition of the tissues in which the vessels 

 are situated, for example, the intestinal vessels during the absorption of alimentary 

 juices; the muscular vessels during the contraction of the muscles (vessels in in- 

 flamed parts). 



The most important factor influencing the quantity of blood in 

 an organ is the activity of the latter. In this connection the ancient 

 dictum "ubi irritatio, ibi affluxus" is applicable. Examples are 

 afforded by the salivary glands, the stomach, and the muscles during 

 activity. As, however, under normal conditions of the body, the 

 individual organs in many ways relieve one another, one organ may 

 in the course of a day be found in a condition of greater plethora at 

 one time and another organ at another time. The variations in the dis- 

 tribution of the blood coincide with the alternations in the functional 

 activity of the organs. Thus, while one organ is in a state of increased 

 activity, the remainder often are resting: the process of digestion is 

 attended with muscular lassitude and mental relaxation; severe mus- 

 cular exertion delays digestion; when the skin is reddened and secreting 

 freely, the action of the kidneys is temporarily in abeyance. Some 

 organs (the heart, the respiratory organs, and certain nerve-centers) 

 appear to maintain a constant level of activity and contain the same 

 quantity of blood at all times. 



While an organ is active, the amount of blood present may increase 

 up to 30 per cent, or even to 47 per cent. The organs of locomotion in 

 young and vigorous individuals are likewise relatively more plethoric 

 than those of older individuals with a feebler muscular system. 



During mental activity the carotid is dilated, and the dicrotic elevation of 

 the carotid curve is increased, while the radial exhibits reverse conditions, and 

 the pulse is accelerated. 



In this condition of greater activity the increased amount of blood 

 usually undergoes more rapid renewal at the same time; for example, 

 after muscular exertion the duration of the circulation is diminished. 

 This circumstance may be affected by a great variety of influences that 

 govern the movement of the blood. 



