58 THE BODY AT WORK 



worn-out red blood-corpuscles are in a sort digested) break 

 up in the liver into a second set of small vessels. The pseudo- 

 capillary vessels of the liver reunite to form the hepatic veins, 

 which add the blood that has passed through that organ to 

 the -rest of the blood which is passing up the inferior vena 

 cava to the heart. A second capillary circulation is found 

 in the kidney also. 



The heart is four -chambered (Fig. 10). Its left ventricle 

 drives the blood round the systemic or greater circulation, the 

 blood returning to the right auricle. The right ventricle 

 drives the blood through the lesser or pulmonary circulation, 

 from which it returns to the left auricle. The walls of all 

 bloodvessels, except capillary tubes, are sufficiently thick to 

 prevent the escape of any of the constituents of blood. To 

 support the pressure of the blood which they contain, the 

 arteries and the larger veins need walls of considerable thick- 

 ness. The walls of the capillaries allow an interchange between 

 blood and lymph in the manner already described (cf. p. 39). 



Blood fresh from the lungs, whether still in the pulmonary 

 veins or in the systemic arteries, is scarlet in colour. Venous 

 blood is darker and purple-red, the depth of its tint varying 

 with the extent to which it has parted with its oxygen. It 

 looks less opaque than arterial blood. With this exception, 

 the physical properties and chemical composition of blood are 

 remarkably constant in all parts of the body. Arterial blood 

 contains more oxygen, venous blood more carbonic acid. 

 Other chemical differences can be recognized, but they are 

 relatively very small. The constancy in the constitution of 

 blood is its most notable character. Bleeding, unless exces- 

 sive, does not greatly affect it. The number of corpuscles 

 is of course diminished, but even these are replaced with great 

 rapidity. The plasma, after bleeding, soon recovers its proteins 

 and salts. A similar readjustment occurs if normal saline 

 solution (water containing 0-9 per cent, spdic chloride), or even 

 a strong solution of salt, is injected into the blood. Within 

 certain limits it is very difficult to disturb the balance of its 

 constituents. It gets rid of substances added in excess, or 

 replaces substances removed, with remarkable facility. If 

 sugar (glucose) be injected into a vein, it escapes through the 

 capillary walls into the lymph. After a short interval the 



