88 THE INDIVIDUAL ORGANISM 



to the lungs. In the lungs the blood enters the capillaries that are in 

 contact with the alveoli, loses its load of carbon dioxide, and becomes 

 oxygenated. The capillaries of the lungs lead to veins that unite to form 

 the large pulmonary veins. These lead back to the heart and empty into 

 the left auricle. 



The Systemic Circulation. The blood that returns to the left side 

 of the heart from the pulmonary circulation is the bright-red oxygenated 

 blood, that is often termed arterial blood. Contractions of the left ven- 

 tricle force it out into the aorta, the largest artery of the body, which 

 sends branches to all parts of the body except the lungs. The capillaries 

 supplied from the systemic arteries bring arterial blood into intimate 

 contact with the tissues. Blood leaving the capillaries passes into the 

 veins, and the two venae cavae finally return the now deoxygenated 

 dark-red venous blood to the right auricle, from which it will be taken 

 into the pulmonary circulation. 



The routes that the blood may traverse in the systemic circulation 

 are varied, but except for a considerable portion carried to the digestive 

 system, the blood from each region returns directly to the heart after 

 traversing a single set of capillaries. 



The Portal System. Certain of the large abdominal arteries that 

 lead from the aorta carry blood to the stomach, intestines, liver, pancreas, 

 and spleen. Here the blood supplies the needed oxygen and takes up the 

 excess carbon dioxide and nitrogenous wastes and also the digested foods 

 that have passed through the walls of the intestine. From the capillaries 

 of these organs, this blood passes finally into a large vein, the hepatic 

 portal, which carries the blood to the liver, where it enters a second set of 

 capillaries that form a network in the tissues of the liver. In the liver the 

 surplus monosaccharides are taken from the blood and stored in the liver 

 cells. From the capillaries of the liver the blood again passes into a 

 venous system, which leads to the heart. 



The Lymphatic System. Although all the materials used by the cells 

 of the body are obtained from the blood and all the waste products of 

 cell metabolism find their way into it, blood as such does not come into 

 direct contact with the cells. It remains within the capillaries, and these 

 capillaries in most instances are not even in immediate contact with 

 the cells. Between the capillaries and the cells, and between the cells, 

 there exists a multitude of tissue, or lymph, spaces, mostly microscopically 

 small but varying much in size and shape. Filling these spaces and bathing 

 the cells is a tissue fluid, the lymph. 1 Lymph is formed by a "leakage" 

 of water, salts, glucose, amino acids, etc., from the blood capillaries into 

 the surrounding tissue spaces. The red blood cells do not pass through 



1 A distinction is sometimes made between tissue fluid and lymph, the latter term 

 being then restricted to the fluid contained within the lymph vessels. 



