Thus in passing through the two great separative and constructive organs of the body, the lungs and the liver, the venous blood, charged both with the products of decomposition (carbonic acid gas), and with the materials for repair, undergoes such changes by casting out the products of tissue destruction, and by modifying the materials of reconstruction, that it issues both from the lungs and the liver in a renovated condition, and charged with those materials which are necessary for the growth and repair of the tissues.

The blood, as it issues from the lungs, is carried by the pulmonary artery, and as it issues from the liver by the hepatic artery, into the aorta, and is then conveyed by the circulation into the furthermost parts of the body, where it nourishes the tissues. But the lungs and the liver have not done all that is necessary for the scavenging and renovation of the blood. After leaving the liver it passes all through the tissues, is again collected by the veins, and passed on to the right side of the heart. It is thence pumped into the lungs, where it parts with its carbonic acid gas, and receives a new and revivifying supply of oxygen. Passing again into the heart, it is pumped from the left ventricle into the aorta, thence to be distributed to the body. But though apparently cleansed by its passage through the lungs, it is still laden with the products of decomposition, of incomplete oxidation, or retrogressive changes which have taken place in the tissues in the course of tissue change or growth, and in the production of energy. These effete products are more particularly those which result from the incomplete oxidation of albumen, and they are found in the blood in the form of urea and uric acid. As these substances are most deleterious in their effects,and even poisonous in their action if allowed to circulate in the blood, it becomes a matter of the greatest importance to get rid of them. This is accomplished by the action of the kidneys.

Fig. 13. - Glomeruli of the Kidney; Origin of the Uriniferous Tubules.

1, 1. Glomeruli surrounded by their capsules, or the funnel-shaped terminations of the uriniferous tubules. 2, 2, 2. Uriniferous tubules springing from the capsules and much contorted in their course. 3, 3, 3. The interlobular branch of the renal artery. 4, 4. Its branches or the afferent vessels of the glomeruli. 5, 5. Two glomeruli in which are convoluted the afferent vessels. 6, 6. Glomerulus with the capsule partly removed. 7, 7. Efferent vessels of the glomeruli. 8. Efferent vessel the branches of which, 9, break up into the capillary network of the kidney, 10.

The kidneys are two bean-shaped bodies, which lie at the back of the abdominal cavity, on either side of the vertebral column in the lumbar region. The arteries which conduct the blood to them come off at right angles to the aorta, from which arrangement it is obvious that the blood passes with considerable force into the kidneys from the main channel of the circulation. Within the kidneys the artery at once divides into a number of vessels which end in what is called a glomerulus. In the glomerulus the vessel breaks up into a great number of finer vessels folded one upon another in a tangled ball. The arteriole communicates with a vein similarly constructed and arranged, but of smaller calibre. It is apparent from the arrangement that the return of the blood from the glomerulus must be somewhat hindered. This convoluted ball of blood vessels is pushed into the globe-like distension of a fine tube. (Fig. 7.) The delicate transparent double walls of the sack-like end of the tube envelop the glomerulus on every side. Here we have all that is necessary for the process of filtration: namely, blood carried at high tension from the full current of the circulation suddenly brought almost to a condition of stasis in the tangle of the glomerulus, and a bag or filter furnished with a conducting tube in immediate contact with the distended blood vessels. What happens is that the watery constituents of the blood, together with the urea and other extractive and colouring matters, are filtered from the capillary vessels into the sack-like termination of the uriniferous tubule. In a state of health the albumen and fibrine of the blood do not pass this filter. The urine thus excreted from the blood in the glomerulus passes by a series of looped vessels into a single tube which opens into a basin-like cavity called the pelvis of the kidney. The fluid which is being constantly forced out from the uriniferous tubules is finally conveyed from both kidneys by long, narrow, muscular tubes called the ureters to the bladder, which is emptied at will.

Fig. 14. - Course of the Uriniferous ducts, Diagrammatic Plan.

1, 1. Rectilinear uriniferous tubule; a collecting duct passing from the periphery of the lobes towards the papilli of the kidney towards which it opens. 2. Lower end of the tubule, which has been cut off a little above its mouth for the convenience of the drawing. 3, 3, 3. Other collecting tubules opening into the cavity of the preceding. 4, 4, 4, 4, 4. Malpighian bodies or glomeruli. 5, 5, 5, 5, 5. Contorted tubules springing from the glomeruli and forming the greater part of the cortical substance of the kidney. 6, 6, 6, 6, 6. Straight tubes succeeding the contorted tubes and descending from the cortical into the medullary substance. 7, 7, 7, 7, 7, Larger branches forming loops. 8, 8, 8, 8, 8. Other ascending branches.