Excretion, The kidneys, The bladder And Homeostasis



THE processes that make a creature alive can be generally described as chemical reactions that perpetuate themselves.

Many of these reactions, respiration for example, release energy that is used in setting off other reactions. All these reactions give rise to end-products, some of which are poisonous or could affect the normal chemical reactions in the body if they were allowed to accumulate.

Even the apparently permanent structures of the body such as the muscles, blood, skin and internal organs are, in fact, changing from day to day. The chemical units of living protoplasm are constantly being renewed. New molecules are being added, degenerate molecules or entire cells are being digested away. For example, an amino acid in some protein eaten one day may be built into the living protoplasm of a muscle fibre the next day. Later, the same amino acid may be broken down and the products carried off in the blood stream. The products of of this kind of protein decomposition contain nitrogen, ammonia being one of the most common compounds.

If nitrogenous compounds were allowed to accumulate in the body they would cause death in a matter of days or weeks. Excess amino acids absorbed after a meal containing protein are deaminated in the liver as described giving rise to ammonia and other nitrogenous compounds. Excretion is the process by which such harmful products are removed from the body as fast as they exceed a certain concentration.

Excretory products. The main excretory products in animals are carbon dioxide and water from respiration, and nitrogenous compounds from the breakdown of excess amino acids. The nitrogenous compounds such as ammonia are converted in the liver into urea and uric acid which are less poisonous.

Excretory organs. In man, the excretory organs are the lungs, the liver and the kidneys. The lungs excrete carbon dioxide and water vapour; the liver excretes bile pigments derived from the decomposition of haemoglobin; the kidneys remove nitrogenous compounds from the blood and eliminate excess water and salts.

The kidneys.

Gross structure. The two kidneys are fairly solid, oval structures, with an indentation on their innermost sides. They are, enclosed in a transparent membrane, and attached to the back of the abdominal cavity. The renal artery, branching from the aorta, brings oxygenated blood to them, and the renal vein takes deoxygenated blood away to the vena cava. A tube, the ureter, runs from each kidney to the base of the bladder in the lower abdomen.

The kidney tissue consists of many capillaries and tiny tubes, called renal tubules, held together with connective tissue. A section through a kidney shows a darker, outer region the cortex, and a lighter inner zone, the medulla. Where the ureter leaves the kidney is a space called the pelvis and into this project cones or pyramids of kidney tissue.

Detailed structure. The renal artery divides up into a great many arterioles and capillaries, mostly in the cortex. Each arteriole leads to a glomerulus, which is a capillary repeatedly divided and coiled, making a little knot of vessels. The glomerulus is almost entirely surrounded by a cup-shaped organ called a Bowman's capsule, which leads to a coiled renal tubule. This tubule, after a series of coils and loops, joins other tubules and passes through the medulla to open into the pelvis at the apex of a pyramid.

Mechanism of excretion in the kidney. The tortuous capillaries of the glomerulus offer resistance to the flow of blood, so that a high pressure is set up. This pressure causes fluid to filter out through the capillary walls and collect in the Bowman's capsule. The filtered fluid, glomerular filtrate, contains glucose, salts and nitrogenous waste dissolved in water, but fibrinogen and other proteins remain in the blood. In man, 180 litres per day of this filtrate, carrying 145 g glucose and l100 g sodium chloride pass into the Bowman's capsules. As the filtered serum passes down the renal tubule, all the glucose, some of the salts and much of the water are absorbed back into a network of capillaries surrounding the tubule. This selective reabsorption prevents the loss of useful substances from the blood serum. The remaining liquid, now called urine, contains only the waste products such as inactive hormones, urea and excess salts and water. This liquid passes down the collecting tubule where more water is reabsorbed and the concentration of the blood is regulated. If the blood is too dilute, e.g. after drinking a great deal, less water is absorbed back into the blood and the urine is dilute. If the blood is too concentrated, e.g. after sweating profusely, more water is re- absorbed from the collecting tubule, making the urine more concentrated. From the collecting tubes, the urine enters the pelvis of the kidney where it collects and continues down the ureter to the bladder as the result of waves of contraction in the ureter.

The capillaries from the glomeruli and the renal tubules unite to form the renal vein. It is the cells of the kidney tubules which selectively reabsorb substances from the glomerular filtrate. They do this often against a diffusion gradient by methods which are not fully understood but which certainly need energy supplied by respiration within the cells. In consequence, the blood leaving the kidneys in the renal vein contains less oxygen and glucose, more carbon dioxide and, as a result of excretion, less water, salts and nitrogenous waste.

The bladder.

The bladder is an extensible sac with elastic and muscular tissue in its walls. The volume of accumulating urine entering the bladder from the ureters, extends its elastic walls to a volume of 400 cm3 or more. At intervals the sphincter muscle which closes the outlet to the bladder relaxes, and the bladder contracts, aided by the muscles of the abdomen, expelling the urine through a duct called the urethra. In babies, the sphincter muscle is controlled by a reflex action triggered off by nerve endings in the stretched walls of the bladder. After about two years or less the muscle can be controlled voluntarily.

Water balance and osmo-regulation 

Water is lost from the body in urine, faeces, sweat and exhaled breath. It is gained by eating and drinking. These losses and gains will produce corresponding changes in the blood. Changes in the concentration of the blood are detected by an area in the brain, the hypothalamus. If the blood passing through the brain is too concentrated, the hypothalamus stimulates the pituitary gland beneath it to secrete into the blood a hormone called antidiuretic hormone (ADH)

When this hormone reaches the kidneys, it causes the kidney tubules to absorb more water from the glomerular filtrate back into the blood. Thus the urine becomes more concentrated and the further loss of water from the blood is reduced. 1f blood passing through the hypothalamus is too dilute, production of ADH from the pituitary is suppressed and less water is absorbed from the glomerular filtrate. The mechanism which produces the sensation of thirst is not well understood but it undoubtedly serves to regulate the intake of water and so maintain the concentration of the blood.


The kidneys play a part in the homeostasis of the body, that Is they help to regulate the composition of the internal environment.

If a mobile, single-celled animal such as Amocba or Paramecium finds itself in conditions which are unfavour able, e.g. too acid, too warm or too light, it is capable of moving until it encounters conditions more amenable to its vital activities.

The cells in a multi-cellular organism cannot move to a fresh environment but are no less dependent on a suitable temperature and pH for the chemical reactions which maintain life. It is therefore crucial to their efficient functioning that the medium round them does not alter its composition very much. A fall in temperature will slow down the chemical reactions in the cell; a drop in pH may inhibit some enzyme systems; a rise in the concentration of solutes may withdraw water from the cell by osmosis. Homeostasis is the name given to the process by which such changes of the internal medium are kept within narrow limits and many organ systems of the body contribute to this control.

The internal medium of most animals is the tissue fluid which is in contact with all living cells in the body. The constitution of the tissue fluid depends on the composition of the blood from which it is derived and, therefore, the homeostatic mechanisms of many animals act by adjusting the composition of the blood.

The skin helps to regulate blood temperature, the liver adjusts its glucose concentration, the lungs keep the carbon dioxide concentration down to a certain level and the kidneys control its composition in three principal ways: (a) they eliminate harmful compounds such as urea, (b) they remove excess water and (c) they expel salts above a certain concentration. These activities are both excretory, in that they remove the unwanted products of metabolism, and osmo-regulatory, in that they keep the osmotic potential of the blood more or less constant


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