THE innumerable chemical processes occurring in any cell result in the production of substances which, if they remained in solution in the cell, would poison it. These waste products or metabolites include carbon dioxide and solid waste substances, notably nitrogen compounds. Their removal from the organism is termed excretion. In so far as respiratory surfaces get rid of carbon dioxide, they are, strictly speaking, excretory organs, but since that is not their sole function, we take the term excretory organ to mean one which is concerned with the removal of other waste substances which have been produced by cell activity.
Since animals are so much more active than plants, they expend much more energy, especially during locomotion. They therefore produce much more waste material as a result of tissue respiration.
A further difference arises from the fact that plants make proteins as they require them, whereas the diet of an animal may contain far more protein than it needs. Although all the cells of an animal can store a small quantity of protein, there are no special protein-storing tissues comparable, for example in a vertebrate, with the liver and muscles and fat depots, e.g. under the skin and around muscles. Any excess of amino-acids produced by digestion of protein in solution is therefore split up, e.g. in a vertebrate by the liver, into a nitrogenous part which is converted into a harmless form, e.g. urea, suitable for excretion, whilst the rest of each molecule is converted into useful carbohydrate or lipide to be used for respiration or storage. This process is termed deamination. For these two reasons, animals produce much more nitrogenous waste than plants. This is reflected in their structure, for animals possess structures or organs which are concerned solely with excretion, whereas no comparable structures are to be found in plants.
The insoluble and therefore unabsorbable parts of food in an animal have never been inside any of its cells and cannot rightly be called excreta. Their removal, therefore, is termed egestion and NOT excretion.
EXCRETION BY VERTEBRATES
The liver and kidneys are organs common to all vertebrates, and although in, shape, internal complexity and blood supply they vary, their functions are similar. Solid wastes are brought in solution by the blood stream to the liver, entering it by an hepatic artery, or by the portal vein from the alimentary canal. In the latter blood vessel are carried many nitrogen compounds absorbed from the canal and which are taken to the liver to be made into harmless excretory products. Hence the value of a direct connection—the portal vein—between the canal and the liver. Leaving the liver by hepatic veins, the blood containing urea and other solid wastes in solution enters the general circulation once more, ultimately reaching the kidneys by a renal artery.
Urea and other unwanted solids are filtered out of the blood by the kidneys, forming the fluid termed urine, which drains away from each kidney by a duct, called the kidney duct or ureter, to be stored temporarily in a urinary bladder l and voided at intervals. The liquid contains about 2 per cent, of solids, of which about one-half is urea, one-sixth common salt and the remainder other compounds in far smaller quantities. The urinary system is intimately connected with the reproductive system in vertebrates, but in our account of the former we shall neglect the latter entirely.
The two kidneys in a mammal such as a rabbit are of a characteristic shape and lie attached to the dorsal wall of the body cavity, the right one more forward than the left. They are deep red in colour and are often embedded in fat, which helps to hold them in place. Each when examined in section is seen to consist of three areas : the cortex ; the medulla ; and the pelvis, whence the ureter leads the urine to the bladder. Where this ductthe kidney, at its inner border, are to be found the renal artery and vein connecting with the dorsal aorta and posterior vena cava respectively. Urine passes from the bladder along a duct, the urinogenital canal or urethra, which lies dorsal to the pubis of the pelvic girdle. In the male the urethra is continued into a thick-walled tube, the penis, and in the female it opens to the surface at the slit-like vulva.
The kidneys consist of a mass of kidney tubules. Each tubule commences in the cortex as a thin-walled bladder called a Malpighian capsule, one side of which is indented by a knot of capillaries called a glomerulus derived from a branch of the renal artery. The remainder of the tubule twists around the capsule and then passes into the medulla, where it forms a U-shaped loop, and then returns again to the cortex before finally opening into a collecting tube. The blood leaving the glomerulus passes through a network of capillaries overlying the tubule before returning to a branch of the renal vein. From the blood in the glomerulus there filters off into the capsule a water ’ exudate containing waste matter and other solutes. During the passage of this fluid through the remainder of the tubule, it becomes concentrated and forms the urine, partly by the absorption of water back into the blood and partly by the addition of more waste matter from the blood. At the same time, useful substances, e.g. glucose, filtered off in the glomerulus are reabsorbed.
It must be realized that in all vertebrates the kidneys act as osmo-regulators as well as excretory organs. They operate so as to keep the composition of the plasma of the blood constant. Excess of water is excreted when necessary and also any soluble substance, even if useful, e.g. sodium chloride or sugar, should this be present in more than the normal amount. Since the composition of the plasma affects its osmotic pressure, the kidneys serve to maintain a constant osmotic pressure in the plasma. That is why they are called osmo-regulators. It is necessary that the osmotic pressure of the plasma should be kept fairly constant to prevent unbalanced osmotic pressure between the blood and the cells through the medium of the lymph. An increase in the osmotic pressure of the blood would lead to the withdrawal of water from the cells ; a decrease to excessive absorption of water by the cells from the blood. The composition of the plasma is liable to fluctuation by reason of water and soluble substances absorbed from the alimentary canal or by loss of the same through sweating, and it is the task of the kidneys to correct this fluctuation as far as possible.
The skin also acts as an excretory organ, for it possesses sweat glands which pour forth a watery solution containing principally salt. This occurs particularly in hot weather or during violent activity when the animal is hot, for evaporation of the water cools the surface of the animal. Each sweat gland is a long but coiled tube from which a duct runs to the surface, where it opens at a pore which is just visible with a strong magnifier — especially across the forehead and on the nose. If not kept clean these pores tend to get blocked up.
The kidneys in a frog are relatively large, flat, oval, plum-coloured bodies lying attached to the dorsal wall of the body cavity. A kidney duct runs backwards from the outer border of each to join the cloaca, where it runs dorsal to the pubic region of the pelvic girdle. The ventral surface of the cloaca has opening into it a thin-walled bilobed bladder wherein the urine collects when the cloacal aperture is closed. A large renal border of each kidney, whilst the dorsal aorta gives off several minute renal arteries to the inner border. Thus most of the blood passing through capillaries in the kidney has come from the legs. Bloodby four or five large renal veins which join to form the posterior vena cava.
The urinary system and blood vessels connected with it in a cartilaginous fish such as the dogfish are very similar in arrangement to those in a frog, except that the caudal vein splits into two to form the renal portal veins. In bony fishes such as the cod the urinary system is distinct from the genital system—otherwise it is like that of cartilaginous fishes.
Solid waste products from various organs readily pass into the blood in insects, since the organs lie bathed in blood. The excreta are removed in three ways :—
The Malpighian tubules are fine and ramify throughout the cavity of the abdomen. They absorb wastes and convert them into uric acid which passes along the tubules to leave the animal by way of the hind gut. A mass of uric acid is often found in the hind gut of insect larva? when they pupate, to be left behind when the imago emerges from the pupa case.
The mass of white material known as the fatty body and which occupies much space includes insoluble nitrogen compounds.
The exoskeleton is made of chitin, which is an insoluble nitrogen compound. Pupation is a period of intense activity and possibly the shedding of the pupa case enables the organism to get rid of useless excreta produced as a result of this activity. Moulting by the larva may also serve a similar purpose. The scales on the wings of butterflies and moths are excretory products.
The intestine of the earthworm has a covering of cells which contain yellow granules and which give this part of the gut its yellow colour. Some of these ’yellow cells ’fall off, float about in the coelomic fluid and ultimately break up, liberating the granules which are excretory products from the blood in the vessels of the gut. The removal of these granules is carried out by nephridia, of which there is a pair in each segment. Each nephridium is a long tube coiled in three sets of loops, and opening from the coelom at one end by a funnel-shaped nephrostome.
This is ciliated, gathering up the excreta which are conveyed along the nephridium by the cilia of the narrow portion. The hinder end of the tube is muscular and opens to the exterior. A complex network of blood capillaries covers the nephridium, which is able to extract therefrom wastes directly from the blood. Yellow cells of a different type make their way through the wall of the gut and their excretory granules leave the earthworm with the fasces.
There are no special cells or parts of cells which are concerned with excretion. We must therefore conclude that the external and internal surface of this animal is large enough to get rid of all its waste products.
Amoeba and Paramecium
These organisms possess contractile vacuoles which are supposed to collect waste matter in solution, to be excreted when the vacuoles burst through the surface. Contractile vacuoles of Paramecium pulsate alternately, the appearance of each one being preceded by formative vacuoles. Since the protoplasmic contents of the cell form a more concentrated solution than the surroundings, water will pass in by osmosis. Any excess of this may possibly be collected in the contractile vacuole to be got rid of, the vacuole in this case acting as an ’osmo-regulator. ’ This is supported by the fact that many marine and parasitic Protista lack contractile vacuoles. It would seem that the surface must also act as an excretory area.