Exretory System Essay, Research Paper
Aim: To examine and understand the structure, function and differences between different types of excretory systems.
The difference between excretion and egestion is that whereas excretion is the process of removing toxic waste by-products of the body s metabolism, egestion is the removal of indigestible remains/substances through the alimentary canal. Through egestion, substances that were not able to be used in the body are removed, and through excretion, waste products of used substances which are potentially harmful are removed.
The principal waste products generated by metabolic activities in most animals are water, carbon dioxide, and nitrogenous wastes. Carbon dioxide is mainly excreted by gills, lungs, or other respiratory surfaces. Excretory organs such as kidneys excrete most of the water and nitrogenous wastes. Nitrogenous wastes include ammonia, uric acid, and urea. In a process called deamination, the amino group is removed from amino acids and is converted to ammonia. Ammonia is highly toxic and it may harm the organism if permitted to accumulate inside the body. Since it is very soluble, in many organisms it s quickly converted to some less toxic nitrogenous waste product such as uric acid or urea. Other waste products of the body include carbon dioxide, which is the product of cellular respiration in which oxygen is used up. Carbon dioxide is carried by the plasma, in the form of hydrogen carbonates, or a smaller portion in the haemoglobin. It is carried to the lungs, where it is excreted from the body.
All these compounds, especially ammonia, are harmful if stored in high concentrations in the body, and therefore must be removed. Even water, which is another by-product, is excreted through the kidneys, the lungs, and the skin, where it serves to cool us down, or to lower the concentration of water molecules in the blood. This is why it is necessary for us to have some form of an excretory system.
Here is a table of the excretory organs found in mammals and the excretory substances which they remove:
Organ Substance removed
Kidney Ammonia, removed as urine in humans
Skin Salts/urea, heat, excess water
Lungs Carbon Dioxide, water vapour
In humans, kidneys are situated one on each side of the spine, and are embedded in fatty tissue. They are bean-shaped organs, with a convex outer border and a concave inner border. The inner border presents an indentation, the hilum, at which the blood vessels enter and leave. In front is the renal vein carrying blood from the kidney; behind it lies the renal artery carrying blood to the kidney. At the very bottom is the ureter, a tube that conveys urine to the bladder. The hilum arises from a deeper indentation, the sinus of the kidney, in which the ureter dilates to form a small sac, the renal pelvis. The kidney also embodies glomeruli, which can be described best as loops of capillaries enclosed within Bowman’s capsules, located at the blind ends of the renal tubules.
Urine is produced in the glomeruli and renal tubules and carried to the renal pelvis by collecting tubules. The glomeruli act as filters, through which water, salts, and waste products from the blood pass into the spaces of Bowman’s capsules and from there down into the renal tubules. Most of the water and salt is reabsorbed from these tubules; the remainder is excreted as urine. The renal tubules also secrete other salts and waste products from the blood into the urine.
The kidneys are also important in maintaining a balance of fluid and salt and a normal degree of acidity. When disorders upset these delicate balances, the kidneys act to restore them by excreting more or less water, salt, and hydrogen ions. The kidneys help maintain normal blood pressure by secreting the hormone renin and elaborate a hormone that stimulates the production of red blood cells.
A table showing the similarities and differences in structures and products of birds and mammals excretory systems:
Birds Mammals Differences
Substance Removed Organ Substance Removed Organ
Excess salt Modified tear gland Excess Salt Kidney, Skin In birds, excess salt is removed through a modified tear gland. In sea birds, it is excreted in a salt rich fluid from the nostrils or mouth.
Ammonia Kidney Ammonia Kidney Birds produce almost solid urine, uric acid
Water Modified tear gland, also kidney and lungs Water Kidney, Skin, Lungs Excess water in birds is removed to a greater extent through their modified tear gland, which is also used to excrete excess salt
The Nephridium and Malpighian tubules are the excretory organs found in earthworms and insects respectively.
In insects, digestion takes place primarily in the midgut, and the products are absorbed in the midgut and the hindgut. The food waste passes to the hindgut, or intestine, for elimination. Connected to the forepart of the hindgut are a large number of small tubes, called the Malpighian tubules that float in the blood of the body cavity. Waste matter in the blood passes through the walls of these tubes which open into the alimentary tract at the caudal end of the stomach and are blind at the other end.
The nephridium is a structure found in earthworms, and most other segmented worms. They usually have a pair of excretory tubes in almost every segment of the body. Fluid from the body cavity enters the funnel shaped opening in one segment, then passes back into the next segment where the tube is surrounded by many blood vessels. Then the tube opens to the outside.