EXCRETORY SYSTEMS AND MECHANISMS, EXCRETION IN MAMMALS, SIMILARITIES

Group of organs concerned with the removal of waste metabolic products are called The Excretory System. These helps to remove waste products from our body and helping free the body of toxins that could be detrimental to the body when accumulated in excess.
Excretion is defined as the removal of waste products of metabolism from the body. Examples of excretory organs are kidney, stomata, flame cells, etc.
Waste products if not removed from the body may be toxic and harmful thereby causing illness.

Listed below are examples of organisms and their excretory systems;
a. Some unicellular organisms; their excretory systems is the Contractile vacuole.
b. Flat worms; their excretory system is the Flame Cells.
c. Annelids (e.g. earthworm); their excretory system is the Nephridia.
d. Insects; their excretory system is the Malpighian tubule.
e. Vertebrates: their excretory systems are Kidneys, Lungs, Skin and Liver.
f. Plants; their excretory systems are the Stomata and Lenticels.

Excretory Organs and Substances they excrete in Vertebrates

In animals
Lungs; Carbon (IV) Oxide and water vapour
Skin; Sweat, Salts, and nitrogenous wastes.
Kidney; Urea, excess water, excess salts, ammonium compounds and acids.
Liver; Bile Pigments.

In Plants
Stomata; Oxygen, Carbon (IV) oxide and water vapour.
Bark; Tannins, Resins and Latex.
Leaves; Crystals of calcium oxalate, calcium carbonate, and anthocyanin pigment which give red, blue, and violet colours to many flowers.
Lenticels; Oxygen, carbon (IV) Oxide, and water vapour.

EXCRETORY MECHANISMS

EARTHWORMS
The excretory structures are called Nephridia. Each nephridium consists of a ciliated filament called the Nephrostome. The fluid which contains useful and useless materials enters the nephrostome and passes down the tubule. The fluid is pushed along by cilia and muscular contractions of the tubule. As the fluid moves along, useful materials are reclaimed by the cells lining the tubule while the waste materials leave the worm through the nephridiopores.

INSECTS
The excretory organ in insects is the Malphigian tubule. The malphigian tubule lies in the Hemocoel and are bathed in blood. The tubules extracts substancesfrom the bloodand passs them into the lumen of the tubule. There, water and useful materials are reabsorbed. As water concentration within the tubule decreases, uric acid precipitates. Since the malphighian tubule leads into the intestine, more water is reabsorbed there. The uric acid is then passed out with faeces.

MAMMALS
The excretory organ in mammals is the Kidney. The mammalian kidneys are two reddish brown, bean-shaped organs located in the dorsal part of the abdominal cavity. Each kidney is supplied with oxygenated blood by the renal artery while the renal vein carries away deoxygenated blood from the kidney. The ureters which are narrow tubes arise from the pelvis into the bladder. The bladder opens to the outside through a tubular urethra. In males, the urethra opens into the penis while in the females, it opens into the vulva.

The pelvis lead to the ureter. The kidney is made up of excretory units, the kidney tubules or the Nephrons. Each nephron consists of malpighian body and it is attached to the uriniferous tubules. The malpighian body consists of a cup-shaped Bowman’s Capsule and a knot of capillaries called the Glomerulus. The glomerulus originates from the renal artery.

EXCRETION BY THE MAMMALIAN KIDNEY
The size of different vessel entering the Bowman’s capsule is greater than the efferent vessel leaving it and so the incoming blood containing water, sugar, amino acids, urea and mineral salts passes through the glomerulus by Ultrafiltration. Water, glucose, nitrogenous wastes, mineral salts filtrates pass into the capsule. Useful products like glucose, some water and mineral salts are reabsorbed into the blood capillaries leaving a fluid called Urine.
The urine containing excess water, nitrogenous wastes e.g. urea and excess mineral salts leave the kidney to the bladder from where it is passed out.

NOTE
Urea has the formula (NH2)2CO2 and hence contains nitrogen, hydrogen, carbon and oxygen.
Compositions of sweat are water, some urea and traces of some salts.

OTHER FUNCTIONS OF THE KIDNEY

1. MAINTENANCE OF ACID-BASE BALANCE (HOMEOSTASIS): The kidney maintains the acid-base balance of the body by excreting acid radicals, when the body fluid is getting acidic. On becoming more basic, more of the basic radicals are excreted, in this way, the pH of the body fluid is maintained.

2. OSMOREGULATION: Osmoregulation helps keep the osmotic environment of each cell constant. When the amount of sugar, mineral salts and urea which determine osmotic concentration vary above the normal range, the kidney excretes the excess to maintain the normal range. When there is excess water in the body, more water is removed by the blood plasma.

3. Elimination of toxins, drugs, and other harmful substances.

SIMILARITIES BETWEEN THE EXCRETORY ORGAN OF EARTHWORM AND THAT OF MAMMALS
1. Each is well supplied with blood.
2. Each is made up of a complex network of coiled tubes or tubules.
3. Both excrete urea or nitrogenous and water.
4. Both have structures for filtering excretory products.

DIFFERENCES BETWEEN THE EXCRETORY ORGAN OF EARTHWORM AND THAT OF MAMMALS.
1. In earthworm, the excretory organ is made up of ciliated tubule while in mammals there is no ciliated tubule.
2. Excretory organ of earthworm is segmentally arranged while that of mammals the excretory organ is not segmentally arranged.
3. In earthworm, it has one pair of nephridia in each segment while in mammals, their nephrons are not segmentally arranged.
4. Each unit of the earthworm’s excretory organ discharges directly outside the body while in mammals, the units are united and discharged through a single duct.
5. In earthworm, some excretory products are stored in special tissues while in mammals, there is no special tissue for storage of waste products.

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