EXCRETIO

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Excretion is the process by which the waste products produced during metabolic activities is removed from the body. In unicellular organism, the waste products

are removed from the cells into the surroundings by diffusion.

In multicellular organism, the waste products are removed through specialized organs.

Excretion takes place in all organisms.

Introduction (Excretion)

Compared to animals, plants do not have a well-developed excretory system to throw out nitrogenous waste materials. This is because of the differences in their physiology. Therefore, plants use different for excretion. The gaseous waste material produced during respiration (CO2) and photosynthesis (O2) diffuse out through stomata in the leaves and through lenticels in other part of the plants. Excess water evaporates mostly from stomata and also from the outer surface of the stem, fruits, etc., throughout the day. This process of getting rid of excess water is called transpiration.

Excretion in Plants

Waste products may be stored in leaves that fall off. Other waste materials that are exuded by some – resins, saps, latexes, etc. are forced from the interior of the plant and by absorptive forces of plant cells. Plants also excrete some waste substances into the soil around them.

Plants do not have mechanism to collect, transport and throw out their waste products. They have adopted varied strategies to protect their living cells from waste products. Old leaves : Waste products are stored in older leaves which soon fall off. Old Xylem : Resins, gums, tannins and other waste products are deposited in the old xylem which soon becomes non-functional , e.g., heart wood.

Mechanism of Excretion in Plants

Bark : Bark consists of dead cells which is peeled off periodically. Tannins and other wastes are deposited in the bark. Incidentally, tannins are raw material for dyes and inks. Central Vacuole : Most plant waste product are stored in central vacuole of their cells. They are unable to influence the working of cytoplasm due to presence of a selectively permeable membrane called tonoplast. Root Excretion : Some wastes substances are actually excreted by the plant in the region of their root. Detoxification : The toxic oxalic acid is detoxified by formation calcium oxalate which gets crystallized into needles, prisms, stars and crystal sand. Excess of calcium is also precipitated as calcium carbonate crystals. Salt Glands : They excrete excess salts obtained from the habitat. Hydathodes also have an excretory function.

Diffusion of oxygen in a leaf

Many unicellular organism like Amoeba throw out their wastes by diffusion from their body surface. Protozoan’s have no organs for excretion. As they live in an aquatic habitat, their wastes are eliminated by diffusion through the plasma membrane. Simple multicellular organisms like Hydra throw out their solid waste matter through mouth. High multicellular organism have well defined specialized excretory organs. Those organs could be simple tubular structures as in flatworms and leech.

Excretion in Animals

The excretory organs of insects (e.g., grasshopper, cockroach and housefly) are also tubular. They remove nitrogenous wastes from the body fluid and help in maintaining the water balance in the body. In vertebrates, the main organs of excretion and maintenance of water balance are the kidneys.

The excretory system of human beings includes pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are located in the abdomen, one on either side of the backbone, Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.

Excretion in Human Beings

Structure of Human Excretory System

Our Excretory system consists of kidneys, blood vessels that join them, ureters, urinary bladder and urethra. They help produce and excrete urine. There are two bean-shaped kidneys that lie in the abdominal cavity, one on either side of the vertebral column. The kidneys are reddish brown, Each of them is about 10 cm long and weighs about 150 g. Although they weigh less, they receive a lot f blood for filtration.

Human Excretory System

A volume of blood nearly equivalent to that in the whole body passes through the kidneys every four or five minutes. The kidneys produce urine to filter out the waste products, like urea and the uric acid, from the blood. Urine leaves each kidney through a tube called ureters. The ureters from both the kidneys are corrected to the urinary bladder that collects and stores urine. Ureters carry urine from the kidneys into the urinary bladder. The urethra is a canal that carries urine from the bladder and expels it outside the body.

Internal Structure of Kidney

Each kidney is enclosed in a thin fibrous covering called the capsule. A renal artery brings blood into the kidney, along with nitrogenous waste materials, After filtration in the kidney, the purified blood leaves the kidney through a renal vein.Two distinct regions can be seen in the section of a kidney : (1) An outer, dark, granular cortex and (2) an inner, lighter medulla. The hollow space from where the ureter leaves the kidney is called the pelvis. Each kidney is made up of numerous (about one million) coiled excretory tubules, known as nephrons, and collecting ducts associated with tiny blood vessels. A nephron is the structural unit of a kidney, having three functions – filtration, absorption and secretion.

Kidney

A cluster of thin-walled blood capillaries remains associated with the cup-shaped end of each nephron tubule. These capillaries bring blood from the body to the nephron for filtration. The network of capillaries spreads over the nephron tubules also. These capillaries finally carry purified blood to the body.

Structure of Nephron

A nephron consists of a long coiled tubule and the Malpighian corpuscle. The tubule of the nephron is differentiated into the proximal convoluted tubule. Henle’s loop and the distal tubule opens into the collecting duct. At the proximal end of the nephron is the Malpighian corpuscle, which consists of Bowman’s capsule is a double-walled cuplike structure which surrounds the dense network of blood capillaries called the glomerulus.

Nephron

Glomerular filtration - When the blood enters into the glomerular through the afferent arteriole, a part of the water and some dissolved constituent of the blood of low molecular weight like nitrogenous waste, glucose and mineral salts filter out through the capillary walls into the surrounding Bowman’s capsule by a process referred to as glomerular filtration. The filtered fluid or glomerular filtrate resembles the blood plasma in its chemical composition except for the absence of large molecules. In man, about 180 liters of fluid is filtered from the blood plasma through glomerular capillary walls every 24 hours.

Mechanism of Excretion in Human

Beings

Tabular Reabsorption - The glomerular filtrate flows on through the convoluted tube, collecting tubule and then into the pelvis of the kidney and down the ureter into the urinary bladder. As the filtrate flows the proximal convoluted tubule some water and physiologically important salts like glucose, amino acids, sodium chloride and sodium bicarbonate are reabsorbed into the blood through the capillaries around this portion leaving only the wastes to be excreted out.Active secretion – As the glomerular filtrate flows through the distal convoluted tubes, the unwanted substances, which could not be filtered out in the glomerular are actively secreted out by the tubular walls into the filtrate from the blood. As the result of this entire process, homeostasis of the blood is maintained and all the waste products remained in the tubular fluid constitute the urine, which is ready for excretion from the body.

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