Life Processes | Excretion
EXCRETION IN PLANTS
Plants get rid of excess water by transpiration. Some waste products are removed along with falling leaves and other plant parts. Other waste products are stored as resins and gums in old Xylem tissue. Plants also excrete some waste substances into the soil around them.
EXCRETION IN ANIMALS
It is the biological process of elimination of harmful metabolic waste products from the body of an organism. The mode of excretion is different in different organisms. Many unicellular organisms remove these wastes by simple diffusion from the body surface into the surrounding water, while complex multicellular organisms use specialized organs for excretion. The organs that are involved in this process constitute the excretory system.
(i) Human Excretory System :
Human excretory system includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethera. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.
(a) Kidney
Excretory system in human beings
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Each kidney has large numbers of filtering units called nephrons which are packed close to each other.
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Kidneys are vital organs of our body. They filter the poisonous waste from blood. Any damage, infections, injury and restricted blood flow leads to accumulation of poisonous wastes in the body which can even lead to death.
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Kidneys remove the poisonous substances such as urea, other waste salts and excess water from the blood and excrete them in the form of yellowish liquid called urine.
It performs following functions :
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It regulates the osmotic pressure/water balance of the blood.
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It regulates pH of the blood.
Structure of Nephron
Structure of a nephron
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Each kidney is made up of a large number of excretory filtration units called nephrons or ureniferous tubules.
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These are considered as functional unit of kidney.
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It consists of a long coiled tubule whose one end is connected to the double walled cup shaped structure of Bowman’s capsule and its other end to a urine-collecting duct of a kidney.
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The Bowman’s capsule contains a bundle of blood capillaries which is called glomerulus.
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The function of glomerulus is to filter the blood passing through it.
The function of tubular part of nephron is to allow the selective reabsoption of useful substances into the blood capillaries.
Blood having Metabolic Waste ® Afferent Arteriole ® Glomerulus ® Bowman’s Capsule ® PCT (Proximal Convoluted Tubule) ® Loop of Henle ® DCT (Distal Convoluted Tubule) ® Collecting Duct ® Ureter ® Urinary bladder ® Urethra ® Urine excreted out
(b) Formation of urine : The purpose of excretion is to filter out waste products from the blood.
(i) Filtration :
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The nitrogenous waste such as urea and uric acid are removed from blood in the kidneys, thus kidneys are the basic filtration unit.
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Each capillary cluster in the kidney is associated with the cup-shaped end of a tube that collects the filtered urine.
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Each kidney has large numbers of these filtration units called nephrons.
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Afferent arteriole brings the blood into bowman’s capsule and efferent arteriole takes the pure blood. Filtrate having mainly urea / uric acid moves through the tubule.
(ii) Reabsorption :
Some substances in the initial filtrate such as glucose, aminoacids, salts and a major amount of water are selectively reabsorbed as the urine flows along the tube. This depends on how much excess water is there in the body and on how much of dissolved waste is there to be excreted. The remaining filterate goes to the collecting duct. Smaller ducts combine together to form a larger collecting duct.
(iii) Secretion :
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Secretion of water and salts from blood capillaries into the tubular parts of Nephron.
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The urine formed in each kidney enters a long tube the ureter which connects the kidney with the urinary bladder.
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Urine is stored in the urinary bladder until the pressure of the expanded bladder leads to pass out through the urethra.
(c) Artificial kidney : It is a device to remove nitrogenous waste products from the blood through dialysis. In case of kidney failure an artificial kidney can be used.
Dialysis :
It is the procedure used in artificial kidney to do the work of a non-functional or damaged kidney. In the process blood of the patient is allowed to pass through the long cellulose tubes dipped in a tank containing dialysing solution having same ionic concentration as plasma. The waste substance diffuse out of blood into the tank and the cleansed blood is returned back into the patient through a vein. This procedure is also known as haemodialysis.
Artificial kidney (Hemodialysis)