Life Process-4 Excretion
Removal of all harmful, unwanted products (especially nitrogenous wastes) from the body of an organism is called excretion.
Different organisms use various strategies to perform the process. For example- Many unicellular organisms remove wastes by simple diffusion from the body’s surface into the surrounding water.
The process of excretion is very important as these waste substances include CO2, water, urea, uric acid, and ammonia can be harmful if not removed from the body.
- The excretory system is primarily associated with the removal of nitrogenous wastes in organisms.
- Urea is the main waste product formed by the breakdown of surplus amino acids and nucleic acids in the liver. Blood transports urea to the kidneys for filtration and removal in the form of urine.
- Removal of the excess water or its retention in case of shortage of water. This is to maintain the required quantity of water (osmoregulation) in the body. Maintaining the solute concentration of the body fluids is called osmoregulation.
Classification of Animals on the basis of the excretion of nitrogenous wastes
Depending upon the nitrogenous wastes excreted, animals can be classified as follows:
- Ammonotelic- In the Ammnotelic organism, the waste product is ammonia.
- Ureotelic – These animals’ waste product is Urea. For example- mammals (humans)
- Uricotelic – In Uricotelic animals, the waste product is Uric acid. For example- Birds, reptiles, and insects
Excretion in humans:
The human excretory system includes organs that facilitate the excretion of nitrogenous wastes from the body.
Excretion is performed by various organs in humans including a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
Human consists of two kidneys, bean-shaped organs located on either side of the vertebral column in the lower abdominal cavity.
Structure of kidney:
Each kidney of an adult human weighs around 120 to 170g. and length 10 to 12 cm. These have an inner concave structure associated with the hilum. The blood vessels, ureters, and nerves enter the kidneys through the hilum.
The kidney consists of these structures:
- Henle’s loop
Main functions of kidneys:
The kidneys eliminate the waste from the bloodstream by the production of Urine. They perform several homeostatic functions such as:
- kidneys maintain the volume of extracellular fluid
- they maintain ionic balance in extracellular fluid
- Kidneys also maintain the pH and osmotic concentration of the extracellular fluid,
- These excrete toxic metabolic by-products such as urea, ammonia, and Uric acids.
Processes of excretion in humans:
The process of excretion in humans is in the following steps:
- Urine formation
- Glomerular filtration
- Tubular Reabsorption
The kidney contains a large number of minute tubular structures called nephrons. Nephrons are the structural and functional units of the kidney associated with blood vessels and capillaries. The nephrons act as filters inside the kidney.
Urine formation takes place by nephrons and has three steps: ultra-filtration, re-absorption, and tubular secretion.
- They form urine and drain it. The kidney has a cluster of very thin-walled blood capillaries with the cup-shaped end of a coiled tube called Bowman’s capsule.
- that is located partly in the renal cortex and partly in the renal medulla
- Some substances in the initial filtrate, such as glucose, amino acids, salts, and a major amount of water, are selectively re-absorbed as the urine flows along the tube (bowman’s capsule)
- As kidneys form the urine, they also maintain the normal composition of blood, fluid, and salt balance throughout the body tissues
- Urine formed in the kidney is brought to the urinary bladder by two hollow muscular tubes called ureters.
- The urethra is the small tube that leads urine to excrete outside of the body. From the urinary bladder, urine is passed outside via the urethra during urination voiding of the urinary bladder is called micturition
- The bladder can hold 400-500 cm³ of urine. When about 200 cm3 or more urine gets collected in the urinary bladder, stretch receptors are stimulated leading to the desire to discharge urine.
Haemodialysis and kidney transplantation
The blood urea level rises abnormally (uremia) in patients suffering from kidney failures. This leads to the accumulation of poisonous wastes in the body, which can even lead to death. In such patients, an artificial kidney is used for removing excess urea from the blood by a process called hemodialysis.
An artificial kidney is a device to remove nitrogenous waste products from the blood through the process of dialysis.
- Artificial kidneys consist of various tubes with a semi-permeable lining deep in the tank filled with dialyzing fluid which has some solutes like those in blood plasma but no nitrogenous molecules like urea, and uric acid.
- Blood coming out of the artificial kidney is warmed to the body temperature and returned to the vein of the patient.
- This is similar to the function of the kidney, but it is different since there is no reabsorption of particles.
Organ donation – it is an act of donating an organ to a person who suffers from the non-function of the organ(s).
- The donated kidney may come from a living person or a donor who has recently died.
- The genetic makeup of the donor should be as close to the patient as possible, that is, if it is donated by a close relation, it reduces the chances of rejection.
- Drugs are, however, used to prevent the rejection of the transplanted kidney by the body.
- The main organs which may be donated as – the corneas, kidneys, heart, liver, pancreas, lungs, intestines, and bone marrow.
Role of the liver in excretion:
- It excretes bile pigments, cholesterol, drugs, and some vitamins.
- It excretes all the above-mentioned substances in bile, which flows into the small intestine, and from there these get removed with the feces.
- The formation of urea and uric acid (from ammonia) also takes place in the liver. These are removed from the body by the kidneys
Excretion in Plants
In the manner of excretion, plants are totally different from animals. Here oxygen may be called waste products. They deal with O2 and self-produced CO2.
- They excrete an excessive amount of water by transpiration.
- In terms of other wastes, such as dead tissues and also they lose leave.
- Many plant waste products are stored in cellular vacuoles.
- Waste products may be stored in leaves that fall off.
- Other waste products are stored as resins and gums, especially in old xylem. Plants also excrete some waste substances into the soil around them.
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