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Excretion - Lesson Summary

Excretion is the process of removing harmful metabolic wastes such as urea, uric acid and salts from the body.

Excretion in organisms

  • Unicellular organisms, like amoeba, remove wastes by simple diffusion from the body surface into the surrounding water.
  • Lower multi-cellular organisms like flat worms use flame cells while earthworms use nephridia for excretion.
  • Higher multi-cellular organisms like fish, frogs, lizards, birds and humans use kidneys for excretion. Homeostasis is the tendency of the physiological system of higher animals to maintain internal stability of the body.

Excretory system in humans
The excretory system in human beings includes a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. This is also known as the urinary system.
1) Kidney 
Kidney is an excretory organ present in the human body. Kidneys act as excretory organs and also control the balance of water and mineral ions in the body. Kidney is divided into two parts namely, external region and internal region.

  • External region is made up of a thick layer followed by a layer of fat. Fat acts as a shock absorber.
  • Inner region comprises of renal cortex and renal medulla. The medulla is composed of conical masses of tissue that lead to pelvis. The cortex has a random arrangement of tiny tubules called nephrons which are the functional units of the kidney.
Nephrons are the microscopic basic filtration units of the kidney. Nephrons carry out an important process of urine formation in the kidney. Ultra-filtration is the first step in the urine formation in which nephrons filter minerals, waste and water but retain red cells, proteins and large molecules. A single nephron is made up of renal corpuscle and a renal tubule.

a) Renal corpuscle is made up of tangled clusters of tiny blood capillaries called glomerulus and Bowman’s capsule. 
  • Glomerulus is called the filtration unit. 
  • Bowman's capsule surrounds the glomerulus.
b) Renal tubule is made up of proximal convoluted tubule, Henle’s loop and distal convoluted tubule. 
  • Renal tubule leads away from Bowman's capsule and becomes highly coiled to form the proximal convoluted tubule.  
  • The tubule makes a hairpin loop, called the loop of Henle. It joins the distal convoluted tubule.
  • The distal convoluted tubule opens into a collecting duct, which passes into the renal medulla.

2) Ureter
Each ureter is a tubular structure which arises on from each kidney and join to form a single tube which opens From each kidney, a ureter arises which opens into the urinary bladder.

3) Urinary bladder
It is a muscular sac situated at the lower part of the abdomen which receives urine from the kidney through ureters. It stores this urine until it becomes full. Urinary bladder opens out through urethra. Urethra passes urine to the outside of the body. The urethra emerges through the penis in males and close to the vagina in females.

Excretory pathway in kidney – Blood is carried by the renal artery to the nephron from there to renal pyramid leading to pelvis to the ureter, then to the urinary bladder and finally to the urethra which passes it to outside.

Other organs of excretion
a) Skin

  • Sweat glands in the skin allow evaporation of excess water along with some salts which have to be excreted out of the body. Sweating is a vital function of your body. Sweating helps in thermoregulation of the body. Sweat glands secrete fluids that will cool your body in cases of extreme heat. 
  • Sebaceous glands in the skin excrete excess oil outside in the form of sebum.

b) Lungs 
Lungs are the respiratory organs in human beings. Pulmonary artery carries impure blood to the lungs from the heart.The alveolar cells of the lungs help in exchange of oxygen with carbon dioxide  to provide the former to the bodily tissues. Lungs excretel carbon dioxide out of the body.

c) Anus
Anus is the opening of the alimentary canal to the exterior. Undigested waste is stored in the rectum for defecation. Anus helps in the excretion of faeces by the process of egestion.

Excretory pathway in kidney – Blood is carried by the renal artery to the nephron from there to renal pyramid leading to pelvis, to the ureter, then to the urinary bladder and finally to the urethra which passes it to outside.

Steps in urine formation
The kidney performs three functions as steps involved in urine formation - ultra-filtration, selective re-absorption and tubular secretion.

  • Ultra-filtration is the process in which nephrons filter minerals, waste and water but retain red cells, proteins and large molecules.  
  • Selective re-absorption involves reabsorption of some substances of the initial filtrate, such as glucose, amino acids, salts, and a major amount of water. Urine is allowed to flow along the tube.
  • Tubular secretion involves the secretion of substances not required by the body into the filtrate by the cells of the distal convoluted tubule before it leaves the kidney.  

Excretory substances
Excretory sunstances can be ammonia, urea and uric acid.

a) Ammonia is generated as a result of oxidative deamination reactions occurring in the body. Ammonia is a toxic nitrogeneous waste and need to be eliminated. It is very toxic to the body and requires more amount of water to excrete it. Aquatic organisms excrete ammonia as they have water around them. Terrestrial animals either convert ammonia into urea or uric acid. Ammonia reaches the liver, the key site which converts this ammonia into urea. 

b) Urea is the excretory waste of human beings. As a result of amino acid metabolism, many nitrogeneous wastes like ammonia.   As ammonia is toxic to human body when retained, it is coverted into urea by the liver.   Urea is also called as carbamide. Urea , the main excretory product formed in the liver is carried away by the blood to kidneys.

c) Uric acid is a bicyclic, heterocyclic compound made by the combination of carbon, hydrogen, oxygen and nitrogen. It is formed in the body during protein metabolism. Animals that excrete their waste in the form of nucleic acid form shelled eggs which are permeable to soluble gases. Most of the terrestrial organisms which do not have much access to water excrete uric acid as their excretory product. e.g.Reptiles, Birds. In human beings, it is formed as a result of metabolism of food mainly containing purines. Uric acid formed dissolves in the blood and taken to kidneys for excretion. It is very concentrated and requires very less amount of water to be excreted along with it. Excess of uric acid in the blood causes the disease called as gout.

Dialysis is the process which involves separation of nitrogeneous wastes from the blood artificially. Dialysis is performed using a device which removes nitrogenous wastes from blood in case of kidney failure.

  • An artificial kidney contains a number of tubes with a semi-permeable lining suspended in a tank filled with dialysing fluid.
  • The patient’s blood is passed through these tubes.
  • During this passage, waste products from the blood diffuse into the dialysing fluid.
  • The purified blood is pumped back into the patient.

Excretion is also observed in plants. Plants get rid of excess water by the process of transpiration. Resins, gums and dead leaves are some excretory products of plants.


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