Excretory Products and Their Elimination

All the living organisms eliminate their body waste and other toxic or excess waste in various ways that reside on the earth. Removal of nitrogenous waste products formed during metabolism from the body of a human is called excretion. Normally excretion means the release of nitrogenous excretory substances like urea Ammonia uric acid etc. Ammonia is excreted by some organisms called ammonotelic such as aquatic amphibians. Urea is excreted by some organisms called ureotelic such as terrestrial amphibians. While uric acid is excreted by some organisms called uricotelic such as reptiles. From all the excretory waste, urea is the least toxic and ammonia is the most toxic.

Excretory Products In Different Organisms

Animals 

Excretory organs

Unicellular animals

Contractile vacuole

Arthropods

By Malpighian tubules

Vertebrate

Kidneys 

Mollusca

Urinary organ

Crustaceans

Antennal gland


Human Excretory System

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Kidney

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  • The main excretory organ in human and other mammals is a pair of kidneys which is bean-shaped and reddish-brown in color.

  • It is an excretory and homeostatic organ which is mesodermal in origin.

  • It is located between the last thoracic and lumbar vertebra. Its weight is 140 grams.

  • There are two parts of the kidney. Cortex and medulla are the outer and inner parts respectively. 

  • Each kidney is made of approximately 1,30,00000 kidney ducts which are called nephrons.

  • The functional and structural unit of the kidney is considered as Nephron. There is a notch on the inner side called hilum in each kidney through which ureters, blood vessels, and nerves enter.

  • There is a cup-like structure in every nephron called Bowman's capsule.

  • The glomerulus is made up of thin blood vessels found in the Bowman's capsule which is made up of two types of arterioles.

  • Afferent arteriole: It takes blood to the glomerulus.

  • Efferent arteriole: By which the blood is taken out of the glomerulus.

  • The process of filtration of liquid into the cavity of the bowman's capsule is called ultrafiltration.

  • The main function of the kidney is the purification of blood plasma i.e to extract the unwanted nitrogenous waste substances through urination.

  • The supply of blood to the kidney takes place in large quantities in comparison to other organs.

  • In the Kidney average, 125 ml per minute blood is filtered i.e 180 liters per day.

  • Out of it, 1.45 urine is formed daily and the remaining is absorbed back by the cells of the nephron and is mixed into the blood.

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Ureter

  • A whitish and thin tube emerges from the hilum of each kidney which is known as the ureter. These tubes are about 28 cm long.

  • Both the ureters open separately in the bladder but they are placed closely carrying urine to the urinary bladder.


Urinary Bladder

  • The urinary bladder is a sac-like structure that is made up of detrusor muscles.

  • This pear-shaped organ stores urine until urine is expelled out from the body.


Urethra

  • The neck of the urinary bladder leads to a tube-like structure which is the urethra.

  • It also helps in urination. It is usually shorter in females as compared to males.


Ammonia

  • Excess amino acids are deaminated by most of the animals as these are the first metabolic waste of protein metabolism.

  • These are degraded into their keto groups which are utilized in catabolism for generating ATP and ammonia groups which are excreted in other forms.


Urea

  • In the body, urea can stay for a short time as it is less soluble in water and less toxic.


Uric Acid

  • They are almost insoluble in water and their crystals are non-toxic. They can remain in the body for a considerable time.


Urine

  • Urine is an aqueous solution in which 95% is water. Other constituents are urea, uric acid, chloride, sodium, potassium, organic and inorganic potassium.

  •  The color of urine is light yellow due to the presence of urochromes in it. Urochromes are formed by the dissociation of hemoglobin.

  • Urine is acidic with a ph value of 6.


Other Excretory Organs in The Human Body

Skin: The oil gland and sweat glands found in the skin respectively secretes sebum and sweat.

Liver: Liver cells play the main role in excretion by converting more and more amino acids and ammonia of blood into urea.

Lungs: The lungs excrete two types of gaseous substance carbon dioxide and water vapor. The excretion of some substances like garlic, onion, and some species in which the vapor component is present is excreted by the lungs.

FAQ (Frequently Asked Questions)

1. What are the excretory system functions? What are the functions of tubules?

Excretory System Functions

  • Body fluids are filtered and water is collected.

  • Concentrate waste products and also eliminate them from the body fluids. 

  • Other substances are returned to body fluids as it is necessary for homeostasis. 

  • Removal of excretory or waste products from the body.


Functions of Tubules

  • A large amount of water is reabsorbed by collecting duct to make the urine concentrated.

  • To maintain the electrolyte balance of the body fluid potassium and hydrogen ion is regulated by distal convoluted tubules. 

  • Descending the Limb of Henle's loop absorbs water to make the filtrate concentrated.

2. Explain the mechanism of urine concentration?

Mechanism of urine concentration (Countercurrent mechanism of urine concentration).

  • Both humans and animals form hypertonic urine. 

  • A countercurrent multiplier system is used for making urine a hypertonic. 

  • This procedure occurs in the Henle's loop and vasa recta. It involves mainly two ions i.e Sodium (Na+) and Chlorine (Cl-). 

  • Urine is isotonic in the proximal convoluted tubule. 

  • Henle's loop has a descending limb which is permeable to water. The tissue fluid surrounding it is hypertonic. 

  • Therefore, both the ions move in and the water moves out from the descending limb by passive transport. 

  • Finally, the filtrate becomes hypertonic in the descending limb.

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