Excretory Products and their Elimination Notes NEET, Download PDF

NEET Short Notes

By Poojashree AgarwalUpdated : Sep 9, 2020 , 22:40 IST

Excretory Products and their Elimination Notes! This article contains short notes on Excretory Products and Their Elimination for NEET examination 2020. This chapter generally holds minimum 8+ marks in every NEET/AIIMS/JIPMER examination. It is also an important chapter from the unit Human Physiology for the students who are sitting in the Board examination next year. The article includes the important points for revision purpose. Let's begin with the brief introduction of Excretory Products and Their Elimination.

Excretory Products and their Elimination Notes

The cells of the living body perform vital metabolic processes to generate energy from the food consumed. These metabolic processes generate cellular wastes which are removed in the process called excretion. The excretion of urea, uric acid, creatinine occurs during the process. These are nitrogenous compounds.

Classification of Animals on the Basis of Excretory Products

Ammonotelic Animals	These animals excrete ammonia and ammonium ions. Mainly aquatic animals are ammonotelic because ammonia is toxic and is readily miscible in water.	Examples: Crustaceans, Echinoderms, Fishes etc.
Ureotelic Animals	These animals excrete urea in the form of urine. Terrestrial animals are ureotelic because urea is less toxic than ammonia and does not need a large quantity of water.	Examples: Mammals.
Uricotelic Animals	These animals excrete uric acid in the form of pellets. It needs very less quantity of water.	Examples: Birds and Reptiles.
Aminotelic Animals	These animals excrete amino-acids.	Examples: Limnaea, and Asterias
Guanotelic Animals	These animals excrete guanine.	Example: Spiders.

Human Excretory System

The excretory system in humans is made up of the following organs:

A pair of kidneys: Located on each side of the abdomen, kidneys are the major excretory organs. The Kidney is covered with renal capsule and shows an outer region of cortex and the inner region of medullary pyramids internally. Nephrons are the structural and the functional units of kidneys. Each nephron comprises of Malpighian Corpuscle (glomerulus and Bowman’s capsule), a series of tubules called proximal convoluted tubules (PCT), Loop of Henle, Distal Convoluted Tubule (DCT) and the Collecting duct. The blood is filtered and urine is made in nephrons.
A pair of ureters: Transfer of urine from kidneys to the urinary bladders occurs via muscular tubes called ureters.
Urinary Bladder: It is the thin-walled muscular organ that stores the urine. It is lined by transitional epithelium that allows the stretching of the bladder.
Urethra: It leads to the process of micturition (removal of urine from the body). Urethral sphincter is present between the bladder and urethra. It is a voluntary sphincter that controls the micturition.

Urine Formation

Urine formation is the function of kidneys. It is transferred to the urinary bladder via the ureters. The urinary bladder stores the urine and the sensation for micturition are generated by the stretch receptors on the wall of the urinary bladder, which is made up of transitional epithelium.

Urine formation occurs through the following three steps in nephrons:

Glomerular Filtration	The blood in the glomerulus is forced to pass through the glomerular capillaries to the Bowman’s Capsule. All plasma component except for cells and proteins are filtered across the membranes of glomerulus and Bowman’s Capsule. This is called ultrafiltration. In the Bowman’s capsule, the plasma is now called the filtrate. Around 1000-1200 mL of blood is filtered here. The epithelium of the glomerulus, the epithelium of the Bowman’s capsule (podocytes) and the basement membrane between these two epithelia help in ultra-filtration.
Tubular reabsorption	The tubules of the nephrons perform the reabsorption of essential solutes from the filtrate. Glucose, amino acids, Na⁺, H₂O, bicarbonate ions and other nutrients are reabsorbed in the PCT, Loop of Henle, DCT and Collecting duct. Around 99% of the components of the filtrate are reabsorbed.
Tubular secretion	The tubules of the nephrons actively secrete the substances from the peritubular capillaries into the lumen. The substances secreted are either in excess or are toxic. K⁺, ammonium ions, H⁺, urea, creatinine etc. are secreted.

Functions of the Tubules

The tubules of the nephrons and their functions in urine formation are as follows:

Proximal Convoluted Tubule (PCT)	It is main reabsorption site. Lined with simple cuboidal epithelium. Reabsorbs glucose, amino acids, ions etc.
Loop of Henle	It is concerned with the counter-current balance of ions.
Distal Convoluted Tubule (DCT)	Sodium and water absorption occur here.
Collecting Duct	It performs the absorption of water. It also secretes urea in the medullary region.

Mechanism of Concentration of the Filtrate

It occurs by the process of counter-current mechanism that operates between the Loop of Henle (acting as counter-current multiplier) and the Vasa recta (acting as counter-current exchanger), as follows:

The filtrate in the two limbs of the Loop of Henle and vasa recta flow in opposite direction, hence, counter-current. The filtrate is maintained at a nearly constant gradient.

The exchange of water and electrolytes is maintained due to this opposite flow. NaCl and urea play an important role in concentration.

The ascending loop of Henle provides NaCl to descending vasa recta and urea enters into the loop Henle.

Depending on the concentration of the blood, the NaCl returns to the descending loop of Henle from the ascending vasa recta and urea can go back to the vasa recta.

Regulation of Function of Kidneys

It occurs by the following:

Anti-Diuretic Hormone (ADH)	It is released by the hypothalamus when the blood pressure is low. It triggers water absorption.
Juxta-Glomerular Apparatus	It releases a hormone called renin, that activates the aldosterone and allows reabsorption of sodium ions in DCT.
Heart	Heart secretes ANF (Atrial Natriuretic Factor) when the blood pressure is high. ANF prevents water absorption, leading to dilution of the urine.

Disorders of the Excretory System

Uremia: Build-up of urea in the blood is called uremia.

Renal Calculi: Formation of the crystal of calcium oxalate in kidneys.

Glomerulonephritis: It is the inflammation of the glomerulus due to drugs, infections etc.

Urinary Tract Infection (UTI): Bacterial infection in the urethra and urinary bladder is called UTI. It makes micturition painful and if left untreated, it can cause kidney failure.

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