The UPSC mains GS-3 paper syllabus has various subjects like Economics, Science and Technology. Science and technology in UPSC is mostly concerned with current Affairs. but it is necessary to understand the basics of science. Human Body consists of some of the most complex systems. Human Excretary system is one of the essential parts of the body, which helps in throwing waste out of the body.


  • What is Excretary System
  • Parts of Human excretary system
  • Main points on human excretary system.


The human excretory system functions to remove waste from the human body. This system consists of specialized structures and capillary networks that assist in the excretory process. The human excretory system includes the kidneys and their functional unit, the nephron. The excretory activity of the kidneys is modulated by specialized hormones that regulate the amount of absorption within the nephron.


  1. It consists of a pair of Kidneys, a pair of ureters, Urinary bladder and Urethra.
  2. The Kidneys are located in the abdominal cavity, situated below the level of last thoracic and third lumbar vertebra close to the dorsal inner wall of abdominal cavity.
  3. Each kidney is been shaped reddish brown. The right kidney is lower and smaller than left kidney because the liver takes up much space on the right side. From each kidney, one ureter arises, and the two Ureters open obliquely into the Urinary bladder, which is a hollow, muscular sac-like structure that stores urine.
  4. Urethra is the membranous tube that arises from the neck of the bladder and conduct the urine to the exterior.


The human kidneys are the major organs of bodily excretion. They are bean-shaped organs located on either side of the backbone at about the level of the stomach and liver. Blood enters the kidneys through renal arteries and leaves through renal veins. Tubes called ureters carry waste products from the kidneys to the urinary bladder for storage or for release.

The product of the kidneys is urine, a watery solution of waste products, salts, organic compounds, and two important nitrogen compounds: uric acid and urea. Uric acid results from nucleic acid decomposition, and urea results from amino acid breakdown in the liver. Both of these nitrogen products can be poisonous to the body and must be removed in the urine.

Urine formation:

Urine formation involves three major steps:

  1. Glomerular filtration
  2. Tubular reabsorption/ selective reabsorption
  3. Tubular secretion

 Glomerular filtration:

  1. The first step in urine formation is the filtration of blood which is carried out by the glomerulus and is called glomerular filtration. On an average 1100 to 1200 ml of blood is filtered by the kidney per unit that is one fifth of the blood pumped  by the ventricles of heart per minute.
  2. The glomerular capillary blood pressure causes filtration of blood through three layer that is endothelium of glomerular blood vessels, epithelium of bowman's capsule and the basement membrane between these two layers.
  3.  The epithelial cells of bowman's capsule are called podocytes and arranged in such a manner so as to leave some minute space called filtration slits.
  4. Blood is filtered so finally through these membranes  that almost all the constituent of Plasma except proteins pass on the lumen of bowman's capsule. Therefore, this process is considered as of ultrafiltration.

Tubular reabsorption:

A comparison of the volume of filtrate formed by per day that is 180 litre with that of urine released that is 1.5 litre, suggest that nearly 99% of the filtrate has to be reabsorbed by the renal tubule. This process is called selective reabsorption.
Water and urea reabsorbed by passive transport, glucose and amino acids are reabsorbed by active transport. The reabsorption of sodium Ions (Na+ occur both by active and passive transport.

Tubular secretion

Certain chemicals in the blood that are not removed by filtration from the glomerular capillary are removed by the process of tubular secretion. It helps in the maintenance of ionic and acid base balance of the body fluid by removing ions like H+, K+, NH4+ etc. Potassium ions, hydrogen ions, and ammonia are secreted out to maintain the equilibrium between the body fluids.

The functions of the various tubules involved in the process are:

  1. Glomerulus- filters the blood
  2. Proximal Convoluted Tubules (PCT)- reabsorb water, ions and nutrients. They remove toxins and help in maintaining the ionic balance and pH of the body fluids by secretion of potassium, hydrogen and ammonia to filtrate and reabsorbing bicarbonate ions from the filtrate.
  3. Descending Loop of Henle- is permeable to water and the filtrate gets concentrated as it is impermeable to electrolytes.
  4. Ascending Loop of Henle- it is impermeable to water and permeable to electrolytes. The filtrate gets diluted due to the movement of electrolytes from the filtrate to the medullary fluid.
  5. Distal Convoluted Tubule (DCT)- allows reabsorption of water and sodium ions. It also helps in maintaining pH and ionic balance by secretion and reabsorption of ions like PCT.
  6. Collecting Duct- a large amount of water is reabsorbed from the filtrate by the collecting duct.
  7. Micturition
  8. The urinary bladder is stretched and gets filled with urine formed in the nephrons. The receptors present on the walls of the urinary bladder send signals to the Central Nervous System, thereby, allowing the relaxation of sphincter muscles to release urine. This is known as micturition.


  1. The functional and structural unit of the kidney is the nephron. The nephron produces urine and is the primary unit of homeostasis in the body. It is essentially a long tubule with a series of associated blood vessels. The upper end of the tubule is an enlarged cuplike structure called the Bowman’s capsule. Below the Bowman’s capsule, the tubule coils to form the proximal tubule, and then it follows a hairpin turn called the loop of Henle. After the loop of Henle, the tubule coils once more as the distal tubule. It then enters a collecting duct, which also receives urine from other distal tubules.
  2. Within the Bowman’s capsule is a coiled ball of capillaries known as a glomerulus. Blood from the renal artery enters the glomerulus. The force of the blood pressure induces plasma to pass through the walls of the glomerulus, pass through the walls of the Bowman’s capsule, and flow into the proximal tubule. Red blood cells and large proteins remain in the blood.
  3. After plasma enters the proximal tubule, it passes through the coils, where usable materials and water are reclaimed. Salts, glucose, amino acids, and other useful compounds flow back through tubular cells into the blood by active transport. Osmosis and the activity of hormones assist the movement. The blood fluid then flows through the loop of Henle into the distal tubule. Once more, salts, water, and other useful materials flow back into the bloodstream. Homeostasis is achieved by this process: A selected amount of hydrogen, ammonium, sodium, chloride, and other ions maintain the delicate salt balance in the body.
  4. The fluid moving from the distal tubules into the collecting duct contains urine. The urine flows through the ureters toward the urinary bladder. When the bladder is full, the urine flows through the urethra to the exterior.


  It is the act of voiding or releasing urine from the urinary bladder. This is accomplished by simultaneous contraction of smooth muscles of urinary bladder wall and relaxation of the skeletal muscle of sphincter of the bladder into the urethra.


  • Human excretory system includes organs that facilitate the removal of nitrogenous wastes from the body.
  • The main excretory organs include kidney, ureter, urinary bladder and urethra.
  • Kidneys filter the blood and urine is the filtrate obtained.
  • Urine passes to the urinary bladder via ureter and is expelled out of the body. This is known as micturition.
  • Kidneys also regulate the osmotic pressure of a mammal’s blood through excessive purification and filtration. This is known as osmoregulation.



The digestive system is made up of the gastrointestinal tract—and the liver, pancreas, and gallbladder.



The respiratory system is the network of organs and tissues that help you breathe. It includes your airways, lungs, and blood vessels


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