Ravela Da Cruz

CBSE Class 11 Biology Chapter 19 Revision Notes

Excretion is the process of removing metabolic waste products from the animal body to control the composition of bodily fluids and tissues.  The waste products obtained from this process are called excretory products

Chapter 19: Excretory Products and their Elimination Revision Notes

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Multiple Choice Questions

  1. Which excretory product requires maximum water for its elimination? ________
  2. The extension of cortex in medulla is known as ___________.
  3. Glomerulus along with Bowman’s capsule is called ____________.
  4. Brush border epithelium is a characteristic of which of the following? ________
  5. Where does filtration of blood occur? __________
  6. Which of the following regulates the functioning of the kidney? ___________
  7. Neural mechanism of micturition is called ____________.
  8. Kidneys play a significant role in its removal of ammonia directly. __________
  9. Which of the following can stimulate the glomerular blood flow and thereby help in bringing the GFR back to normal? _____________
  10. Which of the following substances are secreted by tubular cells during urine formation? ______________
  11. Which of the following parts has minimum reabsorption? ______________
  12. Which of these will be completely reabsorbed from glomerular filtrate under normal conditions in the nephrons? ________________
  13. Which of the following is to be observed if Henle’s loop were to be absent from mammalian nephrons? ______________
  14. Assertion: Ammonia is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Reason: Ammonia is readily soluble.
  15. Assertion: During micturition, urine is prevented from flowing back into the ureters. Reason: Urethral sphincters contract during micturition.

Human Excretory System

  • Excretion is the act of elimination of wastes and other metabolites from the body of an animal, which is generally related to the process of maintaining osmotic levels or osmoregulation
  • Both excretion and osmoregulation are crucial for homeostasis or maintaining a steady internal environment in the body, which is required for regular living functions. 
  • The main nitrogenous wastes emitted by mammals are ammonia, urea, and uric acid.
  • These compounds accumulate in the animal's body through metabolic activity or other mechanisms such as excessive consumption
  • The most hazardous substance is ammonia, whereas the least toxic substance is uric acid.
  • Ammonotelism is the process of eliminating ammonia, and species that excrete ammonia are termed ammonotelic
  • Ureotelic organisms produce urea as a nitrogenous waste product (mammals, terrestrial amphibians).
  • Uric acid is excreted by uricotelic organisms (reptiles, birds).




  • The kidneys are a pair of reddish-brown bean-shaped structure between the last thoracic and lumbar vertebrae.
  • The ureter, blood arteries, and nerves all enter through a notch on the inner side of each kidney called the hilum
  • The renal pelvis, a large funnel-shaped area inside the hilum, bears projections termed calyces
  • The kidney is divided into two zones: the outer cortex and the inner medulla.
  • The medullary pyramids extending into the calyx divide the medulla. 
  • Between the medullary pyramids, the cortex forms the renal column known as the Columns of Bertini
  • The nephron is the functional unit of the kidney. A million nephrons are found in each kidney. 
  • The glomerulus and renal tubules are two elements of each nephron.
  • The tuft of capillaries created by the afferent arteriole is the glomerulus.
  • The efferent arteriole transports blood out from the glomerulus. 
  • Bowman's capsule is the beginning of the renal tubules, split into Proximal Convoluted tubules, Henle's loop, and Distal Convoluted tubules
  • The nephron's malpighian tubules, PCT, and DCT, are located in the cortical area, whereas Henle's loops are located in the medulla.


Kidney Source: Kidney Anatomy

Nephrons are classified into two categories based on their location in the kidney. 

Cortical nephron

  • The loop of Henle is too short in most nephrons, extending just a few millimeters into the medulla and lying in the renal cortex.


Juxtamedullary Nephrons

  • Juxtamedullary Nephrons are a kind of nephron that is found between the medulla
  • The Henle loop is particularly lengthy in certain nephrons and extends far into the medulla.


The cortical nephron accounts for around 80% of the overall nephron count, with the juxtamedullary nephron accounting for the remaining 20%

Urine formation

The three primary phases in the production of urine are glomerular filtration, reabsorption, and secretion.

Glomerular filtration

  • Glomerular filtration takes place in the glomerulus, which filters blood.
  • The epithelium of Bowman's capsule, the endothelium of glomerular blood vessels, and a membrane in between these two layers are all involved in this process.
  • The blood is filtered such that all of the plasma's elements, excluding proteins, enter the Bowman's capsule.
  • As a result, this procedure is called ultra filtration.


  • The renal tubules reabsorb around 99% of the filtrate collected.
  • This is referred to as reabsorption. This is accomplished by active and passive transportation.


  • Tubular secretion is the next phase in the urine production process.
  • Tubular cells release hydrogen ions, potassium ions, and other ions into the filtrate.
  • The ionic, acid-base and other bodily fluid balances are maintained by this mechanism.
  • Urine is formed when the released ions interact with the filtrate.
  • The urine exits the nephron tubule and flows into a collecting duct.

Function of the tubules

PCT (Proximal Convoluted Tubule)

  • This section reabsorbs nearly all important nutrients and 70-80% of electrolytes and water.
  • PCT also aids in the maintenance of bodily fluid pH and ionic equilibrium by selective secretion.

Loop of Henle

  • In the ascending limb of Henle's Loop, resorption is at a minimum.
  • On the other hand, this area is important for maintaining the high osmolarity of medullary interstitial fluid.
  • The loop of Henle's descending limb is permeable to water but nearly impervious to electrolytes.

Distal Convoluted Tubule (DCT)

  • Conditional reabsorption of Na+ and water occurs in the DCT.

Urine formation

Sources: Urine formation


  • Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its release through the urethra, i.e., micturition.
  • The signal for this process begins by the stretching of the urinary bladder as it gets filled with urine.
  • The neural mechanism causing micturition is known as Micturition reflex

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