Here we are providing the short notes on Biology which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.


Excretory Systems in Various Animals

Components of this system in vertebrates include the kidneys, liver, lungs, and skin.

Water and Salt Balance

The excretory system is responsible for regulating water balance in various body fluids.

Osmoregulation refers to the state aquatic animals are in: they are surrounded by freshwater

and must constantly deal with the influx of water.


Excretory System Functions

  1. Collect water and filter body fluids.
  2. Remove and concentrate waste products from body fluids and return other substances to body fluids as necessary for homeostasis.
  3. Eliminate excretory products from the body.


The Human Excretory System

The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney’s functional unit.

The nephron has three functions:

  1. Glomerular filtration of water and solutes from the blood.
  2. Tubular reabsorption of water and conserved molecules back into the blood.
  3. Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.


Kidney Stones

In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful irritant that may require surgery or ultrasound treatments.


Kidney Functions

  1. Maintain volume of extracellular fluid
  2. Maintain ionic balance in extracellular fluid
  3. Maintain pH and osmotic concentration of the extracellular fluid.
  4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.


Kidneys, The Fascinating Filters

Nephron is the filteration unit of kidney.

  • Excessive eating (polyphagia), excessive drinking (polydipsia) and too much of urine (polyusia) are three cardinal symptoms of diabetes. The ‘hypothesis’ produces a chemical substance called ‘antidivretic hormone (ADH).
  • The Adrenal gland maintains the regulating salt in the body and is located in an organ lying just over the kidney. As soon as the salt (sodium) concentration become just a little less than normal, it release into the blood stream a substance called ‘aldosterone’.
  • Renal transplantation or dialysis (artificial kidney) are the supportive measure when the damage to kidney reaches a certain point.


Hormone Control of Water and Salt

Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback.

ADH is released from the pituitary gland in the brain. Dropping levels of fluid in the blood signal the hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to increase water absorption in the kidneys.

Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood. This causes water to flow into the blood by osmosis. Renin is released into the blood to control aldosterone.



  • The raw materials of photosynthesis, water and carbon dioxide, enter the
  • cells of the leaf, and the products of photosynthesis, sugar and oxygen leave the leaf.
  • Water enters the root and is transported up to the leaves through specialized plant cells known as xylem.
  • Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening flanked by two guard cells.
  • Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.


Chlorophyll and Accessory Pigments

  • Chlorophyll, the green pigment common to all photosynthetic cells absorbs all wavelengths of visible light except green, which it reflects to bedetected by our eyes.
  • Black pigments absorb all of the wavelengths that strike them.



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