excretion

• Major forms of nitrogenous wastes excreted by animals:

  • Ammonia
  • Urea
  • Uric acid
• Ammonia is the most toxic form and requires a large amount of water for its elimination
• Uric acid is the least toxic form and can be removed with a minimum loss of water
• Ammonotelism is the process of excreting ammonia
• Ammonia is excreted by diffusion because it is readily soluble
• Animals that excrete ammonia (ammonotelic):

  • Many bony fishes
  • Aquatic amphibians
  • Aquatic insects
• Animals that mainly excrete urea (ureotelic):

  • Mammals
  • Many terrestrial amphibians
  • Marine fishes
• Urea is produced in the liver from ammonia and is excreted by the kidneys
• Animals that excrete uric acid (uricotelic):

  • Reptiles
  • Birds
  • Land snails
  • Insects
• Uric acid is excreted in the form of pellets or paste with a minimum loss of water
• Protonephridia or flame cells are the excretory structures in Platyhelminthes
• Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation
• Kidneys are reddish brown, bean-shaped structures situated between the levels of the last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity
• Excretory system consists of:

  • A pair of kidneys
  • One pair of ureters
  • A urinary bladder
  • A urethra
• The outer layer of the kidney is a tough capsule
• Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels, and nerves enter
• Inner to the hilum is a broad funnel-shaped space called the renal pelvis with projections called calyces
• The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces
• Inside the kidney, there are two zones:

  • An outer cortex
  • An inner medulla
• The cortex extends in between medullary pyramids as renal columns called Columns of Bertini
• Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units
• Each nephron has two parts:

  • The glomerulus
  • The renal tubule
• Glomerulus is a tuft of capillaries formed by the afferent arteriole – a fine branch of the renal artery
• The renal tubule begins with a double-walled cup-like structure called Bowman’s capsule, which encloses the glomerulus
• The tubule continues further to form:

  • Proximal convoluted tubule
  • Henle’s loop (with descending and ascending limbs)
  • Distal convoluted tubule (DCT)
• The DCTs of many nephrons open into a straight tube called the collecting duct, many of which converge and open into the renal pelvis through medullary pyramids in the calyces
• In majority of nephrons, the loop of Henle is too short and extends only very little into the medulla, these are called cortical nephrons
• In some nephrons, the loop of Henle is very long and runs deep into the medulla, these are called juxta medullary nephrons
• The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries
• A minute vessel of this network runs parallel to the Henle’s loop forming a ‘U’ shaped vasa recta
• Kidneys filter blood to remove waste products such as urea and excess water from the body.
• Urine formation involves glomerular filtration, reabsorption, and secretion
• The first step in urine formation is the filtration of blood by the glomerulus, known as glomerular filtration
• Glomerular capillary blood pressure causes filtration of blood through 3 layers:

  • Endothelium of glomerular blood vessels
  • Epithelium of Bowman’s capsule
  • Basement membrane between these two layers
• Filtration is so fine that almost all constituents of plasma except proteins pass onto the lumen of Bowman’s capsule, considered ultrafiltration
• Glomerular filtration rate (GFR) is the amount of filtrate formed by the kidneys per minute, approximately 125 ml/minute in a healthy individual
• Nearly 99% of the filtrate has to be reabsorbed by renal tubules, called reabsorption
• Tubular epithelial cells in different nephron segments perform reabsorption by active or passive mechanisms
• Tubular cells secrete substances like H+, K+, and ammonia into the filtrate during urine formation
• Tubular secretion is important for maintaining ionic and acid-base balance of body fluids