Excretion

• Substances animals accumulate

  • Ammonia
  • Urea
  • Uric acid
  • Carbon dioxide
  • Water
  • Ions like Na+, K+, Cl–, phosphate, sulphate
• These substances have to be removed totally or partially
• Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by animals
• Ammonotelism


  The process of excreting ammonia
• Animals that are ammonotelic

  • Many bony fishes
  • Aquatic amphibians
  • Aquatic insects
• Ammonia excretion


  Readily soluble, generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions, kidneys do not play a significant role
• Ureotelic animals

  Mammals, many terrestrial amphibians and marine fishes that mainly excrete urea
• Urea production and excretion

  Ammonia produced by metabolism is converted into urea in the liver, released into the blood which is filtered and excreted out by the kidneys, some urea may be retained in the kidney matrix
• Uricotelic animals


  Reptiles, birds, land snails and insects that excrete nitrogenous wastes as uric acid in the form of pellet or paste with minimum loss of water
• Excretory structures in invertebrates

  • Protonephridia or flame cells (Platyhelminthes, rotifers, some annelids, Amphioxus)
  • Nephridia (earthworms and other annelids)
  • Malpighian tubules (insects)
  • Antennal glands or green glands (crustaceans)
• Human excretory system

  • Consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra
  • Kidneys are reddish brown, bean shaped structures situated between the levels of last thoracic and third lumbar vertebra
  • Each kidney has nearly one million complex tubular structures called nephrons which are the functional units
• Nephron


  Consists of a glomerulus and a renal tubule
• Glomerulus


  • A tuft of capillaries formed by the afferent arteriole, blood from the glomerulus is carried away by an efferent arteriole
• Renal tubule

  • Begins with a double walled cup-like structure called Bowman's capsule, continues as proximal convoluted tubule, Henle's loop, distal convoluted tubule and collecting duct
• Cortical nephrons


  • Nephrons with a short loop of Henle that extends only very little into the medulla
• Juxtamedullary nephrons


  • Nephrons with a very long loop of Henle that runs deep into the medulla
• Urine formation

  Involves glomerular filtration, reabsorption and secretion
• Glomerular filtration

  Filtration of blood carried out by the glomerulus
• Glomerular filtration rate (GFR)


  Amount of the filtrate formed by the kidneys per minute, approximately 125 ml/minute
• Juxtaglomerular apparatus (JGA)


  • A special sensitive region that can regulate GFR
• Nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules
• Reabsorption


  Process where tubular epithelial cells reabsorb substances from the filtrate either by active or passive mechanisms
• Tubular secretion


  Process where tubular cells secrete substances like H+, K+ and ammonia into the filtrate
• Proximal convoluted tubule (PCT)


  • Reabsorbs nearly all essential nutrients, 70-80% of electrolytes and water, also helps maintain pH and ionic balance
• Henle's loop


  • Plays a significant role in maintaining high osmolarity of medullary interstitial fluid
• Distal convoluted tubule (DCT)

  • Conditionally reabsorbs Na+ and water, reabsorbs HCO3- and secretes H+ and K+ ions
• Collecting duct

  • Allows reabsorption of large amounts of water to produce concentrated urine, allows passage of small amounts of urea, secretes H+ and K+ ions
• Counter current mechanism

  Involves the flow of filtrate in opposite directions in the two limbs of Henle's loop and the flow of blood in opposite directions in the two limbs of vasa recta, helps maintain an increasing osmolarity towards the inner medullary interstitium
• Human kidneys can produce urine nearly four times concentrated than the initial filtrate
• Medulla


  This gradient is mainly caused by NaCl and urea
• NaCl transport

  1. Transported by the ascending limb of Henle's loop

  2. Exchanged with the descending limb of vasa recta

  3. Returned to the interstitium by the ascending portion of vasa recta
• Urea transport

  1. Small amounts enter the thin segment of the ascending limb of Henle's loop

  2. Transported back to the interstitium by the collecting tubule
• Counter current mechanism

  The transport of substances facilitated by the special arrangement of Henle's loop and vasa recta
• The counter current mechanism helps to maintain a concentration gradient in the medullary interstitium
• Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine)
• Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed
• Osmoreceptors


  Activated by changes in blood volume, body fluid volume and ionic concentration
• Antidiuretic hormone (ADH)


  Facilitates water reabsorption from latter parts of the tubule, thereby preventing diurosis
• Increase in blood pressure

  Can increase the glomerular blood flow and thereby the GFR
• Renin-Angiotensin mechanism

  1. JGA activates JG cells to release renin

  2. Renin converts angiotensinogen to angiotensin I and II

  3. Angiotensin II increases glomerular blood pressure and GFR

  4. Angiotensin II activates adrenal cortex to release aldosterone

  5. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule