Endocrine system

• Substances animals accumulate

  • Ammonia
  • Urea
  • Uric acid
  • Carbon dioxide
  • Water
  • Sodium ions
  • Potassium ions
  • Chloride ions
  • 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
• Ureotelic


  Mammals, many terrestrial amphibians and marine fishes that mainly excrete urea
• Uricotelic


  Reptiles, birds, land snails and insects that excrete nitrogenous wastes as uric acid
• Ammonia is the most toxic form and requires large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water
• Excretory structures in invertebrates

  • Protonephridia or flame cells
  • Nephridia
  • Malpighian tubules
  • Antennal glands or green glands
• Protonephridia


  • Primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation
• Nephridia


  • Help to remove nitrogenous wastes and maintain a fluid and ionic balance
• Malpighian tubules


  • Help in the removal of nitrogenous wastes and osmoregulation
• Antennal glands or green glands

  • Perform the excretory function in crustaceans like prawns
• Components of the human excretory system

  • Kidneys
  • Ureters
  • Urinary bladder
  • Urethra
• Kidneys

  • Reddish brown, bean shaped structures
  • Situated between the levels of last thoracic and third lumbar vertebra
  • Close to the dorsal inner wall of the abdominal cavity
  • Each kidney measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g
• Hilum

  Notch on the inner concave surface of the kidney through which ureter, blood vessels and nerves enter
• Renal pelvis

  Broad funnel shaped space inside the kidney with projections called calyces
• Cortex


  Outer layer of the kidney
• Medulla


  Inner layer of the kidney, divided into medullary pyramids
• Columns of Bertini

  Renal columns that extend from the cortex in between the medullary pyramids
• Nephron


  Functional unit of the kidney, consisting of a glomerulus and a renal tubule
• Glomerulus

  • Tuft of capillaries formed by the afferent arteriole, a fine branch of renal artery
• Bowman's capsule


  Double walled cup-like structure that encloses the glomerulus
• Malpighian body or renal corpuscle


  Glomerulus along with Bowman's capsule
• Proximal convoluted tubule (PCT)

  Highly coiled network that continues from Bowman's capsule
• Henle's loop


  Hairpin shaped part of the renal tubule with a descending and an ascending limb
• Distal convoluted tubule (DCT)


  Another highly coiled tubular region that continues from Henle's loop
• Collecting duct

  Straight tube that many DCTs open into, which converge and open into the renal pelvis
• Types of nephrons

  • Cortical nephrons
  • Juxta medullary nephrons
• Peritubular capillaries

  Fine capillary network around the renal tubule formed by the efferent arteriole
• Vasa recta

  Minute vessel of the peritubular capillary network that runs parallel to the Henle's loop forming a 'U' shape
• Urine formation

  1. Glomerular filtration

  2. Reabsorption

  3. Secretion
• Glomerular filtration

  Filtration of blood carried out by the glomerulus
• On average, 1100-1200 ml of blood is filtered by the kidneys per minute, which is roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute
• Glomerular filtration rate (GFR)


  Amount of the filtrate formed by the kidneys per minute, approximately 125 ml/minute or 180 litres per day
• Juxta glomerular apparatus (JGA)


  Special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole, regulates 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)

  • Lined by simple cuboidal brush border epithelium
  • Reabsorbs nearly all essential nutrients, 70-80% of electrolytes and water
  • Helps maintain pH and ionic balance by selective secretion and absorption