11.3 The Kidney

• • Water is produced via condensation reactions (anabolism) and is consumed during hydrolysis reactions (catabolism)
  • The concentration of water within cells = osmolarity
  • Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment, less energy (mammals)
  • Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions (jellyfish)
• There are two key functions that an excretory system performs:

  • Removes nitrogenous wastes that may be toxic to the body in large concentrations
  • Most aquatic animals eliminate their nitrogenous wastes as ammonia (highly toxic but soluble) but mammals eliminate their nitrogenous wastes as urea (less toxic for storage) and reptiles and birds eliminate wastes as uric acid (non toxic and less water needed)
  • Removes excess water to maintain a suitable osmolarity within the tissues and cells
• • In insects, the excretory system (Malpighian tubules) connects to the digestive system of the animal
  • Insects have a circulating fluid system called hemolymph (think blood system in mammals)
  • Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph
  • The tubules pass these materials into the gut to combine with the digested food products
  • Solutes, water and salts are reabsorbed into the hemolymph at the hindgut, whereas nitrogenous wastes (uric acid) and undigested food materials are excreted

  
• • Blood enters the kidneys via the renal artery and exits the kidneys via the renal vein
  • Blood is filtered by specialised structures called nephrons which produce urine
  • The urine is transported from the kidneys via the ureter, where it is stored by the bladder prior to excretion
  • Blood in the renal vein has less urea, less water and solutes/ions, less glucose and oxygen (used by kidney to generate energy and fuel metabolic reactions) and more carbon dioxide (by-product of kidney reactions)

  
• Nephrons:

  • Bowman’s capsule (enter afferent arteriole, leave efferent arteriole) – first part of the nephron where blood is initially filtered in glomerulus and vasa recta reabsorbs
  • Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs
  • Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient
  • Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs
  • Collecting duct - shared by nephrons, feed to renal pelvis

  
• Ultrafiltration:

  • blood enters kidney via afferent arterioles, into capillary tuft called glomerulus ( pores restricts blood, branches for surface area)
  • inside of Bowman’s capsule has podocytes with extensions called pedicels (have gaps for free-moving fluid but restrict blood) that wrap around the blood capillaries of the glomerulus
  • Between podocytes and glomerulus is a glycoprotein matrix called the basement membrane that filters the blood at high pressure(restricts blood and big proteins)
  • wide afferent arteriole and narrow efferent arteriole = high hydrostatic pressure in glomerulus

  
• Selective Reabsorption:

  • reuptake of useful substances from the filtrate in the convoluted tubules (mainly proximal but also distal)
  • proximal convoluted tubule has a microvilli cell lining to increase surface area for absorption from filtrate (thin tubular surface with no gaps)
  • large number of mitochondria for active transport
  • Substances are actively transported across the apical membrane then passively diffuse across the basolateral membrane to blood
  • Reabsorb ions and vitamins (actively by pumps), glucose and amino acids (co-transported across apical membrane with sodium), water (osmosis)

  
• Osmoregulation:

  • loop of Henle creates a high solute (hypertonic) concentration in medulla
  • The descending limb of the loop of Henle is permeable to water but not salts, ascending limb is permeable to salts but not water (more salty filtrate)
  • vasa recta flows in the opposite direction (counter-current)so salts released from the ascending limb are drawn into medulla, establishing a salt gradient
  • animals in desert have longer loops of Henle for more water reabsorption

  
• ADH - anti-diuretic hormone/vasopressin:

  • The amount of water released from the collecting ducts to be retained by the body is controlled by ADH
  • released from posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
  • increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
  • less water in the filtrate, urine is concentrated, individual urinates less
  • if hydrated then ADH levels decrease and less water is reabsorbed, negative feedback
• • Dehydration is a loss of water from the body such that body fluids become hypertonic, thirst and heavily concentrated urine, blood pressure drops and heart rate increases, lethargic
  • Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
  • Individuals will produce excessive quantities of clear urine, cells swell leading to cell lysis and tissue damage, headaches and disrupted nerve functions

  
• • Kidneys prevent the excretion of blood cells and proteins (during ultrafiltration), as well as glucose (selective reabsorption)
  • glucose in urine is a common indicator of diabetes
  • high quantities of protein in urine may indicate disease (e.g. PKU) or hormonal conditions (e.g. hCG = pregnancy)
  • blood in urine can indicate a variety of diseases, infections and cancer
  • drugs pass into urine and can be detected (e.g. performance enhancing drugs)
• • Hemodialysis - external filtering of blood in order to remove metabolic wastes in patients with kidney failure, dialyzer has a porous membrane that is semi-permeable (restricts passage of certain materials), fresh dialysis fluid and removes wastes to maintain an appropriate concentration gradient
  • Kidney transplant: grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels, donors must be a close genetic match (no rejection), survive with one kidney