kidneys

• The homeostatic control of the water potential of the blood is called osmoregulation
• The kidney is made of:

  • Fibrous capsule: an outer membrane that protects the kidney
  • Cortex: a lighter colored outer region made of the renal [Bowman’s] capsule, convoluted tubules, and blood vessels
  • Medulla: a darker colored inner region made up of loops of Henle, collecting ducts, and blood vessels
  • Renal pelvis: a funnel-shaped cavity that collects urine into the ureter
  • Ureter: a tube that carries urine to the bladder
  • Renal artery: supplies the kidney with blood from the heart via the aorta
  • Renal vein: returns blood to the heart via the vena cava
• • Nephrons: span the cortex and medulla and are the basic structural and functional units of the kidney
• Renal [Bowman’s] capsule:

  • Closed end at the start of the nephron
  • Cup-shaped structure that surrounds the glomerulus, a mass of blood capillaries
  • Inner layer made up of specialised cells called podocytes
• Proximal convoluted tubule:

  • Series of loops surrounded by blood capillaries
  • Walls made of epithelial cells with microvilli
• Loop of Henle:

  • Long hairpin loop extending from the cortex into the medulla of the kidney and back again
  • Surrounded by blood capillaries
• Distal convoluted tubule:

  • Series of loops surrounded by blood capillaries
  • Walls made of epithelial cells, but surrounded by fewer capillaries than the proximal tubule
• Collecting duct:

  • Tube where distal convoluted tubules from multiple nephrons empty
  • Lined by epithelial cells
  • Becomes wider as it empties into the pelvis of the kidney
• Blood vessels associated with nephrons:

  • Afferent arteriole: a tiny vessel that arises from the renal artery and supplies the nephron with blood
• Glomerulus:

  • Formed when the afferent arteriole enters the renal capsule of the nephron
  • A many-branched knot of capillaries from which fluid is forced out of the blood
• Efferent arteriole:

  • Formed as the glomerular capillaries recombine
  • A tiny vessel that leaves the renal capsule
  • Has a smaller diameter than the afferent arteriole, causing an increase in blood pressure within the glomerulus
  • Carries blood away from the renal capsule
• Blood capillaries:

  • Formed as the efferent arteriole branches
  • A concentrated network of capillaries surrounding the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule
  • Responsible for reabsorbing mineral salts, glucose, and water
  • Capillaries merge into venules, which then merge to form the renal vein
• Stages of osmoregulation in the nephron: 

  • Formation of glomerular filtrate by ultrafiltration 
  • Reabsorption of glucose and water by the proximal convoluted tubule 
  • Reabsorption of water by the distal convoluted tubule and collecting ducts
• formation of glomerular filtrate

  Blood enters the kidney through the renal artery which branches into afferent arterioles, forming the glomerulus inside the renal [Bowman’s] capsule Glomerular capillaries merge to form the efferent arteriole, which then sub-divides into capillaries winding around the various tubules of the nephron before combining to form the renal vein
• formation of glomerular filtrate pt2

  Walls of glomerular capillaries are made up of endothelial cells with pores between them. Diameter of afferent arteriole is greater than that of the efferent arteriole, leading to a build-up of hydrostatic pressure in the glomerulus. Result of the pressure build-up in the glomerulus: water, glucose, and mineral ions are squeezed out of the capillary to form glomerular filtrate. Blood cells and large proteins are too large to pass through the pores in the glomerular capillaries
• The movement of this filtrate out of the glomerulus is resisted by the: 

  • Capillary endothelial cells 
  • Connective tissue and endothelial cells of the blood capillary 
  • Epithelial cells of the renal capsule 
  • The hydrostatic pressure of the fluid in the renal capsule space 
  • The low water potential of the blood in the glomerulus
• modifications in the glomerular capillaries to reduce the resistance to the flow of filtrate: 

  • The inner layer of the renal capsule is made from podocyte. These have spaces between them which allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes between these cells rather than through them 
  • The endothelium of the glomerular capillaries has spaces between its cells. Fluid can therefore pass between these cells rather than having to go through them 
• reabsorbtion of glucose and water by proximal convoluted tubule

  Sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries This process causes the sodium ion concentration of these cells to decrease. Sodium ions then diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells through special carrier proteins by facilitated diffusion Carrier proteins in the cells carry another molecule (glucose, amino acid, or chloride ions) along with the sodium ion through cotransport
• reabsorption of glucose and water by the proximal convoluted tubule pt2

  Molecules co-transported into the proximal convoluted tubule diffuse into the blood Valuable molecules like glucose and most other substances are reabsorbed into the blood, along with water
• The two regions of the loop of Henle: 

  • The descending limb - this is narrow, with thin walls that are highly permeable to water 
  • The ascending limb - this is wider, with thick walls that are impermeable to water
• loop of henle as a countercurrent multiplier

  1. Sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells of its wall 
  2. This creates a water water potential in the interstitial region [the region of the medulla between the two limbs]. Under normal circumstances, water would pass out of the ascending limb by osmosis. However the thick cell walls mean that very little water escapes 
• the loop of henle as a countercurrent multiplier pt3

  1. At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out so the filtrate gains a progressively higher water potential. 
  2. In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential the further into the medulla one goes. 
• the loop of henle as a countercurrent multiplier pt4

  
  

  1. The collecting duct is permeable to water and so as the filtrate moves down it, water passes out of it by osmosis. This water passes into the blood vessels that occupy this space
  2. As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing out of the tubule
• Osmoregulation part 1- 

  1. Cells called osmoreceptors in the hypothalamus of the brain detect the fall in water potential 
  2. When the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis 
  3. Due to this water loss the osmoreceptor cells shrink, which causes the hypothalamus to produce ADH 
  4. ADH passes to the posterior pituitary gland where it is secreted into capillaries
• osmoregulation part 2-

  1. It passes in the blood to the kidney, where is increases the permeability to water of the cell surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct 
  2. Specific protein receptors on the cell surface membrane of these cells bind to ADH, leading to the activation of the enzyme phosphorylase within the cell
• osmoregulation part 3-

  1. The activation of phosphorylase causes vesicles in the cell to move to, and fuse with, the cell surface membrane 
  2. These vesicles contain pieces of plasma membrane that have numerous water channel proteins [aquaporins] sp when they fuse with the membrane, the number of  water channels is increased which makes the whole structure more permeable to water. 
• osmoregulation part 4-

  1. ADH increases the permeability of the collecting duct to urea, which passes out to further lower the water potential of the fluid around the duct 
  2. This will not increase water potential in the blood but prevent it from getting lower. Osmoreceptors send nerve impulses to the thirst centre of the brain to encourage the individual to seek water. 
• osmoregulation part 5-

  1. The osmoreceptors in the hypothalamus detect the rise in water potential and send fewer impulses to the pituitary gland 
  2. The pituitary gland reduces the release of ADH and the permeability of the collecting ducts to water and urea everts to its former state. This is an example of homeostasis and negative feedback