Kidneys

• What is osmoregulation?

  The homeostatic control of the water potential of blood
• What is the fibrous capsule?

  Outer layer of protection for kidney
• What is the cortex?

  Lighter coloured outer region made up of renal (Bowman's) capsules, convoluted tubules and blood vessels
• What is the medulla?

  Darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels
• What is the renal pelvis?

  Funnel shaped cavity that collects urine into ureter
• What is the ureter?

  Tube that carries urine into bladder
• What is a nephron?

  The functional unit of the kidney
• How long is a nephron?

  14 mm long
• What is the Renal (Bowman's) capsule? (Describe it's structure)

  Closed end of nephron - cup-shaped and surrounds mass of blood capillaries known as glomerulus. Inner layer made of specialized cells called podocytes
• Describe proximal convoluted tubules

  Series of loops surrounded by blood capillaries - it's walls are made up of epithelial cells which have micro-villi
• Describe the Loop of Henle

  Long hairpin loop that extends from cortex into medulla, then back again
• Describe distal convoluted tubules

  Same as proximal but fewer capillaries surrounding it
• What is the function of the collecting duct?

  Collects contents of distal convoluted tubules and transports it into pelvis of kidney
• Name 4 vessels associated with a nephron + their function

  1) Afferent arteriole - tiny vessel from renal artery and enters renal capsule of nephron where it forms the

  2) Glomerulus - Branch/knotted capillaries where fluid is forced out of the blood. Capillaries recombine to form

  3) Efferent arteriole - Tiny vessel with smaller diameter than afferent one so increases blood pressure within glomerulus

  4) Blood capillaries - surround proximal and distal convoluted tubules + loop of Henle. Reabsorb mineral salts, glucose and water . Merge to form venules --> Veins (renal vein)
• Give an important function of the kidney in terms of water potential?

  Ensures that water potential in plasma and hence tissue fluid is kept constant
• How is the glomerular filtrate formed? What is this process called?

  Ultrafiltration - Diameter of the afferent artery is larger than that of the efferent artery, so there's a build of pressure within the glomerulus so water, glucose and mineral ions are squeezed out into the renal capsule to form the glomerular filtrate
• List some of the factors which reduce the movement of the filtrate out of the glomerulus

  1) Endothelial cells of capillary

  2)Connective tissue and endothelial cells of the blood capillary

  3) Epithelial cells of renal capsule

  4) Hydrostatic pressure of fluid in renal capsule space

  5) Low water potential of blood in glomerulus
• Where is most of the filtrate reabsorbed back into the blood? How much is?

  Proximal convoluted tubule - 85%
• How are the proximal convoluted tubes adapted to reabsrob substances?

  1) Microvilli provide large SA to reabsorb substances from filtrate

  2) Infoldings at bases give large SA to transfer reabsorbed substances into blood capillaries

  3) High mitochondria density for active transport
• Outline the steps of reabsorption

  1) Sodium ions actively transported out of cells lining proximal convoluted tubule and into capillaries which carries them away Sodium ion concent. of cells is therefore lowered.

  2) Sodium ions now diffuse down concentration gradient from lumen of proximal convoluted tubules into epithelial lining cells through special carrier proteins by facilitated diffusion

  3) Carrier proteins are co-transport ones which means they carry another molecule (i.e. glucose/amino acids) with sodium ions

  4) Co-transport molecules then diffuse into blood
• How does the loop of Henle work?

  1) Sodium ions actively transported out of ascending limb of loop of Henle using ATP provided by many mitochondria

  2) Low water potential created in region of medulla between the two limbs (interstitial region). Water can't pass out of ascending tube as walls are too thick. Water can pass out of descending limb - this water is then carried away by blood capillaries in this region

  3) Filtrate progressively loses water in this way as it moves down descending limb lowering its water potential - reaches lowest water potential at tip of hairpin.

  4) At base of ascending limb, sodium ions diffuse out of filtrate and as it moves up ascending limb these ions are also actively pumped out and so filtrate develops progressively higher water potential

  5) Water potential gradient apparent as interstitial space between ascending limb and collecting duct has a high ion concentration. Highest water potential in cortex - decreases as you go down collecting duct

  6) Collecting duct permeable so water moves into interstitial space and taken away by vessels

  7) As water moves down filtrate, water potential is lowered however water potential is also lowered in interstitial space so water moves out via osmosis throghout the whole length of the collecting duct
• How is the 'counter-current' multiplier involved?

  Water potential gradient maintained down whole length of collecting duct so osmosis takes place continuously. Due to filtrate in collecting duct with a lower water potential meets interstitial fluids with an even lower water potential
• What are channel proteins specific to water called?

  Aquaporins
• What is the function of the distal convoluted tube? How is this function achieved?

  Controls pH of blood by selecting which ions to reabsorb - permeability of walls become altered by hormones in order to achieve this
• What may cause a rise in solute concentration and lowers water potential?

  1) Too little water being consumed

  2) Much sweating occurring

  3) Large amounts of ions (Sodium Chloride) being taken in
• How does the body respond to a fall in water potential?

  1) Osmoreceptors in hypothalamus of brain detect fall in water potential

  2) When water potential of blood is low, water leaves osmoreceptor cells by osmosis

  3) Due to this water loss, osmoreceptor cells shrink which stimulates hypothalamus to produce ADH (antidiuretic hormone)

  4) ADH passes to posterior pituitary gland, from where it is secreted into the capillaries

  5) ADH reaches kidneys and increases permeability. Specific protein receptors on cell-surface membrane of these cells bind to ADH molecules leading to activation of an enzyme called phosphorylase within the cell

  6) Activation of phosphorylase causes vesicles within cell to move to, and fuse with, its cell-surface membrane

  7) Vesicles conatain pieces of plasma that have aquaporins

  8) ADH also increases permeability to urea which also passes out, further lowering water potential of fluid around the duct

  9) Combined effect is that more water leaves duct via osmosis

  10) Osmoreceptors send impulses to thirst centre of brain

  11) When WP too high, osmoreceptors reduce frequency of nerve impulses to pituitary gland to reduce its release of adh
• How does blood enter the kidney?

  Renal artery
• What is the function of the kidney?

  Filter out urea and excess water/ions from the blood
• What is osmoregulation?


  The regulation of the water potential of the blood
• What is ultrafiltration?


  The filtering of substances out of the blood
• What is selective reabsorption?

  The reabsoprtion of useful substances and the right volume of water into the blood
• What is a nephron?


  Long tubules with bundles of capillaries where the blood is filtered
• What happens during ultrafiltration?

  1. Blood enters from the renal artery into smaller arterioles

  2. The afferent arteriole takes blood to the glomerulus

  3. High pressure forces out liquid and small molecules into the Bowman's capsule

  4. The efferent arteriole transports the blood away which now contains only large proteins/blood cells
• Why is there a high pressure in the glomerulus?

  The efferent arteriole is smaller in diameter than the afferent arteriole
• What is the glomerular filtrate?


  The substances from the blood that enter the Bowman's capsule
• Which substances are not filtered out of the blood and why?

  Blood cells

  Proteins

  Too large to pass through capillary endothelium
• What cells makes up the epithelium of the Bowman's capsule?

  Podocytes
• What happens during reabsorption in the proximal convoluted tubule?

  1. Glucose and solutes are reabsorbed into the blood by active transport

  2. Water moves into the blood by osmosis down the water potential gradient
• How are the cells in the proximal convoluted tubule adapted for reabsorption?

  Microvilli provide large surface area
• Where is the loop of Henle?


  The medulla (inner layer of the kidneys)