CONTROL OF BLOOD WATER POTENTIAL

• Osmoregulation is the control of the water potential of the blood
• What could make the water potential of the blood become lower (more negative)?

  More solute

  Less water
• What could raise the water potential of the blood?

  Less solute

  More water
• If water potential is higher outside red blood cell, water moves into cell by osmosis, causing cells to swell (and possibly burst/lyse).
• If water potential inside cell is same as outside, there is no net movement of water into or out of cell by osmosis.
• If water potential is lower outside of red blood cell, water moves out of cell by osmosis, causing cells to shrink.
• The kidneys play a key role in osmoregulation
• Kidneys produce urine, which is filtered and then excreted from the body
• Kidney system

  A) Aorta

  B) Inferior vena cava

  C) left kidney

  D) renal vein

  E) renal artery

  F) urethra

  G) ureter

  H) urinary bladder

  
• Excretion – urea is a toxic product of amino acid breakdown so could damage cells. It is excreted via the kidneys
• Amino acids → ammonia → urea
• • Osmoregulation – maintains optimum blood water potential
• The kidneys contain a complex tubular structure called the nephron, which is used for osmoregulation.
• How the nephron achieves osmoregulation:

  • Ultrafiltration: blood is first filtered by a structure called the glomerulus to produce “glomerular filtrate”
  • Glucose and water (useful substances) are reabsorbed from the filtrate back into the blood by proximal convoluted tubule.
  • The loop of Henle maintains a concentration gradient of sodium ions in the medulla.
  • The distal convoluted tubule and collecting ducts also allow reabsorption of water back into the blood, but under the control of a hormone called ADH.
• The capillaries that surround the nephron are called peritubular capillaries
• What drives ultrafiltration?

  The blood in the glomerular capillaries is under high hydrostatic pressure.

  This forces fluid out of the capillaries.

  The reason for the high hydrostatic pressure is the afferent arteriole (bringing blood into the glomerulus) is wider than the efferent arteriole (carrying blood away from the glomerulus).
• regions of the nephron

  A) distal convoluted tubule

  
• Ultrafiltration: the formation of glomerular filtrate
• Ultrafiltration is achieved using two layers
• Ultrafiltration -

  • Pores of the capillary endothelium are too small to allow blood cells to leave the blood;
  • Pores of the basement membrane are too small to allow plasma proteins to leave the blood;
  • The basement membrane is a porous mesh made of protein that acts as a fine filter.
  • Everything else can leave the blood e.g. water, glucose, amino acids, small ions.
  • The fluid entering the lumen of the Bowman’s capsule is called the glomerular filtrate.
• Ultrafiltration

  A) Afferent

  B) Efferent

  C) Hydrostatic

  D) Glomerulus

  E) Pores

  F) Red blood cells

  G) Glucose

  H) Bowman's

  I) Glomerular filtrate

  J) proximal convoluted tubule

  
• Reabsorption of glucose and water by the proximal convoluted tubule - This reabsorption at the proximal convoluted tubule is sometimes called “selective reabsorption” because it focuses on reabsorbing useful substances back into the blood.
• At the proximal convoluted tubule:

  1. ALL glucose is reabsorbed from glomerular filtrate back into the blood by active transport (via cotransport) and facilitated diffusion.
  2. MOST water is reabsorbed from glomerular filtrate back into the blood by osmosis down a water potential gradient.
• Describe how glucose is reabsorbed into the blood at the proximal convoluted tubule

  1. Sodium ions and glucose absorbed by co-transport;

  2. (Co-transport) via carrier protein;

  3. Sodium ions removed (from epithelial cell) by active transport into blood;

  4. Maintains low concentration of sodium ions (in epithelial cell) / maintains sodium ion concentration gradient (between tubule lumen and epithelial cell);

  5. Sodium ions enter epithelial cells by facilitated diffusion taking glucose with them (from tubule);

  6. Glucose moved by facilitated diffusion into blood (from epithelial cells); 
• Adaptations of the proximal convoluted tubule epithelial cells that maximise reabsorption by facilitated diffusion and active transport

  • Microvilli (facing filtrate) and membrane folds (facing blood) provide large surface area for reabsorption;
  • Many carrier proteins for fast cotransport/active transport/facilitated diffusion;
  • Many mitochondria for enough aerobic respiration to produce large amounts of ATP for active transport;
  • Many ribosomes to produce carrier proteins.
• “Counter current” refers to the way the filtrate flows in opposite directions as it passes through the loop of Henle: down the descending limb and up the ascending limb.
• • At the top of the loop of Henle, release of sodium ions is slower (relying on active transport) and water release by osmosis is rapid. This makes the water potential of the medulla tissue fluid less negative (higher) in the upper medulla.
• • At the bottom of the loop of Henle, there is rapid release of sodium ions into the medulla tissue fluid by facilitated diffusion. There is a fairly low release of water at this point. This makes the water potential very negative deeper in the medulla.
• The multiplier is the way the water potential of the medulla tissue fluid gets lower the deeper you go in the medulla
• Loop of Henle

  A) Medulla

  B) Impermeable

  C) sodium

  D) tissue

  E) active

  F) against / up

  G) lower

  H) permeable

  I) osmosis

  J) Water potential gradient

  K) facilitated diffusion

  L) down

  M) collecting duct

  N) antidiuretic hormone (ADH)

  O) deeper

  P) counter-current mulitipler

  
• Species that live in drier environments tend to have a greater proportion of their kidneys taken up by the medulla.
• Epithelial cells of the upper ascending limb of the loop of Henle in desert rodents have extremely high numbers of mitochondria, and each of these mitochondria have an extremely high number of cristae.

  This maximises the rate of oxidative phosphorylation during aerobic respiration so that very high amounts of ATP can be made to maximise the active transport of sodium ions into the medulla.

  This lowers the water potential of the medulla even more, and maximises water reabsorption into the blood by osmosis from the distal convoluted tubule and collecting duct.
• What happens to all the water and sodium ions coming out of the loop of Henle into the medulla tissue fluid?

  They gradually enter the nearby peritubular capillaries to re-join the blood circulation.
• • How do we gain water in the body (raising water potential)?
  • Drinking
  • Eating
  • Aerobic respiration (water as a product)
• • How do we lose water from the body (lowering water potential)?
  • Urinating
  • Breathing (evaporation from alveoli)
  • Sweating (evaporation from skin)
• • Not enough water in body 🡪 water potential of blood too low �� antidiuretic hormone (ADH) stops it getting even lower!
• Low water potentials in the blood mean low water potentials in the tissue fluid.
• • Excessive water loss in the urine is known as diuresis.
• • An antidote to diuresis is called an antidiuretic.
• • Mammals produce a hormone called antidiuretic hormone (ADH) that reduces water loss in the urine.