Excretion

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Ultrafiltration 1

Ultrafiltration is filtration at a molecular level. Blood flows into the glomerulus from the afferent arteriole, and is carried away from the glomerulus by the efferent arteriole. The afferent arteriole is wider in diameter than the efferent arteriole - this ensures that the blood in the capillaries of the glomerulus in under increased pressure. The pressure in the glomerulus is higher than the pressure in the Bowman's capsule that surrounds the glomerulus. This difference pushes fluid from the blood into the Bowman's capsule.

The barrier between the blood in the capillaries and the lumen of the Bowman's capsule consists of 3 layers that are each adapted to the function of ultrafiltration:

  • The endothelium of the capillaries has narrow gaps between the cells that blood plasma can pass through.
  • The basement membrane is made up of collagen fibres and glycoproteins that act as a filter to prevent the passage of molecules with a relative molecular mass (Mr) of more than 69,000. This means that most proteins are held in the capillaries of the glomerulus.
  • The epithelial cells of the Bowman's capsule are podocytes that have major processes. These ensure there are gaps between the cells that the fluid can pass through.
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Ultrafiltration 2

What is filtered out of the blood?

Blood plasma containing dissolved substances is pushed under pressure from the capillaries into the lumen of the Bowman's capsule. This includes the following substances:

  • Water
  • Amino acids
  • Glucose
  • Urea
  • Inorganic ions (Na, Cl, K)

 

What is left in the capillary?

The blood cells and proteins are left. The presence of the proteins means that the blood has a very low water potential. This ensures that some of the fluid is retained in the blood, and contains some of the water and other substances, and is important to help reabsorb water at a later stage. The total volume of fluid filtered out of the blood by both kidneys is about 125 cm3 min-1.

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Selective Reabsorption 1

As fluid moves along the nephron, substances are removed from the fluid and reabsorbed into the blood. This occurs mostly in the proximal convoluted tubule, where all the glucose, amino acids and some salts are reabsorbed with some of the water. The cells lining the proximal convoluted tubule are specialised to achieve the reabsorption:

  • The cell surface membrane in contact with the tubule fluid is folded to form microvilli - these increase the surface area for reabsorption.
  • The membrane also contains co-transporter proteins that transport glucose or amino acids, in association with sodium ions, from the tubule into the cell. This is facilitated diffusion.
  • The opposite membrane is also folded to increase the surface area. It contains sodium-potassium pumps that pump Na ions out of the cell and K ions into the cell.
  • The cytoplasm contains many mitochondria, which indicates that an active process that requires energy is involved - many mitochondria will produce a lot of ATP.
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Selective Reabsorption 2

How does reabsorption occur?

  • The sodium-potassium pumps remove Na ions from the cells lining the proximal convoluted tubule - this reduces the concentration of Na ions in the cytoplasm.
  • Na ions are transported into the cell along with glucose or amino acids by facilitated diffusion.
  • As the glucose and amino acid concentrations rise inside the cell, these substances are able to diffuse out into the tissue fluid.
  • These substances then diffuse from the tissue fluid into the blood and are carried away.
  • Reabsorption of salts, glucose and amino acids reduces the water potential in the cells and increases the water potential in the tubule fluid. This means that water will enter the cells and then be reabsorbed into the blood by osmosis.
  • Larger molecules such as proteins will be reabsorbed by endocytosis (engulfing the molecules to form a vesicle).
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Water Reabsorption 1

The loop of Henle consists of a descending limb and an ascending limb. Its arrangement allows salts to be transferred from the ascending limb to the descending limb; the overall effect is to increase the concentration of salts in the tubule fluid and consequently, they diffuse out of the ascending limb into the medulla tissue, giving the tissue fluid a very low water potential.

As the fluid in the tubule descends deeper into the medulla, its water potential becomes lower (more negative). This is due to:

  • Loss of water by osmosis to the surrounding tissue fluid
  • Diffusion of Na and Cl ions into the tubule from the tissue fluid

As the fluid ascends back up towards the cortex, its water potential becomes higher (less negative). This is because:

  • At the base of the tubule, Na and Cl ions diffuse out of the tubule into the tissue fluid
  • Higher up the tubule, Na and Cl ions are actively transported out of the tissue fluid
  • The wall of the ascending limb is impermeable to water, so water can't leave the tubule
  • The fluid loses salts but not water as it moves up the ascending limb
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Water Reabsorption 2

  • Arrangement of loop of Henle = hairpin countercurrent multiplier system
  • Tubule arranged into sharp bend so one part of the tubule passes close to the other, with the fluid flowing in opposite directions
  • Allows exchange between the contents, and can be used to create a very high concentration of solutes
  • Increases efficiency of salt transfer from the ascending limb to the descending limb - causes a build up of salt concentration in the surrounding tissue fluid
  • Movement of salts from the ascending limb into the medulla = high salt concentration in tissue fluid of medulla so low (very negative) water potential
  • Removal of ions from ascending limb means that urine is dilute at the top of the ascending limb. Water can then be reabsorbed from urine into distal tubules and collecting ducts
  • Tubule fluid passes along distal convoluted tubule - active transport is used to adjust the concentrations of various salts
  • Fluid flows into collecting duct (the tubule fluid still has a high water potential). The collecting duct carries fluid to the pelvis.
  • Water moves by osmosis from the tubule fluid into the surrounding tissue. It then enters the blood capillaries by osmosis and is carried away.
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Osmoregulation 1

Osmoregulation is the control of water and salt levels in the body. The correct water balance between cells and the surrounding fluids must be maintained to prevent problems with osmosis. Water is gained from 3 sources:

  • Food
  • Drink
  • Metabolism (e.g. respiration)

Water is lost in:

  • Urine
  • Sweat
  • Water vapour in exhaled air
  • Faeces

The walls of the collecting duct respond to the level of antidiuretic hormone (ADH) in the blood. ADH is released from the pituitary gland and acts on the collecting ducts to increase their reabsorption of water. Cells in the wall have membrane-bound receptors for ADH. The ADH binds to these receptors and causes a chain of enzyme-controlled reactions inside the cell. The enzyme-controlled reactions insert vesicles containing water-permeable channels (aquaporins) into the cell surface membrane. This makes the walls more permeable to water.

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Osmoregulation 2

  • More ADH in the blood = more aquaporins inserted - allows more water to be absorbed into the blood.
  • Less ADH in the blood = cell surface membrane creates new vesicles that remove aquaporins from the membrane - makes the walls less permeable and less water is reabsorbed into the blood.
  • Water potential of the blood is monitored by osmoreceptors in the hypothalamus, which are receptor cells that monitor the water potential of the blood.
  • These cells respond to the effects of osmosis - when the water potential of the blood is low (negative), the osmoreceptors lose water by osmosis. This causes them to shrink and stimulate neursecretory cells in the hypothalamus.
  • The neurosecretory cells are specialised neurones (nerve cells) that produce and release ADH. This flows down the axon to the terminal bulb in the posterior pituitary gland, where it is stored until needed.
  • When the neurosecretory cells are stimulated, they send action potentials down their axons and cause the release of ADH, which enters the blood capillaries running through the posterior pituitary gland. It's transported around the body and acts on the cells of the collecting ducts.
  • Once the water potential of the blood rises again, less ADH is released. ADH is slowly broken down (it has a half life of about 20 minutes - the time taken for its concentration to drop to half its original value). The ADH in the blood will be broken down and the collecting ducts will recieve less stimulation.
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Excretion

Excretion = the removal of metabolic waste from the body

Carbon dioxide

Excess carbon dioxide is toxic and has 3 main effects:

  • Carried in the blood as hydrogencarbonate ions (H+ ions are also formed). Occurs in red blood cells with enzyme carbonic anhydrase. H+ ions combine with Hb and compete with oxygen for space - reduce oxygen transport.
  • Combines directly with Hb to form carbaminohaemoglobin - has lower affinity for oxygen than normal Hb.
  • Cause respiratory acidosis - CO2 dissolves in blood plasma and combines with water to produce carbonic acid. H+ ions lower pH and make blood more acidic. Change detected by medulla oblongata, and causes an increase in breathing rate. If pH drops below 7.35, results in slow/difficult breathing, headache, tiredness, tremour and confusion. Also rapid HR and changes in blood pressure = respiratory acidosis.

Nitrogenous compounds

  • Deamination = the removal of the toxic amine group from an amino acid to produce ammonia
  • Ammonia is then converted to urea which is transported to the kidneys to be excreted
  • Remaining keto acid used in respiration for energy or converted to carbohydrate/fat for storage
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