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14.1.514-15 · Homeostasis and Control

Formation of Urine — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

14.1.5 Formation of Urine — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain ultrafiltration and selective reabsorption in the nephron.StageWhereWhat happensUltrafiltrationBowman's capsuleSmall molecules are filtered from the glomerular capillaries into the cap…

  • Capillary narrowing
  • Wider afferent arteriole
  • Forcing out
  • Collecting duct
  • Glomerular filtrate
  • Co-transporter

Part 1 of 7Two stages

StageWhereWhat happens
UltrafiltrationBowman's capsuleSmall molecules are filtered from the glomerular capillaries into the capsule
Selective reabsorptionProximal convoluted tubuleUseful molecules are returned to the blood
  • Urine
    • after reabsorption the remaining filtrate is urine, which passes along the ureters into the bladder.

Part 2 of 7How ultrafiltration happens

Capillary narrowing
  • Capillary narrowing capillaries get narrower in the glomerulus, raising pressure.
Wider afferent arteriole
  • Wider afferent arteriole the afferent arteriole is wider than the efferent, so pressure is high.
Forcing out
  • Forcing out small molecules are forced out of the capillaries into the capsule.
Filtrate
  • Filtrate amino acids, water, glucose, urea and inorganic ions (mainly Na+, K+ and Cl-).
Retained
  • Retained red and white blood cells and platelets are too large; the basement membrane stops large proteins.

Part 3 of 7Three layers crossed

Three layers crossed
The capillary endothelium, with holes in each cell.
The basement membrane, a collagen and glycoprotein mesh.
The epithelium of Bowman's capsule, the podocytes with gaps.

Part 4 of 7Water potential in the glomerulus

Pressure
  • Pressure high pressure raises the water potential of the plasma above that of the filtrate.
Solutes
  • Solutes plasma proteins stay in the blood, lowering plasma water potential below that of the filtrate.
Overall
  • Overall the pressure effect outweighs the solute effect, so water moves from blood to capsule.

Part 5 of 7Selective reabsorption in the proximal tubule

1
Sodium-potassium pumps in the basal membranes move Na+ into the blood, lowering Na+ inside the cells.
2
Na+ diffuses from the filtrate through co-transporter proteins in the luminal membrane.
3
Each co-transporter carries Na+ with another solute such as glucose or an amino acid.
4
The solutes diffuse into the blood through transport proteins in the basal membrane.
Glucose
  • Glucose all is reabsorbed, so none should be in urine.
Others
  • Others amino acids, vitamins and inorganic ions are also reabsorbed.
Water
  • Water water potential of the blood falls and the filtrate rises, so water enters the blood by osmosis.
Urea
  • Urea a significant amount diffuses back into the blood.

Part 6 of 7Loop of Henle and collecting duct

Salts
  • Salts reabsorbed into the blood by diffusion in the loop of Henle.
Water
  • Water follows by osmosis.
Collecting duct
  • Collecting duct water is reabsorbed in different amounts depending on the body's needs.

Part 7 of 7More from the specification

More from the specification
Renal clearance of hepatic urea
  • Renal clearance of hepatic urea deamination in the liver produces urea, which causes subsequent ultrafiltration and selective reabsorption into urine.
Homeostasis and Osmoregulation in the Kidneys
  • The kidneys play a central role in homeostasis by regulating blood water potential and solute concentration through the multi-step process of ultrafiltration and selective reabsorption.
Urine Formation in the Nephron
  • Ultrafiltration occurs under high hydrostatic pressure in the glomerulus, after which selective reabsorption in the proximal convoluted tubule removes glucose, amino acids,
  • and essential ions back into the blood.
Negative Feedback Control of Blood Solutes
  • Osmoreceptors in the hypothalamus trigger ADH release to control water reabsorption in collecting ducts;
    • similarly, negative feedback regulation of blood glucose by insulin and glucagon ensures metabolic waste formation occurs in kidneys without solute loss.
Glucose oxidase
  • Enzyme oxidising glucose to gluconic acid and hydrogen peroxide.
Target cells
  • Cells that respond to a particular hormone.
Insulin
  • A pancreatic hormone that lowers blood glucose.
Control
  • A test on distilled water used for comparison.
Homeostasis
  • The maintenance of a constant internal environment.
Kidney
  • An organ that filters the blood and produces urine.
Cause
  • A factor that directly produces an effect through a known mechanism.
Gradient
  • A difference in concentration.
Negative feedback
  • A mechanism in which a change from the set point triggers a response that returns the factor to the set point.

Key terms

TermMeaning
UltrafiltrationFiltration under pressure through the glomerular barrier.
Glomerular filtrateFluid that enters Bowman's capsule.
Selective reabsorptionReturn of useful substances to the blood.
Co-transporterMembrane protein carrying two solutes together.
Luminal membraneCell surface membrane facing the lumen.
Basal membraneCell membrane facing the blood capillary.

Avoid these mistakes

1
Saying all glucose remains in urine — all of it is reabsorbed.
2
Saying blood cells pass into the filtrate — they are too large.
3
Saying reabsorption of glucose is by simple diffusion alone — it is by co-transport driven by the Na+ gradient.
4
Saying the efferent arteriole is wider — the afferent is wider.

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