11.3 The kidney and osmoregulation

11.3 The kidney and Osmoregulation

Essential questions: how does the kidney regulate the concentrations of substances in the blood? What are the functions of each component of a nephron tubule?

Lesson One: Overall structure and function of the kidney


  • Cortex: Outer part of kidney, this contains the blood capillaries from the renal artery
  • Medulla: the inner part of the kidney, this contains ducts which collect the waste products from blood.
  • Pelvis: a part of the kidney that collects urine
  • Renal Artery: brings blood from the heart to the kidney
  • Renal vein: brings blood from the kidney back to the heart
  • Malphigian tubule system: a blood filtering system found in arthropods, not in mammals

Ultrastructure (large scale) of kidney

kidney_medThe overall purpose of the kidney is to regulate blood composition. That is achieved by excreting excess nutrients, and also excreting waste materials eg. urea.

The kidney works by two processes:

1. ultrafiltration: filtering under high pressures, of almost all blood components (except for large proteins and cells).

2. selective reabsorption: the reabsorption by active and passive processes, of the correct amounts of nutrients.


Deamination is the breakdown of excess amino acids in the liver, removing an amine group to form ammonia, and ultimately urea. This leaves behind a carbohydrate group. Urea is the main waste component of urea.

Comparison of the contents of the renal artery and renal vein.

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Lesson two: The Structure and function of a nephron

Objective: understand how the structure of a nephron allows the kidney to perform it’s functions.


  • Nephrons: tubules inside the kidney which allows it to carry out it’s functions
  • Glomerulus: a conglomerate of blood capillaries, which subject the blood to ultrafiltration. The receiving blood vessel (the afferent arteriole), has a larger diameter than the departing blood vessel (the efferent arteriole). This causes the positive pressure to drive ultrafiltration
  • Bowmans capsule: the cup like head of a nephron, which receives the filtrate from ultrafiltration of blood in the glomerulus. 
  • The proximal convoluted tubule: a part of the nephron that re-absorbs glucose, salts and water by active transport
  • The loop of Henle: a part of the nephron that absorbs salts and water, using a counter current system.
  • The distal convoluted tubule: part of the nephron which changes it’s permeability to water according to the needs to the body, through the action of a hormone ADH (Anti-Diuretic Hormone).
  • The collecting duct: the end of the nephron, which collects urine. Water is also reabsorbed in the collecting duct.
  • The vasa recta: the blood vessel into which useful substances are re-absorbed
  • Peritubular capillaries: capillaries which run received the re-absorbed substances from the convoluted tubules (proximal and distal).

The kidney is composed of many small tubules called nephrons. A nephron carries out the functions of the kidney.

Simplified diagram of a nephron below


Task A: Labelling exercise: In reality the nephron twists so that the flow of fluids goes backwards in the loop of Henle (hence the term countercurrent).

Choice between:

  • -A. Copy and paste the image below and label it.
  •  B. download the document (click here  Labeling Exercise).
  •  C. do the online quiz here http://phs.psdr3.org/science/anatomy/assess/nephronquiz.html nephron

 Lesson three: The flow of nutrients through the nephron.

Introductory video: Higher level you watch this for homework


This wonderful image appears…Handwritten tutorials is a youtube channel where this video comes from

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and the words: ‘the proximal convoluted tubule does most of the absorption, the loop of henle absorbs salt and water, and the distal convoluted tubule does the fine tuning’ are helpful.

Summary: Parts of the Nephron

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1. Ultrafiltration (filtration under high pressure) causes urea and other wastes, water, salts, glucose NOT blood cells or large proteins

2. Selective reabsorption of glucose, salts and water using active transport

3. The loop of Henle is important in absorbing water and salts. It uses a countercurrent system (the fluid flow down the descending limb, and up the ascending limb i.e. in opposite directions, hence countercurrent) to absorb first water (descending), then salts (ascending). Because of the countercurrent system, the water absorption in the descending limb happens because of the active absorption of sodium in the ascending limb.

Why don’t the sodium ions pumped out of the ascending limb enter the descending limb? This would stop water absorption…The answer is the descending limb is not permeable to sodium. 

Why don’t the water molecules enter the ascending limb? This would work against osmotic gradients, as the interstitial fluid has a greater solute concentration. Also the ascending limb is not permeable to water.

Image showing how the countercurrent system increases the osmolarity (the concentration of solutes) in the nephron. The units of osmolarity are osmoles, Osm, or milliosmoles, mOsm; 1 Osm means that there are one mole of solutes contributing to osmosis.


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