The Mammalian Kidney: Function (OCR A Level Biology): Revision Note

The mammalian kidney: function

• The nephron is the functional unit of the kidney; nephrons are responsible for the formation of urine

• The process of urine formation in the kidneys occurs in two stages:

  • ultrafiltration

    • Small molecules are filtered out of the blood and into the Bowman's capsule of the kidney nephron, forming glomerular filtrate

  • selective reabsorption

    • Useful molecules are taken back from the filtrate and returned to the blood

• After reabsorption is complete the remaining filtrate forms the urine

• Urine then flows out of the kidneys, along the ureters and into the bladder, where it is temporarily stored

The processes of ultrafiltration and selective reabsorption

Ultrafiltration

• Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries known as the glomerulus, which sits inside the Bowman’s capsule

• The arteriole entering the glomerulus (the afferent arteriole) is wider in diameter than the capillary leaving the glomerulus (the efferent arteriole), resulting in high blood pressure within the glomerulus

• This high blood pressure causes the smaller molecules being carried in the blood to be forced out of the capillaries of the glomerulus and into the Bowman’s capsule, where they form the glomerular filtrate

• The main substances that pass out of the capillaries and form the glomerular filtrate are:

  • amino acids,

  • water,

  • glucose,

  • urea and

  • inorganic ions (mainly Na⁺, K⁺ and Cl^-)

• Blood cells and large proteins remain in the blood as they are too large to pass through the holes in the capillary endothelial cells

Features that aid ultrafiltration

• The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them:

  • The first cell layer is the endothelium of the capillary; gaps between the endothelial cells allow small molecules to pass through this layer

  • The next layer is the basement membrane, which is made up of a mesh of collagen and glycoproteins; small molecules can pass through the holes in the mesh

  • The second cell layer is the epithelium of the Bowman’s capsule; these epithelial cells have many finger-like projections with known as podocytes, between which there are gaps for small molecules to pass through

Ultrafiltration occurs when small molecules filter out of the blood and into the Bowman’s capsule to form glomerular filtrate. These molecules must pass through three layers during this process: the capillary endothelium, the basement membrane and the Bowman’s capsule epithelium.

Selective reabsorption

• Many of the substances that end up in the glomerular filtrate need to be kept by the body

• These substances are reabsorbed into the blood as the filtrate passes along the nephron

  • This process is knowns as selective reabsorption as only certain substances are reabsorbed

• Reabsorbed substances include:

  • water,

  • salts,

  • glucose and

  • amino acids

• Most of this reabsorption occurs in the proximal convoluted tubule

  • Note that while most water and salts are reabsorbed in the proximal convoluted tubule, the loop of Henle and collecting duct are also involved in the reabsorption of these substances

• The lining of the proximal convoluted tubule is composed of a single layer of epithelial cells, which are adapted to carry out reabsorption in several ways; they have:

  • microvilli

  • co-transporter proteins

  • many mitochondria

  • tightly packed cells

Mechanisms of reabsorption

• Sodium ions (Na⁺) are transported from the proximal convoluted tubule into the surrounding tissues by active transport

• The positively charged sodium ions creates an electrical gradient, causing chloride ions (Cl^-) to follow by diffusion

• Sugars and amino acids are transported into the surrounding tissues by co-transporter proteins, which also transport sodium ions, in the following process

  1. sodium-potassium pumps in the cells that line the proximal convoluted tubule actively transport sodium ions out of the epithelial cells and into the blood, where they are carried away

  2. this lowers the concentration of sodium ions inside the epithelial cells, causing sodium ions in the filtrate to diffuse down their concentration gradient into the epithelial cells

  3. these sodium ions move via co-transporter proteins in the membrane, and as they move the proteins transport another solute at the same time, e. g. glucose or an amino acid

  4. once inside the epithelial cells these solutes diffuse down their concentration gradients into the blood

• The movement of ions, sugars, and amino acids into the surrounding tissues lowers the water potential of the tissues, so water leaves the proximal convoluted tubule by osmosis

• Urea moves out of the proximal convoluted tubule by diffusion

• All of the substances that leave the proximal convoluted tubule for the surrounding tissues eventually make their way into nearby capillaries down their concentration gradients

  
  

  Selective reabsorption in the proximal convoluted tubule occurs via cotransport proteins

Reabsorption of water and salts

• Sodium and chloride ions are pumped out of the filtrate in the ascending limb of the loop of Henle into the surrounding medulla region, lowering its water potential

  • The ascending limb of the loop of Henle is impermeable to water, so water is unable to leave the loop here by osmosis

  • The water potential of the ascending limb increases as it rises back into the cortex due to the removal of solutes and retention of water

• The neighbouring descending limb is permeable to water, so water moves out of the descending limb by osmosis due to the low water potential of the medulla created by the ascending limb

  • The descending limb has few transport proteins in the membranes of its cells, so has low permeability to ions

  • The water potential of the filtrate decreases as the descending limb moves down into the medulla due to the loss of water and retention of ions

• The low water potential in the medulla created by the ascending limb also enables the reabsorption of water from the collecting duct by osmosis

• The water and ions that leave the loop of Henle for the medulla make their way into nearby capillaries