formation-of-urine

Formation of urine in the nephron

• The nephron is the functional unit of the kidney

  • The nephrons are responsible for the formation of urine

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

1. Ultrafiltration

2. Selective reabsorption

• After the necessary reabsorption of amino acids, water, glucose and inorganic ions is complete, the filtrate eventually leaves the nephron and is now referred to as urine

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

Ultrafiltration

• Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule

• The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them

  • This blood is already at high pressure because it is coming directly from the renal artery which is connected to the aorta)

• This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate

• 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—each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes

  • The next layer is the basement membrane—this is made up of a network of collagen and glycoproteins

  • The second cell layer is the epithelium of the Bowman’s capsule—these epithelial cells have many tiny finger-like projections with gaps in between them and are known as podocytes

• As blood passes through the glomerular capillaries, the holes in the capillary endothelial cells and the gaps between the podocytes allows substances dissolved in the blood plasma to pass into the Bowman’s capsule

  • The fluid that filters through from the blood into the Bowman’s capsule is known as 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^–)

• Red and white blood cells and platelets remain in the blood as they are too large to pass through the holes in the capillary endothelial cells

• The basement membrane acts as a filter as it stops large protein molecules from getting through

How ultrafiltration occurs

• Ultrafiltration occurs due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule

  • Water moves down a water potential gradient, from a region of higher water potential to a region of lower water potential

  • Water potential is increased by high pressure and decreased by the presence of solutes

• Overall, the effect of the pressure gradient outweighs the effect of solute gradient

• Therefore, the water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowman’s capsule

  • This means that as blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood into the Bowman’s capsule

Selective reabsorption

• Many of the substances that end up in the glomerular filtrate actually 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

• Glucose reabsorption occurs in the proximal convoluted tubule

• The lining of