Control of blood water potential (A-level only) Flashcards
(32 cards)
Osmoregulation
Osmoregulation is the control of the water potential in the blood.
The kidneys
Osmoregulation takes place in the kidneys.
The kidneys absorb more or less water according to the water potential.
High water potential
If blood water potential is too high, more water must be lost by excretion to return the water potential to normal.
The blood reabsorbs less water from the kidneys.
The urine is more dilute and water potential in the blood decreases.
Low water potential
If blood water potential is too low, less water must be lost by excretion to return the water potential to normal.
The blood reabsorbs more water from the kidneys.
The urine is more concentrated and water potential in the blood increases.
Structures of a nephron
Bowman’s Capsule
Afferent and efferent arterioles
Proximal convoluted tubule (PCT)
Loop of Henle
Collecting duct
Bowman’s capsule
The Bowman’s capsule is the beginning of the tubules that make up the nephron.
The capsule surrounds a network of capillaries.
This network is called the glomerulus.
The first step of filtration of the blood to form urine takes place in the Bowman’s capsule.
This step produces the glomerular filtrate.
Afferent and efferent arterioles
Blood flows into the glomerulus through the afferent arteriole and out of the glomerulus through the efferent arteriole.
The afferent arteriole is much wider than the efferent arteriole.
This means that the blood pressure in the capillaries is very high.
Proximal convoluted tubule (PCT)
The PCT is the site of selective reabsorption.
After the glomerular filtrate has been produced in the Bowman’s capsule, glucose and water are reabsorbed into the bloodstream through the PCT.
Loop of henle
The loop of Henle produces a low water potential in the medulla of the kidney.
The loop of Henle consists of an ascending limb and a descending limb.
The ascending limb is impermeable to water.
The descending limb is permeable to water.
Collecting duct
Water is reabsorbed into the blood through the collecting duct.
The amount of water that is absorbed depends on the water potential of the blood.
If blood water potential is low, more water is reabsorbed.
If blood water potential is high, less water is reabsorbed.
This is osmoregulation.
Steps in producing glomerular filtrate:
Pressure filtration
Capillary endothelium
Basement membrane
Podocytes
Glomerular filtrate
Pressure filtration
The branch of capillary that enters the glomerulus is much wider than the branch that exits the glomerulus.
This creates a high blood pressure in the glomerulus.
The high blood pressure causes the fluid and its solutes (e.g. glucose, amino acids) in the blood to be forced out of the capillary.
This is called pressure filtration.
Capillary endothelium
The fluid flows through the pores in the capillary endothelium.
Basement membrane
Then the smaller molecules filter through slit pores in the basement membrane.
This is a mesh of collagen fibres and glycoprotein.
Most proteins and all blood cells are too big to pass through the slit pores.
Podocytes
The substances finally pass between the epithelial cells of the Bowman’s capsule.
The epithelial cells, called podocytes, have finger-like projections that the substances can flow between.
Glomerular filtrate
The fluid that has filtered from the capillaries to the Bowman’s capsule is called the glomerular filtrate.
The filtrate contains:
Water.
Amino acids.
Urea.
Glucose.
Inorganic ions.
Stages of selective reabsorption into the bloodstream in the proximal convoluted tubule (PCT):
Sodium-potassium pumps
Co-transporter proteins
Reabsorption of glucose and amino acids
Reabsorption of water
Sodium-potassium pumps
Na+ ions are actively transported out of the PCT epithelial cells and into the blood by sodium-potassium pumps.
K+ ions are also transported into the epithelium.
Co-transporter proteins
Active transport of Na+ ions causes the concentration of Na+ ions inside the epithelial cells to decrease.
Na+ ions in the filtrate diffuse into the epithelial cells (down their concentration gradient) through co-transporter proteins.
Co-transporter proteins allow glucose and amino acids to be transported into the epithelial cells along with the Na+ ions.
Reabsorption of glucose and amino acids
As glucose and the amino acids are co-transported into the PCT epithelial cells, their concentration increases inside the cells.
Glucose and the amino acids diffuse down the concentration gradient into the blood.
Blood pressure is relatively high so that the substances in the blood are carried away quickly.
This maintains a steep concentration gradient.
Reabsorption of water
The movement of Na+ ions, glucose and amino acids into the bloodstream causes the water potential to decrease in the blood and increase in the PCT.
Water in the PCT diffuses into the blood through osmosis.
Any substances that are not reabsorbed are excreted as waste.
The loop of henle allows water to be reabsorbed in the collecting duct. The steps involved are:
Top of the ascending limb
Bottom of the ascending limb
The descending limb
Reabsorption of water
Osmoregulation
Top of the ascending limb
Na+ ions are actively transported out of the top of the ascending limb into the surrounding tissue fluid in the medulla.
This causes the solute concentration of the medulla to increase and the water potential to decrease.
The ascending limb is impermeable to water.
This means water inside the tubule cannot diffuse out.
Bottom of the ascending limb
Na+ ions diffuse out of the bottom of the ascending limb into the medulla.
This further increases the solute concentration of medulla.
