What are the 2 main functions of the kidney
Excretion - removal of urea from the body
Osmoregulation- regulation of water content of the body
What is each kidney made up of
Cortical Nephrons and juxtamedullary nephrons
Where are cortical nephrons found and what is there structure
They are found in the renal cortex and they have a U shaped loop of henle
What is the structure of juxtamedullary nephrons
They have very long loops of henle making them efficient at producing very concentrated urine and they penetrate right into the medulla of the kidney
Where does ultrafiltration occur
It occurs in the glomerulus and the bowman capsule
How does ultra filtration work
- The barrier between the blood in the capillary and the lumen of the bowmans capsule consists of three layers (the endothelium of the capillary, basement membrane of the endothelial cells of the capillary and the epithelial cells of the bowmans capsule)
- the endothelium has narrow gaps between its cells so that blood plasma and small molecules can pass through
- the basement membrane consists of a fine mesh of collagen fibres which act as a filter to prevent the passage of larger molecules
- epithelial cells are called podocytes and they have finger like projections called pedicels that wrap around capillaries forming slits to make sure any blood cells that have left the capillary do not get into the kidney tubule
How does selective reabsorption work
- The cells lining the proximal convoluted tubule are specialised
- the cell membrane in contact with the tubule fluid is covered with microvilli which increase S.A for reabsorption.
- the cell membrane also contains special cotransporter proteins which transport glucose or amino acids from the tubule into the cell. Facilitated diffusion
- the membrane close to the tissue fluid and blood capillaries is also folded to increase its S.A. This membrane also contains sodium potassium pumps which allows active transport of sodium ions into the cell
What are the 4 mechanisms in selective reabsorption
Active transport, contransport , passive diffusion and osmosis
How does active transport work in selective reabsorption
- sodium potassium pumps actively pump out sodium ions from the PCT to the tissue fluid
- atp is provided by many mitochondria nearest the capillary
- this reduces conc of sodium ions in pct cells
How does cotransport work in selective reabsorption
- as a result of the fall of sodium ions, sodium ions continuously move into the pct cells from the filtrate
- the sodium ions move into the cells through special proteins called cotransporter proteins
- these allow facilitated diffusion of sodium ions to be accompanied by the transport of larger molecules e.g glucose
How does diffusion work in selective reabsorption
As the glucose and amino acid conc rise inside the cell these molecules are able to diffuse out of the opposite side of the cell into the tissue fluid and then into the capillary
How does osmosis work in selective reabsorption
As sodium ions,glucose and amino acids leave the filtrate the water potential of the pct cells decrease
-as a result water leaves the filtrate and enters the cells and then the blood by osmosis down a water potential gradient
What is the function of the loop of henle
Creates a very high conc of sodium and chloride ions in the tissue fluid of the medulla helping the body to conserve water by reabsorbing It from the nearby collecting duct
What is the function of the vasa recta
It follows the loop of henle and the flow of blood inside them is slow and opposite to the flow of the filtrate. The vasa recta absorb water that has been absorbed into the medulla
What is the structure of the distal convoluted tubule
Cells in this tubule are similar to the pct with microvilli lining the surface to increase s.a for reabsorption and numerous mitochondria to supply energy for active transport
What is the function of the dct
Sodium ions are actively pumped out of this tubule if there’s not enough salt in the body. Chloride ions follow passively. Water can also leave this tubule by osmosis and the dct pumps out hydrogen ions or secretes them to control blood ph it also secretes creatinine
Where is water regulation controlled
The pituitary gland and osmoreceptors in the hypothalamus of the brain
How is water regulated
-Water potential is monitored by osmoreceptors in the hypothalamus
- when water potential of the plasma is low these cells lose water by osmosis causing them to shrink
- this stimulates neurosecretory cells in the hypothalamus which produce and release adh
- adh flows down the axon of the neurosecretory cell to its terminal bulb in the posterior pituitary gland
- adh travels in the blood to the kidney where it increase the permeability of the cell surface membrane of the collecting duct and the dct to water
- it causes water channels (aquaporins) to be inserted into the cell surface membrane
- 3 million water molecules move through each aquaporin per second
- as the medulla has a low water potential water leaves the filtrate by osmosis as it passes through which produces a small conc of urine
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