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Title: Structure of the Kidney, Ultrafiltration and Selective Reabsorption
Description: These notes are for A level Students. these notes talk about the structure of the kidneys, Ultrafiltration and selective reabsorption. This isn't a summary of the topics rather detailed notes providing you with all the knowledge you need for your Level. They are highlighted and have diagrams for easy remembrance and as long as you can remember, you will pass your tests and Exams.
Description: These notes are for A level Students. these notes talk about the structure of the kidneys, Ultrafiltration and selective reabsorption. This isn't a summary of the topics rather detailed notes providing you with all the knowledge you need for your Level. They are highlighted and have diagrams for easy remembrance and as long as you can remember, you will pass your tests and Exams.
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The Structure of the Kidney, Ultrafiltration
and Selective Reabsorption
...
A narrow tube, called the ureter, carries
urine from the kidney to the bladder
...
The whole kidney is covered by a fairly tough fibrous capsule, beneath which is the cortex
...
Where the ureter joins, there is an area called the renal pelvis
...
Homeostatis Page 5
The position of a single nephron, and its structure
...
The glomeruli and capsules of all the nephrons are in the cortex of the kidney
...
The first part of the loop is the descending limb
...
Blood vessels are closely associated with the nephrons
...
The capillaries of the glomerulus rejoin to form an efferent arteriole
...
Blood from these capillaries flows into venules that empty into a branch of the renal vein
...
The first stage, ultrafiltration, involves filtering small molecules, including urea,
out of the blood and into the Bowman’s capsule to form filtrate
...
The second stage, selective reabsorption, involves taking back any useful molecules
from the filtrate as it flows along the nephron
...
Ultra Filtration
The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers and a
basement membrane
...
Next comes the basement membrane, which is made up of a network of collagen and glycoproteins
...
These cells have many tiny finger-like
projections with gaps in between them, and are called podocytes
...
However, the basement membrane stops large protein
molecules from getting through
...
Red and white blood cells and platelets are too large to pass through the perforations in the endothelium, so they remain in
the blood
...
Detail of the endothelium of a glomerular capillary and Bowman’s capsule
...
You will see
that glomerular filtrate is identical to blood plasma except that there are almost no plasma proteins in it
...
In a human, for all the glomeruli in both kidneys, the rate is about 125 cm3 min–1
...
1) Water Potential
What makes the fluid filter through so quickly? This is determined by the differences in water potential between the plasma
in glomerular capillaries and the filtrate in the Bowman’s capsule
...
Water potential is lowered by the presence of solutes, and raised by high pressures
...
Water potential is lowered by the presence of solutes, and raised by high pressures
...
This tends to raise the water potential of the blood plasma above the water potential of the contents of Bowman’s capsule
...
This is because, while most of the contents of the blood plasma can filter through the basement membrane and into the
capsule, the plasma protein molecules are too big to get through, and so they stay in the blood
...
Overall, the effect of differences in pressure outweighs the effect of the differences in solute concentration so that the
water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the capsule
...
Selective Reabsorption
Many of the substances in the glomerular filtrate need to be kept in the body, so they are reabsorbed into the blood as the
fluid passes along the nephron
...
Reabsorption in the proximal convoluted tubule
Most of the reabsorption takes place in the proximal convoluted tubule
...
These cells are adapted for their function of reabsorption by having:
• many microvilli on the surface facing the lumen (the luminal membrane) of the nephron to increase the surface area for
reabsorption of substances from filtrate in the lumen
...
• tight junctions that hold adjacent (neighbouring)
cells together firmly so that fluid cannot pass between the cells (all substances that are reabsorbed must go through the
cells)
...
Blood capillaries are very close to the outer surface of the tubule
...
The basal membranes of the cells lining the proximal convoluted tubule are those nearest the blood capillaries
...
The sodium ions are carried away in the blood
...
However, sodium ions do not diffuse freely through the membrane: they can only enter through special co-transporter
proteins in the membrane
...
The passive movement of sodium ions into the cells down their concentration gradient provides the energy to move
glucose molecules in this way into the cells, even against a concentration gradient
...
Once inside the cell, glucose diffuses down its concentration gradient, through a transport protein in the basal membrane,
into the blood
...
All of the glucose in the glomerular filtrate is transported out of the proximal convoluted tubule and into
the blood
...
Similarly, amino acids, vitamins, and many sodium ions and chloride ions (Cl–) are reabsorbed in the proximal convoluted
tubule
...
The movement of these solutes into capillaries decreases the water potential of the blood
...
Water moves down this gradient through the cells and into the blood by
osmosis
...
Surprisingly, quite a lot of urea is reabsorbed
too
...
Its concentration in the filtrate is considerably higher than in
the capillaries, so it diffuses passively through the cells of the proximal convoluted tubule and into the blood
...
The reabsorption of so much water and solutes from the
filtrate in the proximal convoluted tubule greatly reduces the volume of filtrate remaining
...
Reabsorption in the loop of Henle and collecting duct
The function of these loops is to create a very high concentration of sodium and chloride ions in the tissue fluid in the
medulla
...
As you will see, creating a tissue fluid in the medulla enables a lot of water to be reabsorbed from the fluid in the collecting
duct as it flows through the medulla
...
After leaving the loop of Henle, the filtrate continues through the distal convoluted tubule into the collecting duct, which
runs into the medulla again
...
Therefore, water can move out of the collecting duct by osmosis until the water potential of urine is the same as the water
potential of the tissue fluid in the medulla, which may be much greater than the water potential of the blood
...
The ability of some small mammals, such as rodents, to produce very concentrated urine is related to the relative thickness
of the medulla in their kidneys
...
Reabsorption in the distal convoluted tubule and collecting duct
The first part of the distal convoluted tubule functions in the same way as the ascending limb of the loop of Henle
...
In the distal convoluted tubule and collecting duct, sodium ions are actively pumped from the fluid in the tubule into the
tissue fluid, from where they pass into the blood
...
The rate at which these two ions are moved into and out of the fluid in the nephron can be varied, and helps to regulate the
concentration of these ions in the blood
...
Homeostatis Page 11
Relative concentrations of three substances in the different parts of nephrons and in collecting ducts
Title: Structure of the Kidney, Ultrafiltration and Selective Reabsorption
Description: These notes are for A level Students. these notes talk about the structure of the kidneys, Ultrafiltration and selective reabsorption. This isn't a summary of the topics rather detailed notes providing you with all the knowledge you need for your Level. They are highlighted and have diagrams for easy remembrance and as long as you can remember, you will pass your tests and Exams.
Description: These notes are for A level Students. these notes talk about the structure of the kidneys, Ultrafiltration and selective reabsorption. This isn't a summary of the topics rather detailed notes providing you with all the knowledge you need for your Level. They are highlighted and have diagrams for easy remembrance and as long as you can remember, you will pass your tests and Exams.