The blood in a glomerulus is separated from the space inside the renal capsule by:
• the capillary wall (endothelium) which is one cell thick and has pores in it;
• the basement membrane of the wall of the renal capsule;
• the layer of cells making up the wall of the renal capsule, called podocytes; these cells
have slits between them.
The blood in a glomerulus is at a relatively high pressure, because the efferent arteriole
is narrower than the afferent arteriole. This forces molecules from the blood through
these three structures, into the renal capsule.
The pores in the capsulary endothelium and the slits between the podocytes will let all
molecules through, but the basement membrane acts as a filter and will only let small
molecules pass through.
– Substances that can pass through include water, glucose, inorganic ions such as Na+,
K+ and Cl- and urea.
– Substances that cannot pass through include red and white blood cells and plasma
proteins (such as albumen and fibrinogen).
– The liquid that seeps through into the renal capsule is called glomerular filtrate.
The cells in the walls of the tubule have many mitochondria, to provide ATP for active
transport. Their surfaces facing the lumen of the tubule have a large surface area
provided by microvilli.
Active transport is used to move Na+ out of the outer surface of a cell in the wall
of the proximal convoluted tubule, into the blood.
This lowers the concentration of Na+ inside the cell, so that Na+ ions diffuse into
the cell from the fluid inside the tubule. The Na+ ions diffuse through protein transporters
in the cell surface membrane of the cell
As the Na+ions diffuse through these transporter proteins, they
carry glucose molecules with them. This is called co-transport. The glucose molecules
move through the cell and diffuse into the blood.
The movement of Na+ and glucose into the blood decreases the water potential
in the blood. Water therefore moves by osmosis from the fluid inside the tubule, down a
water potential gradient through the cells making up the wall of the tubule and into the
blood.
Aa a result, the fluid inside the nephron now has:
no glucose
a lower concentration of Na+than the filtrate originally had
less water than the filtrate originally had
About 50% of the urea is also reabsorbed in the proximal convoluted tubule.
The loop of Henle
Some, but not all, nephrons have long loops of Henle that dip down into the medulla and
then back up into the cortex. The function of the loop of Henle is to build up a high
concentration of Na+ and CI- in the tissues of the medulla. This allows highly
concentrated urine to be produced. Note that the loop of Henle itself does not produce
highly concentrated urine.
As fluid flows down the descending limb of the loop of Henle, water moves out of it by
osmosis. By the time the fluid reaches the bottom of the loop, it has a much lower water
potential than at the top of the loop. As it flows up the ascending limb, Na+ and Cl- move
out of the fluid into the surrounding tissues, first by diffusion and then by active
transport.
This creates a low water potential in the tissues of the medulla. The longer the loop, the
lower the water potential that can be produced.
Role of loop of Henle
Creating a Salt Gradient in the Medulla
The function of the loop of Henle is to create a salt bath concentration in the fluid
surrounding the tubule
The descending limb of the loop of Henle is permeable to water, but relatively
impermeable to Na+Cl-.
The ascending limb of the loop of Henle is permeable to salts, but impermeable to
water
This means that as the loop descends into the medulla, the interstitial fluid
becomes more salty (and less salty as it ascends into the cortex)
As the vasa recta blood network that surrounds the loop flows in the opposite
direction (counter-current exchange), this further multiplies the effect.
The distal convoluted tubule and collecting duct
The fluid inside the tubule as it leaves the loop of Henle and moves into the collecting
duct has lost a little more water and more Na+ than it had when it entered the loop.
Because more water has been lost, the concentration of urea has increased.
Now, in the distal convoluted tubule, Na+ is actively transported out of the fluid.
The fluid then flows through the collecting duct. This passes through the medulla, where
you have seen that a low water potential has been produced by the loop of Henle. As
the fluid continues to flow through the collecting duct, water moves down the water
potential gradient from the collecting duct and into the tissues of the medulla. This
further increases the concentration of urea in the tubule. The fluid that finally leaves the
collecting duct and flows into the ureter is urine.
VIDEO
Homeostasis in humans
https://www.youtube.com/watch?v=4WN-cOEdF6I