Let's learn about tubular reabsorption and secretion.
Let's begin by going over a few basic processes.
The functional unit of the kidney is the nephron.
The nephron makes urine.
Blood enters the nephron at the afferent arteriole.
The afferent arteriole brings blood to the first part of the nephron known as the glomerulus
which is the filter.
The blood is filtered by the glomerulus and the filtrate enters the proximal convoluted
tubule of the nephron.
Here substances can move back and forth between the blood in the peritubular capillaries and
the tubule.
In Tubular reabsorption--substances move from the tubule to the peritubular capillaries.
In Tubular secretion--substances move from the peritubular capillaries to the tubule.
Let's take a closer look at how this happens.
Here we see a diagram of a kidney tubule along with the cells lining it, the interstitium
and the peritubular capillaries.
Remember that in tubular reabsorption substances move from the tubule to the peritubular capillaries.
Sodium moves from the tubule into the cell by way of a cotransporter proteins that also
transport glucose, amino acids and vitamins into the cell.
These substances end up back in the blood.
These transporters are sometimes called symporters.
The tubular cells also have sodium potassium pumps that move sodium back into the interstitium.
The sodium potassium pumps use ATP to create an interstitial sodium gradient which helps
to draw sodium out of the tubule.
A very small amount of sodium moves between the cells through leaky cell junctions.
We call this paracellular movement.
Other ions move between cells as well.
These include magnesium, calcium, potassium and chloride.
Water is also reabsorbed by osmosis.
The sodium gradient works to produce an osmotic pressure to pull water out of the tubule.
Water also moves between cells (paracellular route).
Lipid soluble substances are reabsorbed by diffusion.
Remember that lipid soluble substances can move across cell membranes.
In the DCT antidiuretic hormone makes aquaporins (water channels) available for water reabsorption.
Aldosterone promotes more sodium reabsorption and potassium secretion in DCT and collecting
duct by opening sodium and potassium channels.
Hydrogen ions from the blood are secreted by the sodium-hydrogen antiporter which moves
sodium in one direction while moving hydrogen ions in the other.
The secretion of hydrogen ions works to balance blood pH.
Bicarbonate ions are also reabsorbed.
Carbon dioxide diffuses into the cell.
Once inside it combines with water to form carbonic acid which dissociates into hydrogen
and bicarbonate ions.
The bicarbonate ions are cotransported out of the cell with sodium and end up in the
peritubular capillaries.
Urea cycles through the nephron.
Urea flows into the descending limb of the nephron loop and then to the ascending limb
and distal convoluted tubule.
The Collecting duct is permeable to urea so it flows back into interstitium of medulla.
Urea can move back into the descending limb of the nephron loop because it is permeable
to urea.
This creates a cycle which works to maintain the high solute concentration gradient in
the medulla.
We hope you have learned something about tubular reabsorption and secretion and see you next
time.