The mammalian kidney: function 5.2.6 Flashcards
What are the nephrons responsible for?
The formation of urine
What are the two stages of urine formation?
- Ultrafiltration
- Selective reabsorption
Where does ultrafiltration occur?
Bowmans capsule
Where does selective reabsorption occur?
Proximal convoluted tubule
Explain ultrafiltration briefly
Small molecules (such as amino acids, water, glucose, urea, and salt ions) are filters out of the blood capillaries of the glomerulus and into the Bowmans capsule to form filtrate (known as glomerular filtrate)
How is the blood in the glomerular capillaries separated?
The capillaries are separated from the lumen of the Bowmans capsule by two cell layers with a basement membrane in between them
What are the two cell layers that separate the glomerular capillaries and the lumen of the Bowmans capsule?
First = endothelium of the capillary - which are perforated by thousands of tiny membrane-lines holes
Next layer is the basement membrane which is made up of a network of collagen and glycoproteins
Second = epithelium of the Bowmans capsule, which have many tiny finger like projections with gaps in between (like claw clips) called podocytes
What is the function of podocytes
The holes allow substances dissolved in blood plasma (glomerular filtrate) to pass into the Bowmans capsule, such as amino acids, salt, glucose, water.
However red and white blood cells, and platelets remain in the blood as they are too large to pass through the holes in the podocytes
What is the function of the basement membrane
It is a layer between the endothelium and the podocytes and acts as a filter because it stops large protein molecules from getting through
What happens after the necessary reabsorption of filtrate matter (amino acids, salts, glucose etc.)
The filtrate eventually leaves the nephron and is now referred to as urine
Where does the urine flow after the kidneys?
It flows out of the kidneys, along the ureters, and into the bladder where it is temporarily stored
How is the high blood pressure in the glomerulus formed?
The afferent arteriole (entering) is wider in diameter than the efferent arteriole (leaving), resulting in high blood pressure in the glomerulus
What is the result of the high blood pressure in the glomerulus?
It causes the smaller molecules being carried in the blood to be forced out of the capillaries of the glomerulus and into the Bowmans capsule, where they form what is known as the filtrate
Why does ultrafiltration occur (what causes it)?
Due to the differences in water potential between the plasma in the glomerular capillaries and the filtrate in the Bowmans capsule
How does solute concentration affect water potential in the glomerulus and Bowmans capsule
The basement membrane allows most solutes within the blood plasma to filter into the Bowmans capsule however not large protein molecules. As a result solute concentration is higher (water potential lower) in the blood plasma of the glomerulus than in the filtrate of the Bowmans capsule.
What is the result of the high solute concentration In the glomerulus?
Water moves down the water potential gradient from the Bowmans capsule into the blood plasma in the glomerular capillaries
What is the effect of pressure gradient outweighing solute concentration?
The water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowmans capsule therefore there is an overall movement of water down the water potential gradient from the blood into the Bowmans capsule
Where does selective reabsorption occur?
In the proximal convoluted tubule
Why does selective reabsorption occur?
As many of the substances that end up in the glomerular filtrate actually need to be kept by the body
Why is it called selective reabsorption
As only certain substances are reabsorbed
How do microvilli aid reabsorption
Increase the surface area for reabsorption
What is the lining of the proximal convoluted tubule composed of?
A single layer of epithelial cells such as:
- Microvilli - increased surface area for reabsorption
- Co-transporter proteins - transports a specific solute such as glucose or amino acids across the plasma membrane
- High number of mitochondria - provides ATP for Na/K pumps in the basal membranes
- Tightly packed cells - means no fluid can pass between the cells and all substances must pass through the cells
How do many co-transporter proteins in the luminal membrane aid reabsorption
As they co-transport specific solutes such as glucose and amino acids across the membrane
How does many mitochondria aid reabsorption
Provides ATP for the Na/K pumps in the basal membranes of cells
