Flashcards in The Kidneys and Osmoregulation (Chapter 14) Deck (59)
Describe the structure of the 4 tubes in the kidney
1) each kidney receives blood from a renal artery
2) each kidney returns blood via a renal vein
3) the ureter (narrow tube) carries urine from kidney to the bladder
4) from the bladder, the ureter carries urine to the outside of the body
Describe the structure of the kidney
- The whole kidney is covered by a fairly tough capsule, beneath which lies the cortex
- The central area is made up of the medulla
- Where the ureter joins, there is an area called the pelvis
What are the 3 main areas of the kidney?
What is the kidney made up of?
1000s of tiny tubes (nephrons) and many blood vessels
Describe the structure of the nephron
- One end of the tube forms a Bowman’s capsule (cup-shaped structure), which surrounds a glomerulus (tight network of capillaries) in the cortex
- From the capsule, tube then runs towards the centre of the kidney, first forming the PCT (a twisted region) and then the loop of Henle
- The tubule then runs back upwards the cortex, where it forms the DCT (another twisted region) before finally joining a collecting duct that leads down through the medulla and into the pelvis
Describe the two blood vessels closely associated with the nephrons
- Each glomerulus is supplied with blood by an afferent arteriole (a branch of the renal artery)
- The capillaries of the glomerulus rejoin to form an efferent arteriole which leads off the form a network of capillaries running closely alongside the rest of the nephron
- Blood from these capillaries flows into a branch of the renal vein
Describe the 2-stage process in which the kidney makes urine
1) ultrafiltration - involves filtering small molecules including urea, out of the blood and into the Bowman's capsule
2) selective reabsorption - involves taking back any useful molecules from the fluid in the nephron as it flows along
Where do molecules from the Bowman's capsule flow?
Along the nephron towards the ureter
How is the blood in the glomerular capillaries separated from the lumen of the Bowman's capsule?
By 2 cell layers and a basement membrane
Describe the structure of the Bowman's capsule
1) first cell layer = the endothelium (lining) of the capillary, which has far more gaps than in any other capillaries
2) basement membrane - made up of a network of collagen and glycoproteins
3) second cell layer (inner lining the Bowman's capsule) - formed from podocytes (epithelial cells) which have many tiny finer-like projections with gaps between them
Why are the holes in the capillary endothelium and the gaps between the podocytes quite large?
To make it easy for substances dissolved in the blood plasma to get through from the blood into the Bowman's capsule
What is the function of the basement membrane?
- To stop large protein molecules and blood cells from getting through
- ∴ the basement membrane acts as a filter
How are the concentrations of substances in the glomerular filtrate and blood plasma different?
They are almost identical, but almost no plasma proteins in glomerular filtrate
What is the glomerular filtration rate?
The rate at which the fluid filters from the blood in the glomerular capillaries into the Bowman's capsule
What is the glomerular filtration rate determined by?
- The differences in water potential between the blood plasma in the glomerular capillaries and the filtrate in the Bowman's capsule
- Water potential is lowered by the presence of solutes and raised by high pressures
Why is the water potential in the blood plasma higher than the water potential of the contents of the Bowman's capsule?
Inside the glomerular capillaries, the blood pressure is relatively high, bc the diameter of the afferent arteriole is wider than that of the efferent arteriole causing a head of pressure inside the glomerulus
Why is the concentration of solutes in the blood plasma higher than the concentration of solutes in the filtrate in the Bowman's capsule?
- While most of the contents of the blood plasma filter through the basement membrane and into the capsule, the plasma protein molecules are too big to get through and ∴ stay in the blood
- This difference in solute concentration tends to make the water potential in the blood capillaries lower than that of the filtrate in the Bowman's capsule
Why does water move down the water potential gradient from the blood into the Bowman's capsule?
Overall, the effect of differences in pressure outweighs the effect of differences in solute concentration and ∴ the water potential of the glomerulus is higher than the water potential of filtrate in the capsule
Why are substances reabsorbed into the blood as the fluid passes along the nephron?
Bc many of the substances in the glomerular filtrate need to be kept in the body
Why is the process called selective reabsorption?
Bc only certain substances are reabsorbed
Where does most of the reabsorption take place?
In the proximal convoluted tubule
What is the lining of the PCT made of?
A single layer of cuboidal epithelial cells
What do the cuboidal epithelial cells of the PCT have to make them adapted for their function of reabsorption?
1) microvilli - to increase the surface area of the inner surface facing the lumen
2) tight junctions - to hold adjacent cells together so that the fluid cannot pass between the cells
3) many mitochondria - to provide energy for the Na-K pump proteins in the outer membranes of the cells
4) co-transporter proteins in the membrane facing the lumen
5) blood capillaries very close to the outer surface of the PCT
Describe the blood in the capillaries close to the outer surface of the PCT
It has come directly from the glomerulus ∴ it has much less plasma in it than usual and has lost much of its water and many of the ions/other small solutes
What are the basal membranes of the cells lining the PCT and what is a characteristic of them?
- Those nearest the blood capillaries
- They are folded to give a large surface area for carrier proteins
What 2 kind of carrier proteins are in the basal membranes?
1) Na-K pump
2) co-transporter proteins
How does the Na-K pump allow the movement of glucose and other solutes into the cells of the PCT from the lumen?
1) Na-K pumps in the basal membranes move Na+ ions out of the cells, into the blood and these are carried away in the blood
2) this lowers the [Na+] inside the cell, so that Na+ diffuse into the cell, down their concentration gradients from the fluid in the lumen of the PCT via co-transporter proteins
3) the passive movement of Na+ into the cell provides the energy to move glucose molecules against their concentration gradient into the cell
What is the movement of glucose into the cells of PCT using Na+ an example of?
Indirect/secondary active transport, because the energy (as ATP) is used in the pumping of Na+, not in the moving of glucose
How does glucose move from the cells of the PCT into the blood?
Once inside the cell, glucose diffuses downs its concentration gradient, through a co-transporter protein in the basal membrane, into the blood