Kidneys Flashcards
Kidneys functions
1) To filter blood to remove nitrogenous waste - urea
2) Ultrafiltration (in glomerulus and bowman’s capsule)
3) Selective reabsorption (PCT)…?
4) Osmoregulation, the homeostatic function of regulation the water and solute composition of the blood (DCT and collecting duct)
Kidneys facts
They recieve 20-25% of the total output of the heart. They filter 170dm3 of filtrate a day.
They reclaim each day 1300g of NaCl, 180g glucose, almost all the water (180dm3) that is filtered each day.
They produce 1200-2000cm3 urine a day.
Blood supply to/from kidneys
TO each kidney via RENAL ARTERY
FROM each kidney via RENAL VEIN
Blood supply to/from the nephron
TO via an afferent arteriole, which splits into a capillary network, the glomerulus.
FROM by the efferent arteriole to two other capillary structures know as pertubular capillaries.
Pertubular capillaries
A capillary network serving the PCT and DCT and another one running alongside the loop of Henle, called the vasa recta.
Kidney nephron facts
the functional unit of the kidney - about 1 million in each kidney.
Kidney nephron structure
A nephron begins with a cup shaped structure called a renal (Bowman’s) capsules, followed by the proximal convoluted tubule (PCT) and then the loop of Henle, which leads to the distal convoluted tubule (DCT) and finally joins to the collecting duct.
What are the two parts of the kidney nephron?
Cortex (bit on outside of kidney) and medulla (bit on inside of kidney).
Difference between PCT and DCT
PCT has lots of microvilli and DCT does not.
Loop of Henle facts
Descending (going down part) loop has thinner walls than the ascending loop (going up part).
PCT
Proximal convoluted tubule - longest part of the renal tubule. It has a simple tall cuboidal epithelium, with a brush border (of microvilli). The epithelium almost fills the lumen and the microvillli increase the SA by 30-40 fold.
Loop of Henle
This has a thick descending portion (pars recta), a thin descending portion, a thin ascending portion and a thick ascending portion. The lumen is made up of simple squamous epithelium. Hard to tell apart from adjacent capillaries, except that there are no red blood cells in the lumen.
DCT
Distal convoluted tubule - these tubules are less numerous than the PCTs. The epithelial cells are cuboidal, with very few microvilli. The cells stain more palely than those of the PCT.
Collecting tubules
NOT PART OF THE NEPHRON - The eptihelium of these tubules consist of cuboidal or columnar cells. They empty into collecting ducts that are easy to recognise because they have large lumens, with pale staining columnar epithelium.
Ultrafiltration
It is the filtration of small molecules - e.g. water, glucose, urea, amino acids, ions - from the blood plasma into the Bowman’s capsule (occurs here), when under pressure.
Selective filtration
PART OF ULTRAFILTRATION - High filtration pressure in glomerulus pushes out small molecules from the capillaries into the lumen of the Bowman’s capsule, through 3 layers which together form a selective barrier. Any molecules in the blood can cross this barrier, except large plasma proteins and blood cells.
How is there a high filtration pressure in the glomerulus?
High blood pressure in renal artery.
Difference in diameter of the afferent and efferent arterioles - the afferent is much wider, going into the glomerulus.
What are the 3 layers of the selective barrier between the lumen of the capillaries and that of the Bowman’s capsule?
1) Endothelium of the capillaries - contains pores called fenestrae.
2) Capillary basement membrane - main part of the selective barrier, sometimes called ‘molecular sieve’. It is an extra-cellular layer of proteins that only lets very small molecules through.
3) Podocytes - kidney cells that wrap around the capillaries of the glomerulus and have large gaps between them.
Describe the basic process of ultrafiltration
High filtration pressure in glomerulus pushes out small molecules from the capillaries into the lumen of the Bowman’s capsule. By the time the blood leaves the glomerulus via the efferent arteriole, it has a much lower water potential.
Why does blood have a much lower w.p. post-ultrafiltration?
So much water has been lost to the filtrate, that there is now a high concentration of proteins and red blood cells in the blood. High solute concentration so low water potential.
Middle, cloudy bit of a cross section of a kidney
Renal pelvis - carries urine down to ureters…?
What happens in selective reabsorption?
All the important molecules that the blood plasma has lost to the filtrate (but needs to retain), i.e. glucose, amino acids, ions (like Na+ and Cl-), water and vitamins, are reabsorbed.
Where does selective reabsorption happen?
PCT - that’s why it has all those microvilli! At the end of the PCT, the filtrate will be isotonic (equal w.p.s) with the blood plasma.
Water reabsorption details in PCT
About 90% of water is reabsorbed by OSMOSIS. This happens because the ions are removed from the filtrate, causing the w.p. to increase compared with that of the blood. Therefore, water moves back into the blood, down a water potential gradient.
