15.5 Structure & function of kidney Flashcards
What are the two important homeostatic roles of the kidneys in the body?
Excretion (of urea), and osmoregulation (balancing and controlling water potential in the blood)
What are the three main areas of the structure of the kidney?
Cortex, medulla and pelvis
What is the cortex?
The cortex is the dark outer layer. This is where the filtering of the blood takes place and has a dense capillary network carrying blood from renal artery to nephrons.
- contains : glomerulus, the Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule of nephron
What is the medulla?
Medulla is lighter in colour as it contains the tubules of nephrons that form the pyramids of the kidney and collecting ducts.
- contains the loop of Henle and collecting duct of nephron
What is the pelvis?
The pelvis of the kidney is the central chamber where the urine collects before passing out down the ureter.
What are nephrons?
Nephrons are tiny tubules that make up the main functional structures of the kidneys.
In the nephrons the blood is filtered and filtered material is returned to the blood. Responsible for urine formation
What are the main structures of the nephron?
Bowman’s capsule, Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule, collecting duct
What is the Bowman’s capsule?
It is a cup-shaped structure that contains the glomerulus, a tangle of capillaries. More blood goes into the glomerulus than leaves it due to the ultra filtration processes that take place.
What is the proximal convoluted tubule?
The first coiled region of the tubule after the Bowman’s capsule, found in the cortex of the kidney. This is where many of the substances needed by the body are reabsorbed into the blood.
What is the loop of Henle?
- A long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the kidney medulla.
- The descending limb runs from the medulla to bottom of loop.
- The ascending limb runs from the bottom to the cortex.
What is the distal convoluted tubule?
- A second twisted tubule where the fine-tuning of the water balance of the body takes place.
- The permeability of the walls to water varies in response to the levels of the antidiuretic hormone (ADH) in the blood.
- The distal convoluted tubule also regulates ion balance and pH of the blood.
What is the collecting duct?
- The urine passes down the collecting duct through the medulla to the pelvis.
- The walls are also sensitive to ADH.
What is ultrafiltration?
The removal of nitrogenous waste of the blood resulting in the formation of tissue fluid in the capillary beds of the body.
What is the afferent arteriole?
The arteriole that takes blood into each glomerulus.
What is the efferent arteriole?
The arteriole that takes the filtered blood away from the glomerulus.
- it has a smaller diameter
What is the basement membrane made up of?
- It is made up of a network of collagen fibres and other proteins that make up a second “sieve”.
Describe the process of ultrafiltration.
1) The glomerulus is supplied with blood by a relatively wide afferent (incoming) arteriole from the renal artery.
2) The blood leaves through a narrower efferent (outward) arteriole and as a result there is considerable pressure in the capillaries of the glomerulus.
3) This forces liquid and small molecules in the blood out the capillary wall and in the Bowman’s capsule.
4) The liquid and small molecules pass through three layers to get into the Bowman’s capsule and enter the nephron tubule- the capillary endothelium, the basement membrane and the epithelium of the Bowman’s capsule.
5) Most of the plasma contents can pass through the basement membrane but the blood cells and many proteins are retained in the capillary because of their size.
Which three layers does the blood pass through to get into the Bowman’s capsule?
1) The capillary endothelium
2) The BASEMENT MEMBRANE
3) Epithelium of the Bowman’s capsule
What are podocytes?
Special cells on the wall of the Bowman’s capsule that act as an additional filter
What are the extensions on podocytes called? And what do they do?
Pedicels.
- These extensions wrap around capillaries, forming slits that make sure any cells, platelets, or large plasma proteins that have managed to get through the epithelial cells and basement membrane do not get into the tubule itself.
What does the filtrate usually consist of?
- Urea
- Water
- Glucose
- Salt
Why is selective reabsorption needed?
- Ultrafiltration removes urea from the blood but it also removes a lot of water along with glucose, salt and other substances.
- Many of these substances are needed by the body e.g glucose is used for cellular respiration
What substances are moved from the filtrate back into the blood in the PROXIMAL CONVOLUTED TUBULE?
- Glucose
- Amino acids
- Vitamins
- Hormones
How are the substances transported from the filtrate back into the blood by the PROXIMAL CONVOLUTED TUBULE?
By active transport.
How are sodium ions moved back by?
Active transport
How are chloride ions and water transported back?
They follow passively down concentration gradients
What are the adaptations of the proximal convoluted tubule?
- They are covered with microvilli, greatly increasing the surface area over which substances can be reabsorbed.
- They have many mitochondria to provide the ATP needed in active transport systems.
What is the concentration of the filtrate when reaching the loop of Henle at the end of the proximal convoluted tubule COMPARED to the blood & tissue fluid?
Concentration of filtrate is ISOTONIC (same) compared to concentration in tissue fluid and blood
How does the loop of Henle produce concentration gradients to move substances such as water from one area to another?
- The loop of Henle uses energy to produce concentration gradients that result in the movement of substances.
- It acts as a COUNTERCURRENT MULTIPLIER
Describe the events that occur in the loop of Henle.
1) Near the top of the ascending limb, Na+ and Cl- ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla, because theres a high concentration of urine.
2) Because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated (the ions can’t diffuse out- the descending limb isn’t permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.
3) Near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla. (The ascending limb is impermeable to water, so it stays in the tubule)
4) The first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This is essential for the kidney to produce urine that is more concentrated than the blood.
5) The fluid left in the ascending limb becomes increasingly dilute, and by the time it reaches the top of the ascending limb it is hypotonic to the blood again.
What is the role of the distal convoluted tubule?
It balances the water needs of the body
What does the permeability of the walls of the distal convoluted tubule vary upon?
Levels of ADH
How is the distal convoluted tubule adapted to its role?
The cells lining the distal convoluted tubule have many mitochondria so they are adapted to carry out active transport.
What occurs in the DISTAL CONVOLUTED TUBULE?
- If the body lacks salt, sodium ions will be actively pumped out of the distal convoluted tubule with chloride ions following down an electrochemical gradient.
- Water can also leave the distal tubule, concentrating the urine, when the walls of the tubule are permeable as a response to ADH.
What occurs in the collecting duct?
- The collecting duct passes down through concentrated tissue fluid of the renal medulla.
- Water moves out of the collecting duct by diffusion down a concentration gradient as it passes through renal medulla, and so urine becomes more concentrated.
