Lecture 19 - Nephron function Flashcards
Q: What are the key structural components of the nephron?
A:
Renal corpuscle: Includes the glomerulus (capillary network) and Bowman’s capsule (filtration site).
Proximal convoluted tubule (PCT): Reabsorbs ~70% of filtered fluid (ions, nutrients, water).
Loop of Henle: Consists of the descending limb (water-permeable) and ascending limb (solute-permeable).
Distal convoluted tubule (DCT): Important for selective secretion and absorption.
Collecting duct: Regulates final urine concentration
Q: What are the three main renal processes and where do they occur?
A:
Glomerular filtration: Occurs in the glomerulus, where blood plasma is filtered into the Bowman’s capsule.
Tubular reabsorption: Primarily in the PCT and Loop of Henle. Reabsorbs essential substances like water, glucose, and ions back into the blood.
Tubular secretion: Happens mainly in the DCT and collecting duct, where waste products like hydrogen ions (H⁺) and drugs are secreted into the filtrate.
Q: What factors influence Glomerular Filtration Rate (GFR)?
A:
Hydrostatic pressure in glomerular capillaries (55 mmHg): Drives filtration.
Osmotic pressure in capillaries (30 mmHg): Opposes filtration by pulling water back into the blood.
Fluid pressure in Bowman’s capsule (15 mmHg): Opposes filtration.
Q: What happens if hydrostatic pressure in the glomerulus decreases?
A: If glomerular hydrostatic pressure decreases, GFR decreases, reducing the rate at which blood is filtered.
Q: What is the medullary osmotic gradient and why is it important?
A:
It is a gradient of increasing osmolarity from the renal cortex to the renal medulla, crucial for water reabsorption.
The gradient is generated by the countercurrent multiplier system in the Loop of Henle and maintained by the vasa recta.
Q: How does the countercurrent multiplier work?
A:
In the ascending limb of the Loop of Henle, NaCl is actively pumped out, making the surrounding medulla more concentrated.
The descending limb is permeable to water, so water moves out into the hyperosmotic medulla.
Q: How is the gradient maintained by the vasa recta?
A: The vasa recta acts as a countercurrent exchanger, absorbing water and solutes while maintaining the gradient without washing it out.
Loop of Henle
- Thin descending limb
- Permeable to water only
- Thick ascending limb
- Permeable to solute only
- Reabsorbs ~50% of
remaining water and NaCl
Proximal tubule
- Reabsorb 60-70% of filtered volume
- Reabsorption of organic nutrients
- 99% of glucose, amino acids etc
- Active reabsorption of ions
- Na+, K+, HCO3-
- Passive reabsorption of ions
- Urea, Cl-
- Reabsorption of water
- Osmosis
- Secretion
- H+ ions
Q: What is tubuloglomerular feedback and how does it regulate GFR?
A:
TGF is a mechanism that regulates glomerular filtration rate (GFR) by sensing NaCl concentration in the filtrate.
The macula densa cells in the distal convoluted tubule (DCT) detect NaCl levels.
High NaCl: Indicates GFR is too high, so the afferent arteriole constricts, reducing blood flow to the glomerulus, lowering GFR.
Low NaCl: Indicates GFR is too low, so the afferent arteriole dilates, increasing blood flow, raising GFR.
This feedback helps maintain stable kidney function and efficient blood filtration.
- Autoregulation (Intrinsic Regulation) of GFR:
What it is: This is a local, automatic response of the kidneys that adjusts GFR to maintain constant filtration despite changes in systemic blood pressure.
Mechanisms involved:
Myogenic response: When blood pressure increases, the smooth muscles in the afferent arteriole constrict to reduce blood flow into the glomerulus, lowering GFR. When blood pressure drops, the afferent arteriole dilates to increase blood flow, raising GFR.
Tubuloglomerular feedback (TGF): The macula densa detects changes in NaCl concentration in the distal convoluted tubule and adjusts the diameter of the afferent arteriole to regulate GFR (as explained earlier).
- Hormonal Regulation of GFR:
What it is: Several hormones regulate GFR by affecting blood pressure and volume, as well as the kidney’s filtration capacity.
Key hormones involved:
Renin-Angiotensin-Aldosterone System (RAAS): When blood pressure is low, the kidneys release renin, which triggers a chain of events leading to the production of angiotensin II. Angiotensin II constricts blood vessels and stimulates aldosterone release, which increases sodium and water reabsorption, raising blood pressure and GFR.
Atrial Natriuretic Peptide (ANP): Released by the heart in response to high blood pressure, ANP dilates the afferent arteriole, increasing GFR and promoting the excretion of sodium and water, lowering blood pressure.
Antidiuretic Hormone (ADH): Increases water reabsorption in the kidneys, influencing blood volume and GFR.
- Autonomic Regulation (Sympathetic Nervous System) of GFR:
What it is: The sympathetic nervous system controls GFR in response to stress or emergencies (e.g., fight-or-flight response).
How it works:
During high-stress situations, sympathetic activation causes vasoconstriction of the afferent arterioles, reducing blood flow to the glomerulus. This reduces GFR, conserving water and maintaining blood pressure for critical organs like the brain and muscles.
In low-stress conditions, sympathetic activity is reduced, allowing normal GFR and kidney function to resume.
Q: What are the three levels of GFR regulation?
A:
Autoregulation: Local mechanisms like myogenic response and tubuloglomerular feedback maintain GFR despite changes in blood pressure.
Hormonal regulation: Hormones like RAAS, ANP, and ADH regulate blood pressure, blood volume, and GFR.
Autonomic regulation: The sympathetic nervous system can constrict afferent arterioles during stress, reducing GFR to conserve blood flow.
Proximal tubule
- Reabsorb 60-70% of filtered volume
- Reabsorption of organic nutrients : 99% of glucose, amino acids etc
- Active reabsorption of ions : Na+, K+, HCO3-
- Passive reabsorption of ions : Urea, Cl-
- Reabsorption of water : Osmosis
- Secretion : H+ ions
