4 - Renal Physiology II

Question 1

How much water is reabsorbed along the entire length of the nephron?

Answer 1

600-1200 mOsm/L

Question 2

Where along the length of the nephron is water reabsorbed?

Answer 2

Everywhere EXCEPT the ascending loop of Henle

• Proximal convoluted tubule
• Descending tubule
• Distal convoluted tubule
• Collecting duct

Question 3

Describe kidney conservation of water by excreting concentrated urine

Answer 3

The ability of the kidney to form urine that is more concentrated than the plasma is essential for survival of mammals that live on land

The basic requirements for forming a concentrated urine are:

• A high osmolarity of the renal medullary interstitial fluid
• A high level of antidiuretic hormone (ADH) also called vasopressin

Question 4

Describe the counter multiplier system in the loop of Henle for producing a hyperosmotic renal medulla

Answer 4

Loop of Henle is responsible for counter current multiplication which is necessary for concentration or dilution of urine

• Co-transport of Na+ and Cl- and other ions out of the thick ascending limb of the loop of Henle into the medullary interstitium
• Active transport of ions from the collecting ducts into the medullary interstitium
• Facilitated diffusion of large amount of urea from the medullary collecting ducts into the medullary interstitium
• Diffusion of only small amount of water from the medullary tubules into the medullary interstitium, far less than the re-absorption of solutes into the medullary interstitium

IMPORTANT ***

Question 5

Describe how the vasa recta can serve the purpose of counter current exchanges

Answer 5

• The vasa recta serve as counter current exchangers minimizing washout of solutes
  from the interstitium
• The U-shaped structure of the vessels minimizes loss of solute from the interstitium

Question 6

Describe the function of antidiuretic hormone

Answer 6

• An increase in fluid osmolality, plasma sodium concentration caused osmoreceptor cells to shrink
• Shrinkage of the osmoreceptor cells signals the supraoptic nuclei that release ADH
• In the kidney, ADH controls the degree of dilution or concentration of the urine

Question 7

Describe the ADH feedback mechanism

Answer 7

• Regulation of extracellular fluid osmolarity in response to a water deficit
• Water is conserved in the body while sodium and other solutes continue to be excreted in the urine
• Opposite sequence of events occurs when the extracellular fluid become too dilute (hypo-osmotic)
• With excess of water ingestion and decrease
  in extracellular osmolarity, less ADH is formed, the renal tubules decrease their
  permeability, less water is re-absorbed and a large volume of urine is formed

Good flow chart on slide 44

Question 8

Describe the relative osmolality of the tubule fluid along the nephron

Answer 8

• Proximal = urine is same as plasma
• Loop = more concentrated than plasma as it loses a lot of water in the descending limb, then becomes less concentrated than plasma when it comes back down due to losing ions in the ascending limb
• Classic distal tubule = starts off less concentrated than plasma (due to all the ions reabsorbed in the ascending limb), but then the concentration depends on ADH (hydration level)
• This difference in concentration based on level of hydration continues through to the excretion of urine
• If the body is dehydrated, it will reabsorb more water and urine will be concentrated
• If the body is very hydrated, it will secrete more water, making the urine dilute
• This variance is seen in the cortical collecting duct, medullary collecting duct, and urine excretion

Question 9

What is the role of thirst in controlling extracellular fluid (ECF) osmolarity?

Answer 9

• Fluid intake is regulated by thirst mechanism
• The hypothalamus contains thirst osmoreceptors that sense ECF osmolarity
• These cells stimulate other hypothalamic neurons causing thirst and increased water intake in response to hyperosmolality

Question 10

Describe the stimuli for thirst

Answer 10

• Large decreases in circulating blood volume and/or blood pressure
• Hemorrhage
• Dryness of the mouth and throat
• Thirst mechanisms are temporarily suppressed after drinking water. If insufficient water was drunk the person becomes thirsty again until the ECF osmolarity is returned to normal

Question 11

What are the mechanisms of regulation of reabsorption and secretion

Answer 11

1. Local Control (renal auto-regulation)
2. Hormonal Control
3. Nervous Control

Question 12

Describe renal autoregulation (local control)

Answer 12

The kidney regulates fluid and electrolytes (sodium chloride) homeostasis by modulating the rapport between glomeruli and tubules

• The tubules respond to glomeruli with the Glomerulotubular Balance (GTB)
• The glomeruli respond to tubules through the Tubuloglomerular Feedback (TGF)

Question 13

What do the GTB and TGF feedback systems aim to stabilize?

Answer 13

• single nephron GFR

- distal fluid and electrolytes delivery

Question 14

Describe the function of GTB

Answer 14

Glomerulotubular Balance (GTB)
- GTB enables the function of a tubule to compensate automatically for the fluctuation in filtration rate in the glomerulus to which it is attached to

GTB is achieved by tubules via

• changes in hydrostatic and colloid osmotic forces
• luminal factors

Question 15

Describe a calculation of GTB

Answer 15

GFR is about 180 L/day and tubular reabsorption is 178.5 L/day, leaving 1.5L/day of fluid to be excreted in the urine.

An increase in GFR to 225 L/day with no tubular reabsorption compensation would determine an urine flow of 46.5 L/day

This is why it is so important

Question 16

Describe the Tubuloglomerular Feedback (TGF) system

Answer 16

Fluctuation in sodium chloride delivery to the distal tubule sensed via the juxtaglomerular apparatus controls GFR by a feedback mechanism acting on renal arteriolar resistance:

• afferent arteriole
• efferent arteriole

The juxtaglomerular complex consists of macula densa cells in the initial portion of the distal tubule and juxtaglomerular cells in
the wall of the arterioles

Question 17

Explain the TGF system further

Answer 17

The kidney responds to changes in sodium chloride concentration with the control of renal arteriolar resistance

Fluctuation in sodium chloride delivery to the distal tubule sensed via the juxtaglomerular apparatus controls GFR by two feedback mechanisms acting on:

• afferent arteriolar
• efferent arteriolar

The juxtaglomerular complex consists of macula densa cells in the initial portion of the distal tubule and juxtaglomerular cells in the wall of the arterioles

Question 18

Describe hormonal control in the renal system

Answer 18

Precise regulation of body fluid volumes and electrolytes concentrations requires the kidney to excrete different solutes and water at variable rates, sometimes independently of one another

Question 19

What hormones provide this specificity of tubular re-absorption and secretion?

Answer 19

• Antidiuretic hormone (ADH), also called vasopressin
• Renin
• Angiotensin II
• Aldosterone
• Atrial natriuretic peptide

Question 20

Describe the mechanisms of renin release

Answer 20

Baroreceptor mechanism (increased pressure in afferent arterioles inhibits renin release and vise versa)

Sympathetic nerve mechanism (beta-adrenergice nerves stimulate renin release)

Macula densa mechanism (increases NaCl in distal nephron, inhibiting renin release and vise versa)

Question 21

Describe angiotensin II

Answer 21

• Angiotensin II is part of the renin-angiotensin system and is a powerful sodium-retaining hormone
• Angiotensin II formation is regulated by changes in blood
  pressure and extracellular fluid volume

Question 22

What are the main effects of angiotensin II

Answer 22

Angiotensin II increases sodium and water re-absorption and helps to return blood pressure and extracellular fluid volume
to normal by three main effects:
- It constricts the efferent arterioles
- It stimulates aldosterone secretion
- It directly stimulates sodium tubular re-absorption

Question 23

Describe aldoserone

Answer 23

• Aldosterone is secreted by the zona glomerulosa cells of the adrenal cortex
• Aldosterone acts to the cells of the cortical collecting tubule to increase Na+ re-absorption and K+ secretion
• Impaired aldosterone secretion associates with defects in regulation of Na+ & K+ concentration: adrenal destruction or malfunction (Addison’s disease); adrenal tumors (Conn’s syndrome)
• Na+ and K+ intakes affect the level of aldosterone
• High Na+ intake decreases while high K+ intake increases plasma aldosterone***

Question 24

What happens to aldosterone levels if you consume excess Na+?

Answer 24

If you eat salty fries, you will decrease aldosterone production

Question 25

What will happen to aldosterone levels if you consume excess K+? ***

Answer 25

If you have high potassium intake, you will increase aldosterone production

Question 26

Describe the funtion of atrial naturetic peptide

Answer 26

- Atrial natriuretic peptide is secreted by cardiac atria cells in volume expansion conditions - Increased level of atrial natriuretic peptide inhibits re-absorption of sodium and water by renal tubules, especially by the collecting ducts - Decreased sodium and water re-absorption increases urinary excretion and helps the blood volume to return to normal

Question 27

Describe the nervous system control

Answer 27

- Kidney blood vessels are richly innervated by sympathetic nerve fibers - Renal sympathetic nerve activity is fundamental during severe, acute disturbances (e.g. severe hemorrhage, brain ischemia, defense reaction)

Question 28

An increase in activity of renal sympathetic nerves results in an increase of sodium and water due to which factors?

Answer 28

- Reduction in GFR by constriction of renal arterioles - Increase in tubular sodium re-absorption - Increase in renin release and angiotensin II formation, which further stimulates tubular sodium re-absorption

Question 29

Give a summary of the influences of the kidneys

Answer 29

Three factors that directly influence the kidneys - RBC production (erythropoiesis) - ECF volume - Total peripheral resistance The first two (RBC production and ECF volume) influence blood volume, which goes on to influence BP Total peripheral resistance (the last direct influence on the kidneys) also directly effects BP