Osmoregulation and Excretion in Humans part 3

Question 1

Overview

Answer 1

• While much of the absorption and secretion occurs passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. This control is exerted by hormones (directly by ADH and aldosterone, indirectly by renin and angiotensin)
• Most water is recovered in the PCT, loop of Henle, and DCT. But about 10% (about 18 L) reaches the collecting ducts. The collecting ducts, under the influence of ADH, can recover almost all of the water passing through them, in cases of dehydration, or almost none of the water, in cases of over-hydration

Question 2

What do we know?

Answer 2

-Normally PCT reabsorbs all of the glucose and amino acids; about 65% of Na+ and water

• Descending limb of loop only permeable to water
• Ascending limb only permeable to solutes, actively and passively
• Hormones fine-tune reabsorption in DCT and collecting duct
• Most of the filtered water and solutes have bene reabsorbed before DCT

Question 3

Osmoregulation: Reabsorption and secretion in the loop of henle

Answer 3

• The descending and ascending portions of the loop are highly specialized to enable recovery of much of the Na+ and water that were filtered by the glomerulus
• As the forming urine moves through the loop, the osmolarity will change from isosmotic to a very hypertonic solution to a very hypotonic solution
• Solutes and water recovered from these loops are returned to the circulation by way of the vasa recta

Question 4

Solvents

Answer 4

-A solvent is a substance that can dissolve a solute

Question 5

How are solutes measured

Answer 5

In weight

Question 6

How are solvents measured

Answer 6

weight or volume

Question 7

Osmolarity units

Answer 7

Osmoles of solute per liter of solution

In the body, were generally measuring milliosmoles

Question 8

Osmolality

Answer 8

Omsoles of solute per kilogram of solvent

Question 9

Osmolarity vs osmolality

Answer 9

They are NOT interchangeable but since we are looking at the relative differences, we can use either as long as were consistent with our terms

For dilute solutions, the difference between osmolarity and osmolality is insignificant

Question 10

Where do nephrons create an osmotic gradient

Answer 10

Within the renal medulla

Question 11

Long nephron loops and the gradient

Answer 11

The long nephron loops of juxtamedullary nephrons create the gradient

Act as countercurrent multipliers

Question 12

The countercurrent multiplier depends on three properties of the nephron loop to establish the osmotic gradient

Answer 12

• Filtrate flows in the opposite direction (countercurent) through the two adjacent parallel sections of a nephron loop
• The descending limb is permeable to water, but not to salt
• The ascending limb is impermeable to water, and pumps out salt

Question 13

Positive feedback cycle that uses the flow of fluid to multiply the power of the salt pumps

Answer 13

1. Salt is pumped out of the ascending limb
2. Increase in intestinal fluid osmolality
3. Water leaves the descending limb
4. Increase in osmolality of filtrate in descending limb
5. Increase in osmolality of filtrate entering the ascending limb

Question 14

Vasa recta and the gradient

Answer 14

The vasa recta preserves the gradient and they act as countercurrent exchangers.

Question 15

Collecting ducts

Answer 15

use the gradient to adjust urine osmolality

Question 16

Cycle of water and solutes

Answer 16

(As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it)

1. Filtrate entering the nephron loop is isosmotic to both blood plasma and cortical interstitial fluid
2. Water moves out of the filtrate in the descending limb down its osmotic gradient. This concentrates the filtrate
3. Filtrate reaches its highest concentration at the bend of the loop
4. Na+ and Cl- are pumped out of the filtrate. This increases the interstitial fluid osmolality
5. Filtrate is at its most dilute as it leaves the nephron loop. At 100 mOsm, it is hypo-osmotic to the interstitial fluid

Question 17

Glomerular filtration rate

Answer 17

amount of filtrate produced per minute

Average: 125 mL/minute

Question 18

GFR is based on three pressures present in glomerulus

Answer 18

Blood hydrostatic pressure (BHP), colloid osmotic pressure (COP), and capsular pressure (CP)

Question 19

Blood hydrostatic pressure (BHP)

Answer 19

Blood pressure in the glomerulus (out)

Question 20

Colloid osmotic pressure (COP)

Answer 20

Pressure exerted by proteins in the blood (in)

Question 21

Capsular pressure (CP)

Answer 21

Pressure exerted by filtrate in the glomerular capsule (in)

Question 22

Blood pressure and GFR

Answer 22

• Adequate blood pressure (BHP) is required to maintain GFR (Nephron action can alter blood volume, thereby altering pressure)
• GFR needs to be controlled or can lead to kidney failure

Question 23

Too high GFR

Answer 23

Excessive urination, damage to kidneys

Question 24

Too low GFR

Answer 24

Inadequate filtration, build up of toxins

Question 25

How is resting homeostasis in healthy humans maintained when GFR is too high

Answer 25

Too high from blood pressure

Cardio/vasomotor mechanisms try to lower blood pressure/GFR

Question 26

How is resting homeostasis in healthy humans maintained when GFR is too low

Answer 26

From low blood pressure

Cardio/vasomotor mechanisms try to raise blood pressure/GFR

Hormonal regulation

Question 27

Hormones

Answer 27

Hormones are chemicals in the body that cause changes to other tissues in the body

Question 28

What are the different hormones involved in raising GFR when blood pressure is low

Answer 28

• Renin and angiotensin (these work together)
• Aldosterone (The salt hormone)
• Antidiuretic Hormone (ADH)

Question 29

Where is the juxtaglomerular apparatus (JGA) located

Answer 29

At the junction of the DCT and afferent arteriole

Question 30

Juxtaglomerular apparatus (JGA)

Answer 30

Cells that monitor and adjust GFR

Consists of Juxtaglomerular (JG) cells and Macula densa cells

Question 31

Juxtaglomerular (JG) cells

Answer 31

• In the afferent arteriole (blood vessel)

- Secrete renin in response to signal rom macula densa cells

Question 32

Macula densa cells

Answer 32

• In the distal convoluted tubule (nephron)

- Monitor urine in DCT and tell JG cells how fast the urine is moving

Question 33

How do macula densa cells work

Answer 33

1. When GFR decreases, filtrate moves more slowly through the nephron
2. The slower movement of filtrate causes more NaCl to be reabsorbed into the blood from the nephron loop
3. The filtrate then becomes more dilute compared to normal (because NaCl has moved out)
4. Macula densa cells detect the dilute filtrate and respond by triggering JG cells to release renin

Question 34

Hormonal regulation in response to low blood pressure

Answer 34

• Renin released by JG cells
• Renin activates a protein called angiotensinogen in blood which is a precursor to angiotensin I
• Angiotensin I is activated to angiotensin II by ACE, primarily in the lungs

Question 35

Angiotensin II

Answer 35

• Results in increased blood pressure
• Widespread vasoconstriction in body
• Stimulates thirst (target hypothalamus)
• Stimulates secretion of aldosterone
• –Stimulates reabsorption of sodium ions in the DCT and collecting duct
• –Water follows because of osmosis, more water in blood increases pressure
• Stimulates secretion of antidiuretic hormone (ADH)
• –More water reabsorption

Question 36

Antidiuretic hormone (ADH)

Answer 36

• Produced by hypothalamus in response to dehydration
• Causes the production of aquaporins in collecting duct
• Allows for uptake of water faster
• Production of aquaporins decreases as rehydration occurs
• Alcohol blocks ADH production