17. Chapter 20- Fluid and Electrolyte Balance

Question 1

What are the 4 things to maintain for the homeostatic mechanisms to maintain fluid/electrolyte balance?
How much NaCl is taken in the body everyday?

Answer 1

1. Fluid volume
2. Osmolarity
3. Concentrations of individual ions
4. pH

Body is in a state of constant flux
We ingest 2L of fluid with 6-15 grams of NaCl
Whatever comes in must be excreted if not needed (kidneys are primary route) small amount lost in feces and sweat

Question 2

What do H2O and Na+, K+, Ca2+, and H+ and HCO2- do for the body?

Answer 2

H2O and Na+ determine ECF volume and osmolarity (ECF osmolarity affects cell volume, Slide 5 Feb 1)
K+ balance can cause problems with cardiac and muscle function
Ca2+ is involved in many processes in the body
H+ and HCO3- determine the body pH

Question 3

What are the 4 systems that are involved in the balance of fluids and electrolytes?

Answer 3

1. Respiratory
2. Cardiovascular
   These two are under neural control and are quite rapid
3. Renal
   These occur more slowly since kidneys are primarily under endocrine and neuroendocrine control
4. Behaviour responses

Slide 6 Feb 1 overview of what’s to come

Question 4

How is water balanced in our body’s?

Answer 4

Water intake must match water excretion
Water weight makes up to 50%-60% of our body weight (females 50)
Some things change this like sweating which uses lots of water and few ions which changes osmolarity
Slide 8 Feb 1

Question 5

How do kidneys conserve water?

Answer 5

They can remove excess fluid or conserve what is in the body but cannot replace what is lost to the environment
Volume gain is offset with increase loss
Kidneys filter using the handle on slide 9 Feb 1

Question 6

What kind of urine does the renal medulla make?

What is diuresis?

Answer 6

Renal medulla creates concentrated urine
The concentration of urine (osmolarity) is a measure of how much water is excreted by the kidneys
When removal of excess water is required, the kidneys produce large volumes of dilute urine
Diuresis is the removal of excess urine
If kidneys need to preserve water, low volume of concentrated urine is produced
Slide 11 Feb 1

Question 7

How do the kidneys control urine concentration?

Answer 7

Vary the amounts of water and Na reabsorbed in the distal nephron

Question 8

What is vasopressin and what does it control?

Answer 8

Vasopressin is the posterior pituitary hormone (AVP or ADH)
That controls water reabsorption
Slide 12 Feb 1

Question 9

How does AVP (vasopressin) use AQ12 to increase osmosis of water and create concentrated urine?

Answer 9

Slide 13 Feb 1 picture

Question 10

What are the 3 things that activate osmoreceptors?

What is the most potent stimulus of AVP (vasopressin) secretion?

Answer 10

Blood volume, pressure, and osmolarity activate osmoreceptors
Increased osmolarity is the most potent stimulus of AVP
slide 14 Feb 1

Question 11

What are the 4 steps of AVP (vasopressin) being produced and secreted?

Answer 11

Slide 15 Feb 1

Question 12

What is the counter current multiplier?
What creates the hyperosmotic interstitium and why isn’t it reduced as water is reabsorbed? (What is countercurrent exchange systems and Urea, 2 things)

Answer 12

The loop of henle

1. Counter current exchange systems- evolved in mammals and birds to reduce heat loss from flippers tails wings that are poorly insulated and have high surface area to volume ratio
2. Urea contributes to hyperosmotic interstitium

Question 13

What is the countercurrent multiplier and the counter current exchanger?
Main jobs of each?

Answer 13

Countercurrent multiplier- loop of henle is referred to as this
Main job is to create the hypertonic interstitium
Countercurrent exchanger- peritubular capillaries (vasa recta) are refereed to as this
Main job is to prevent the washout (dilution) of the hypertonic interstitium

Slides 17-18 Feb 1

Question 14

What are the 3 steps to active transport in the loop of henle (slide 19 Feb 1)

Answer 14

Slide 19 Feb 1

Question 15

Why doesn’t water entering interstitium via the descending limb dilute the hypertonic medulla?

Answer 15

The opposite direction loop of the vasa recta picks some solute up and loses some water as it travels by the ascending limb creating hyperosmotic blood

Question 16

How does urea contribute to the osmolarity of medullary interstitium?

Answer 16

Half the solute in the medulla interstitium is urea
The high solute concentration in the medulla is only partly due to NaCl
Large amount of urea is reabsorbed in the distal portion of the nephron and creates a recycling loop
Slide 1 Feb 4

Question 17

What are the two things water balance depends on? (Summary of water balance)

Answer 17

1. Hyper osmotic medullary interstitium- counter current exchange/multiply
   Urea
2. AVP mediated insertion of water pores in the collecting duct (AQP2)- driven mainly by plasma osmolarity (osmolarity up then AVP release up)
   High AVP increases AQP2 insertion= increased reabsorption
   Low AVP decreases AQP2= reduced absorption
   Always some level of AVP release

Question 18

How do we calculate how much water we need to consume if our ECF [NaCl] was increased to 155mosmol/litre and we wanted it to stay at 140mosmol/litre?

Answer 18

Slide 3 Feb 4

Question 19

Is Cl absorption regulated when NaCl enters?

Answer 19

No it tends to follow through the electrochemical gradient set up by Na+ movement or cotransported with Na

Question 20

What is aldosterone?

What does it target?

Answer 20

Helps control Na+ balance
The renin-angiotensin-aldosterone system (slide 9 Feb 4) regulates blood Na levels
Aldosterone is a steroid hormone responsible for altering Na+ reabsorption and K excretion

Targets last third of the distal tubule and the portion of the collecting duct located in the cortex of the kidney

Slide 6 Feb 4

Question 21

What are the 2 things aldosterone does once it binds to cytoplasmic mineralocorticoid receptor in P cells?

Answer 21

1. Increases Opening of apical Na+ and possibly K+ channels (increase Na entry to cell speeds up basolateral Na-K pump which increase Na reabsorption)
2. Hormone ligand complex translocate into the cell nucleus, bonds hormone response elements that increase transcription of apical Na channels and basolateral Na-K pumps

Question 22

What is angiotensin II?

Answer 22

Triggers aldosterone release once blood pressure decreases and controls aldosterone secretion

Increased osmolarity inhibits release of aldosterone
Large drops in plasma Na+ directly stimulate aldosterone secretion as well

Question 23

What are the 3 stimuli of the renin-angiotensin pathway?

Answer 23

1. Low blood pressure on renal arterioles (makes granular cells secrete renin)
2. Sympathetic neurons activated by CVCC when blood pressure decreases terminate on granular cells (stimulates renin secretion)
3. Paracrine feedback from macula densa cells signal to granular cells to secret renin

Slide 6 Feb 6

Question 24

What is renins main goal?

Answer 24

Concert angiotensinogen (inactive plasma protein) into angiotensin I

Angiotensin I is then converted to active form of angiotensin II (by enzyme produced in blood vessel endothelium

Then travels to adrenal cortex and stimulates production of aldosterone

Question 25

What are the 5 effects of angiotensin II?

What inhibits angiotensin II?

Answer 25

1. Increases vasopressin secretion
2. Stimulates thirst
3. Potent vasoconstrictor
4. Increase sympathetic output to heart and blood vessels
5. Increases proximal tubule Na+ reabsorption

All help restore blood pressure

ACE inhibitors prevent ANG I converting to ANG II to keep low blood pressure

Question 26

What promotes Na and water excretion?

Answer 26

Atrial natriuretic peptide (ANP)
increased blood volume stretches atrial walls during filling

This does complete opposite of what angiotensin II does

Slide 12 Feb 6

Question 27

What are the effects of atrial natriuretic peptide on the kidneys, hypothalamus, adrenal cortex, and medulla?

Answer 27

Kidneys- relaxes afferent arterioles, reduces renin release, reduces Na reabsorption at collecting duct
Hypothalamus- reduces AVP release
Adrenal cortex- inhibits aldosterone release
Medulla- acts on CVCC to decrease blood pressure

Question 28

How does potassium concentrations correlate with aldosterone release?

Answer 28

Low [K+] decreases aldosterone release, which reduces secretion, which reduces excretion
High [K+] stimulates aldosterone release directly and increases secretion and this excretion

K+ plays a huge role in setting the testing membrane potential so needs to be monitored with small range

Question 29

How do behavioural responses aid salt and water balance?

Answer 29

Behavioural responses are critical in restoring the normal state, especially when ECF volume decreases or osmolarity deviates
Drinking water is normally the only way to replace lost water and eating salt is only way to raid bodies Na content
(The act of drinking relieves thirst, water doesn’t have to be absorbed, receptors in mouth respond to water by decreasing thirst and decreasing AVP release)
Slide 2 Feb 8

Question 30

What are avoidance behaviours?

Answer 30

Help prevent dehydration
Avoid heat of day and become active at night in some areas
Midday nap keep wools indoors during hottest part of the day

Question 31

Study table slide 4 Feb 8

Answer 31

Okay

Question 32

What is severe dehydration and how is the loss in ECF volume, decreased blood pressure, and increased osmolarity restored? (Three things)

Answer 32

1, conserving fluid to prevent additional loss

2. Trigger cardiovascular reflexes to increase blood pressure
3. Stimulate thirst so normal fluid volume and osmolarity can be restored

Decrease ECF volume signals increase aldosterone but increased osmolarity inhibits it at the same time

Question 33

What 5 things result from the body fighting severe dehydration?

Answer 33

1. Carotid and aortic baroreceptors signal CVCC (heart rate goes up as SA node controls shifts, ventricular contractions increases, sympathetic input to arterioles increases)
2. Decreased blood pressure directly decreases GFR
3. Paracrine feedback at macula densa cells causes granular cells to release renin
4. Granular cells respond to decreased blood pressure by releasing renin
5. Decreased blood pressure, volume, increased osmolarity, and increased ANGII all stimulate vasopressin and the thirst centres and hypothalamus