Lecture 11 - Fluid Balance

Question 1

how is water distributed in the body?
percent and fraction based

Answer 1

about 55% of our body holds water — varies with sex, age, mass

2/3 is in the ICF
1/3 is in the ECF –> 75% in the interstitial fluid and 25% in the plasma

Question 2

What are the average daily water gains and losses? provide a value

Answer 2

• water gain: intake and metabolic production 2.5 L (food, drink)
• water loss: 2.5L (urine, sweat, lungs, feces)

Question 3

what is water homeostasis? excessive and little water loss causes and effects

Answer 3

• water homeostasis is when the water gain and water loss when subtracted should equal 0. 2.5L - 2.5L = 0

excessive water loss disrupts homeostasis:
- leads to less ECF which decreases blood pressure
- this can make you confused, have chest pain, make no urine, cause disease (hypertension and shock)

no water loss ie. too much water can:
- back up lungs, legs and abs
- difficulties in breathing, walking

Question 4

when would we change the amount of urine produced?

Answer 4

• urine production is crucial for water homeostasis
• we change the amt of urine produced when we loose a lot of water for ex. in bleeding and diarrhea

Question 5

how is urine formed?

Answer 5

• we have blood that is filtered through the bloodstream into the kidneys
• ends up in tiny sections called nephrons: this is where the blood plasma is filtered
• the filtrate is what produces the urine and leaves through the collecting duct, pelvis, ureter, bladder, urethra, and then out of the body

Question 6

how does the nephron filtrate, reabsorb, and secrete filtrate to produce urine?

how much filtrate passes through?
how much urine is produced?

Answer 6

• the kidney acquires blood (around 1584 L a day) through the afferent arteriole
• the glomerulus in the nephron filters this blood (180L a day) as its collected by the bowman’s capsule
• as this filtrate travels down the tubule, a lot of this filtrate is reabsorbed in the blood stream
• contrairly, some is secreted from the blood stream back in the nephron tubule (depends on conditions)
  filtrate - absorbed + secretion = amt of solute excreted via urine
• around 1.8L of urine is made a day
• this is 1% of the amt of filtrate produced – this is because A LOT of filtrate is reabsorbed in the blood stream

Question 7

what physiological processes is the nephron/ kidney responsible for? (5)

Answer 7

• Excreting waste
• Regulates blood volume
• Controls electrolytes
• Blood pH
• Vitamin D!!!!! (via PTH and calcitrol)

Question 8

what is the collecting duct responsible for

Answer 8

• responsible for the fine tuning of how much urine is produced
• also where a lot of hormones will work to produce their final effect to maintain a physiological process to maintain homeostasis!

Question 9

what are the three hormone that regulate water, Na+, and K+ balance in the kidneys?
list what each hormone does

Answer 9

Vasopressin/ADH:
inc. water reabsorption in blood stream –> concentrated urine in collecting duct

Aldosterone:
inc. Na+ reabsorption
inc. K+ secretion
thus inc. water reabsorption in blood stream –> concentrated urine in collecting duct

Atrial natriuretic peptide:
dec. Na+ and water reabsorption
inc. K+ reabsorption
thus dec. water reabsorption in blood stream –> diluted urine in collecting duct

Question 10

Vasopressin/Antidiuretic hormone/ADH:
- does it promote or demote concentrated urine?
- where is it synthesized
- where is it secreted

Answer 10

• promote concentrated urine because water intake is more but that water goes into the blood stream leaving little water in the urine
• synthesized in the hypothalamus
• secreted by the posterior pituitary into the blood circulation (posterior cannot make hormone, just releases them)

Question 11

what is the primary function of ADH

Answer 11

• Increase water reabsorption in the kidneys
  – Conserves body water
  – needed for water balance and fluid homeostasis
  – Increases blood volume and blood pressure

how does it increase blood pressure:
- it increases water reabsorption in the blood stream –> inc. blood volume –> inc. blood pressure
→ regulates permeability of cells in the kidney
- Increased permeability = increased reabsorption and low urine output

Question 12

how does osmolarity regulate ADH release?
1st stimuli = plasma osmolarity

Answer 12

high plasma osmolarity = inc. ADH
- high osmolarity –> high ions and less water –> need to dilute this –> reabsorb water from nephron to bloodstream –> release ADH
- high plasma osmolarity, is detected by osmoreceptors in the hypothalamus –> trigger the release of ADH from the posterior pituitary –> travel to the kidneys –> collecting ducts of nephrons –> increase water reabsorption in bloodstream

Question 13

how does osmolarity regulate ADH release?
2nd stimuli = low blood pressure

Answer 13

• low blood pressure = release of ADH
• detected by atria walls of the heart as well as special barrell receptors in the aotric and carotid arteries
• also stimuli for ADH release but osmolarity is more important

Question 14

what is a side effect to plasma vasopressin/ADh release and concentration

Answer 14

• when plasma osmolarity is in the normal range the ADH levels are low ish like normal
• but as soon as osmolarity crosses that normal range, there is a huge increase in ADH thus osmolarity is the most potent stimulus for ADH release
• high levels of osmolarity and ADH can lead to thirst - very concentrated water

Question 15

where does the vasopressin/ADH go when its released from the hypothalamus to bloodstream to increase water reabsorption in the bloodstream (6 steps)

Answer 15

1. vasopressin travels all the way from the hypothalamus to the pituitary to the blood stream
2. as it travels through the blood stream it has to find it proper receptor, which are on the membrane of the collecting duct
3. once it binds to the receptor, there is a signal cascade
4. results in the insertion of ADH in aquaporin-2 water pores (like vesicles in the collecting duct cells) which are then going to be exocytosed into the apical membrane (membrane that connects the collecting duct lumen and cells), where the urine filtrate is
5. when aquaporin pores are fused, there is a lot more water that can enter the cell from the collecting duct lumen (water is reabsorbed from the filtrate into the cell – not the aquaporin) .
6. water from the collecting duct lumen travels back through the collecting duct cell, interstitial fluid and then through the blood

thus water is reabsorbed by the bloodstream because of the ADH which allowed the aquaporin-2 water pores to fuse w the collecting duct
- by reabsorbing water we lower the osmolarity back to homeostatic level

Question 16

aldosterone:
- what type of molecule is it
- where is it synthesized
- what does it regulate
- what are its main brief functions

Answer 16

• steroid
• synthesized in adrenal cortex that is above the kidneys
• regulates sodium and thus water homeostasis

Functions:
1) Na+ Reabsorption (retain water/ water reabsorption in the bloodstream)
2) K+ Secretion
3) Acts on distal tubule and collecting duct

Question 17

what are the two stimulators and one inhibitor of aldosterone?

Answer 17

Stimulators
1. High [K+]plasma
2. Angiotensin II (via Blood pressure)
→ RAAS – Renin-angiotensinogen pathway!!

Inhibitor
3. High osmolarity (extracellular fluid)

Question 18

how does aldosterone act on the cells to effect Na+ and K+ (6)

Answer 18

1. aldosterone travels from the adrenal cortex to the bloodstream and diffuses into the cells of the distal tubules and the collecting duct (steroid so can diffuse freely but only acts on cells that have the appropriate receptor – note K+ ions also diffuse in to be secreted later on)
2. once the aldosterone and receptor bind, they translocate into the nucleus where they initiate transcription of new channel genes (on apical membrane) and new pumps, specifically Na+ ATPase pumps
3. the K+ secretion and Na+ reabsorption channels on the apical membrane communicate with the lumen of the distal neuron to modulate the concentrations
4. then the sodium potassium ATPase pumps the sodium back into the bloodstream (reabsorbed – usually pumped out) as needed while the K+ ions are secreted to the lumen (normal)

thus aldosterone increases Na+ reabsorption in the bloodstream and secretion of K+ into the lumen

Question 19

2nd way aldosterone is stimulated: renin-angiotensin pathway

Answer 19

• there are renal juxtaglomerular cells in the glomerulus (where the bloodstream meets the nephron and is filtered)
• these cells have the ability to secrete renin when blood pressure falls

how does it know blood pressure has fallen:
- as blood enters the nephron through afferent arteriole, the cells are well positioned to detect changes in blood pressure (since they are sitting right next to the blood vessels)
- when they sense the blood pressure has fallen, they can secrete renin

Question 20

describe the Renin-Angiotensin - Aldosterone Pathway (5 with last step having 3 sections) in the hypothalamus, blood vessels, adrenal cortex

Answer 20

1. drop in blood pressure
2. the renal juxtaglomerular cells in the glomerous in the kidney secretes renin
3. renin cleaves with angiotensinogen from the liver to create angiotensin 1
4. angiotensin 1 combines with ACE enzyme from lungs to make angiotensin 2
5. angiotensin 2 is the only active form and can stimulate a number of responses:
   a. hypothalamus: make you think your thirsty and thus makes you drink water –> inc. blood pressure and volume (water reabsorption)
   b. blood vessels: vasoconstricts blood vessels –> inc. blood pressure
   c. adrenal cortex: aldosterone secretion –> goes to kidneys where it inc. Na concent. and thus water reabsorption –> inc. blood pressure

Question 21

what are all the relevant functions of angiotensin 2 (+ two more)

Answer 21

• Stimulates thirst
• Potent vasoconstrictor
• aldosterone secretion
• Increases proximal tube Na+ retention
• Increases vasopressin/ ADH

Question 22

atrial natriuretic peptides (ANP): what are the three peptides part of the natriuretic peptides?
what is the general function of this family of peptides?
what secretes these peptides?
what is the main relevant peptide?

Answer 22

• 3 peptides: ANP, BNP, CNP
• general function is to stimulate water absorption in the nephron so its urinated out (opposite of ADH and aldosterone) via a decrease in blood pressure
• secondary endocrine organs release these peptides.
• main focus is on ANP which is secreted by the atria in the heart

Question 23

How does ANP work to lower blood pressure ?
- in kidney (tubule, afferent arteriole (2)) Renin/GFR
- in hypothalamus (ADH)
- in adrenal cortex (aldosterone)
- in medulla oblongata

Answer 23

• atria and ventricles are able to sense the stretch of the heart via myocardial cells in atria
• if they sense a big stretch in the heart –> body thinks there is too much pressure
• too much pressure can mean too much blood volume and thus inc. in blood pressure by having a lot of water reabsorption (must decrease pressure/volume and reabsorption)
• if stretch detected –> natriuretic peptides are released (ANP)

in kidneys:
in the kidney tubules:
- ANP dilates the blood vessels –> increases the renal excretion of fluids (decreasing [Na+] and H2O reabsorption in blood, however increases K+) –> losing salt and water –> decreased blood volume –> lowers blood pressure

in the kidney afferent arteriole:
- decreases renin –> decreased blood pressure
- dilates and causes increased GFR (how much blood is filtered per minute) –> more filtrate in nephron, more water excreted –> lower blood pressure

in hypothalamus:
- lower ADH –> increases the renal excretion of fluids (decreasing [Na+] and H2O reabsorption) –> losing salt and water –> decreased blood volume –> lowers blood pressure

in adrenal cortex:
- since there is less renin secreted from kidney afferent arteriole –> less aldosterone –> increases the renal excretion of fluids (decreasing [Na+] and H2O reabsorption) –> losing salt and water –> decreased blood volume –> lowers blood pressure

in medulla oblongata:
- decreased sympathetic output –> lowers blood pressure

Question 24

summary of what hormones ANP inhibits

Answer 24

suppresses renin, aldosterone, vasopressin/ADH