Lecture 8: Acid Base Balance

Kidneys are a __ response to acid base balance, whereas respiration is an __ response.

long-term; immediate

There is a gain of hydrogen ions when there is a loss of __ in the __.

bicarbonate; urine

There is a loss of hydrogen ions in the __

urine

Alkalosis occurs when loss of __ exceeds gains in the __ , and the pH is [greater/less than] 7.4

H+; ECF; greater

Acidosis occurs when __ of H+ exceeds __ in the ECF and the pH is [greater/less] than 7.4.

gain; loss; less

The kidneys are responsible for balancing H+ ion __ and __.

loss; gains

When HCO3- is lost from the body, it is the same as if the body has:

gained a H+ ion

When CO2 is lost from the body, it is the same as if the body has:

lost a H+ ion

• H+ ion is lost in the formation of carbonic acid

Alkalosis occurs when there is __ of HCO3- to __.

addition; ECF

Acidosis occurs when there is __ of HCO3- in __.

excretion; urine

What is the respiratory response to decreased ECF pH?

inc respiration

(there’s inc arterial [H+] which stimulates chemoreceptor firing → contractions → inc ventilation → dec CO2)

How do kidneys balance pH?

alter plasma HCO3-

When H+ ion decreases in the ECF, the kidney __ HCO3-

expels

• alkalosis

Excretion of HCO3- in urine = __ H+ to __

adding H+ to plasma

When H+ ion increases in ECF, the kidney __ HCO3-

retains

• acidosis

Addition of HCO3- to plasma = __ H+ from __

removing; plasma

Most of bicarbonate handling occurs in the __

proximal tubule

• some in thick ascending limb

What transporters aid in the reabsorption of HCO3-?

AE2: HCO3- in, Cl out

NBCe1: 3HCO3- out with 1 Na+ out

In the tubular lumen, there is a __ of bicarbonate.

loss

(HCO3- that is filtered interacts with H+ → H2CO3 → H2O + CO2)

Why does reabsorption of HCO3- result in no change in plasma HCO3-?

H2O + CO2 in tubular epithelial cells → H2CO3 → H+ & HCO3-

• H+ diffuses to tubular lumen and interacts with a filtered HCO3- → H2CO3 = LOSS of HCO3-

• HCO3- diffuses into interstitial fluid = GAIN / reabsorbed

• the loss and gain cancel each other out

How does a gain in plasma HCO3- affect plasma pH?

increases pH (loses H+)

When does a gain in plasma HCO3- occur?

when H+ from tubular epithelial cells interacts with (HPO4)2- rather than HCO3-

• can occur if not enough HCO3- in filtrate

How does glutamine contribute to an increase in HCO3- in the plasma?

1. filtered glutamine transported into tubular epithelial cell or glutamine from interstitial fluid diffuses into tubular epithelial cell

2. glutamine → NH4+ + HCO3-

3. NH4+ transported into tubular lumen → excreted

4. HCO3- diffuses into interstitial fluid = gain

a-intercalated cells require:

ATP

How do a-intercalated cells contribute to acid balance?

H+ + HCO3-

• H+ transported to lumen with ATP

• HCO3- out to plasma, Cl- in

What 3 organs does calcium homeostasis occur?

1. intestinal absorption of Ca2+

2. kidney: increase Ca2+ reabsorption

3. bone: increase plasma Ca2+ (via osteoclast stimulation)

Most Ca2+ handling occurs in the __. However, Ca here is __.

bone matrix; unavailable

The Ca in the ICF is mostly __

bound

• only 1% here

60% of Ca is __

free

40% of Calcium in the ECF is __

protein bound

Furosemide affects the [region of nephron] by __ Na-2Cl-K channels. This therefore [inc/dec] Ca2+ diffusion through tight junctions.

thick ascending limb; blocking; inc

Thiazide affects the [region of nephron] by __ Na/Cl channels. This therefore [inhibits/enhances] Ca binding protein.

distal tubule; blocking; enhances

__ also increases Ca reabsorption in the distal tubule.

PTH

Vitamin D is processed in the __ and activated in the __ by __.

liver; kidney; PTH

• then goes to plasma → GI tract for absorption of Ca

How does PTH affect the bone?

bone resorption → inc release of calcium into plasma

How does PTH affect the kidney?

inc Ca reabsorption → dec urinary excretion of Ca → inc plasma Ca

How does PTH affect vitamin D?

after vitamin D is processed in liver, PTH activates it in the kidney → intestinal epithelium for absorption of calcium → inc plasma Ca

A sufficient plasma Ca concentration will __ PTH

inhibit

20% of blood plasma Mg2+ is __

protein bound

Where is the majority of Mg2+ reabsorbed in the nephron?

thick ascending limb (60%)

• 30% in proximal tubule

• 5% in distal tubule

__ is the most abundant intracellular ion and maintains intracellular __.

potassium; osmolarity

Potassium is tightly regulated to excrete the __ amount that is ingested.

same

The control of __ is the major mechanism for regulating body K+

urinary K+ excretion

The majority of K+ is reabsorbed in the __

proximal tubule (67%)

• 20% in TAL

How do a-intercalated cells regulate K+?

ATP channel that takes K+ in, H+ out → K+ is diffused to plasma

When is K+ reabsorbed in the distal tubule?

in low K+ diet, H+/K+ ATPase channel is activated to help compensate

Principal cells cause K+ __

secretion

• K+ into lumen, Na/K ATPase filters K+ from blood

Phosphate is mostly found in the __ and is __ there.

bone matrix; unavailable

Phosphate can also be found in the __ (15%) and __ (<0.5%)

ICF; ECF

Phosphate in the ECF is important for __

ICF buffering

Phosphate is inhibited by:

PTH

Most of phosphate is reabsorbed in the __, but this can be __ by PTH.

proximal tubule (70%, 15% in thin descending limb); inhibited

15% of __ is excreted.

1-110% of __ is excreted.

5% of __ is excreted.

• phosphate

• potassium

• magnesium