pH regulation

Blood pH

• pH = measure of hydrogen ion concentration

• pH = -log [H+]

• Blood pH = 7.35-7.45

• pH imbalances are quickly lethal body needs to compensate

• H+ intake:

- food

- drink

- metabolism (carbohydrate, protein, lipid)

- breathing

• H+ output

- urine

Figure 19.23

pH regulation

• Three lines of defense:- Buffers- Respiration- Kidneys

• Buffer reactions are nearly instantaneous. However, they are limited to the amount of buffer available in the blood

• Respiration helps buffer the blood by shifting H+ into HCO3-. However, it can be limiting and take several minutes

• Kidneys are the only organ which can get rid of H+. However, they can take several hours (time enough to be dead!!- therefore, the presence of the other 2 lines of defense)

pH regulation

• Three lines of defense:

- Buffers

- Respiration

- Kidneys

First line of defense: Buffers

• Buffers:

- protein buffer

- bicarbonate buffer

- phosphate buffer

Only free H+ contributes to pH

H+ can be shifted from a strong acid to a weaker one.

• Strong acid complete dissociation

• (100) HCl (100)H+ + (100) Cl-

• Weak acid • (100) H2CO3 (80) H2CO3 +

(20) H+ + (20) HCO3-

pH regulation

• If strong acid intake:

• H+ + Cl- + ( blood Na+ HCO3-)

Na+ Cl- + H+ + HCO3-

• Less H+ is circulating into the blood - smaller shift toward acidosis

• If strong base intake:

• Na+ OH- + (blood H2 CO3) H2O

+ Na+ HCO3-

• Less OH- is circulating in the blood smaller shift toward alkalosis

Second line of defense: breathing

• Three lines of defense:

- Buffers

- Respiration

- Kidneys

CO2 + H2O H2CO3 H+ + HCO3-

• CO2 retention: ↑ blood H+ ↓ pH acidosis

• CO2 loss:• ↓ blood H+ pH ↑ alkalosis

Figure 17.22

Figure 19.24

Third line of defense: kidneys

• Three lines of defense:

- Buffers

- Respiration

- Kidneys

• Kidneys are the only organs able to eliminate H+ from the body slow process

• H+ can be bound to HCO3-

for transport to the bladder and eliminated

• H+ can be added to NH3 to make ammonia (NH4

+) and eliminated

• H+ can be eliminated directly by exchange with Na+

• Urine is usually slightly acid. It’s pH varies with food and drink intakes

Clinical applications:

• Acidosis: Blood pH<7.35

Figure 19.28