A 400-meter sprint pours acid into your blood. Your blood barely notices.
During a 400-meter sprint, your muscles pour lactic acid into your blood. Your blood has to stay almost exactly as acidic as it was, or the enzymes in your heart and brain stop working. Part of the defense is an equilibrium that links your blood to your lungs. This page shows how breathing faster or slower shifts it.
Core Claims
- One equilibrium. Carbon dioxide dissolves in blood and reacts with water: CO2 + H2O ⇌ H+ + HCO3−. More CO2 means more H+, and H+ makes the blood more acidic.
- Breathing faster. You breathe out more CO2. Removing a reactant shifts the equilibrium left. The shift uses up H+, and the blood becomes less acidic (alkalosis).
- Breathing slower. CO2 builds up. The extra reactant shifts the equilibrium right. The shift makes H+, and the blood becomes more acidic (acidosis).
- Paper bag. Rebreathing your own CO2 raises the CO2 in the blood and shifts the equilibrium back to the right.
CO2 in the blood
Retrieval Checklist
- Write the equilibrium for carbon dioxide dissolved in blood.
- Predict how faster breathing shifts it and what that does to the acidity of the blood.
- Predict what slower breathing does.
- Explain why breathing into a paper bag helps during an anxiety attack.
One equilibrium links your blood to your lungs.
Your cells make carbon dioxide all the time. It dissolves in your blood. There it reacts with water and makes a little acid.
This is a reversible reaction at dynamic equilibrium, like the one in the Haber process. More CO2 in the blood means more H+ ions, and H+ is what makes a liquid acidic. Your lungs control how much CO2 stays in your blood. Every breath out removes some.
Thinking CO2 is only a waste gas with no effect on the blood. Dissolved CO2 makes H+, so the amount of CO2 in the blood changes how acidic the blood is.
Fast breathing, slow breathing, and Le Chatelier.
When you change how fast you breathe, you change the CO2 in your blood. By Le Chatelier’s principle, the equilibrium shifts to work against that change.
Breathing faster: the blood becomes less acidic
During a panic attack or at high altitude, fast, deep breathing removes CO2 faster than your cells make it. CO2 is a reactant, so removing it shifts the equilibrium left (←).
H+(aq) + HCO3−(aq) → CO2(aq) + H2O(l)
The shift uses up H+, so the blood becomes less acidic. Doctors call this alkalosis. The person feels tingling and lightheaded, and muscles can spasm.
Treatment: Breathing into a paper bag makes the person rebreathe their own CO2. The CO2 in the blood rises and the equilibrium shifts right (→). The H+ comes back.
Breathing slower: the blood becomes more acidic
During a severe asthma attack, choking or an opioid overdose, breathing is too shallow to remove CO2. The CO2 builds up in the blood. The extra reactant shifts the equilibrium right (→).
CO2(aq) + H2O(l) → H+(aq) + HCO3−(aq)
The shift makes H+, so the blood becomes more acidic. Doctors call this acidosis. Without a ventilator or emergency oxygen, severe acidosis weakens the heart and can cause a coma.
Thinking faster breathing makes the blood more acidic because you take in more air. Faster breathing blows off CO2, and CO2 makes acid. Less CO2 means less H+.
How a buffer absorbs a chemical shock.
EnrichmentBuffers, pH values and the linked equilibria
Extra depth. The rest of the lesson works without it.
A buffer: a weak acid and its conjugate base
Add 1.0 milliliter of 1.0 M hydrochloric acid (about 20 drops) to 1.0 liter of pure water. The pH falls from 7.0 to 3.0. That is a 10,000-fold increase in H+. Add the same acid to 1.0 liter of human blood, with breathing keeping CO2 steady, and the pH changes by less than 0.02. A buffer makes the difference.
A buffer holds large amounts of both a weak acid (a proton donor, HA) and its conjugate base (a proton acceptor, A−).
When strong acid is added
The conjugate base binds the added H+ and makes weak acid:
A−(aq) + H+(aq) → HA(aq)
The free H+ turns into undissociated weak acid, so the pH barely changes.
When strong base is added
The weak acid gives up H+ to neutralize the added hydroxide:
HA(aq) + OH−(aq) → A−(aq) + H2O(l)
The OH− turns into water and conjugate base, so the pH barely changes.
The pH window and the bicarbonate chain
Human blood plasma stays between pH 7.35 and 7.45. Below 7.35 is acidosis, which slows the central nervous system. Above 7.45 is alkalosis, which makes nerves overexcitable and muscles spasm. A pH below 6.8 or above 7.8 is quickly fatal.
The bicarbonate buffer is the main defense. It links the lungs, the blood and the kidneys in one chain of equilibria.
Interactive blood-buffer simulator.
EnrichmentAcid, base and breathing in blood and in pure water
Extra depth. The rest of the lesson works without it.
Add acid or base, change the breathing rate, and compare buffered blood with pure water.
How does blood stay near pH 7.4 when acid is added?
The explanation appears after you predict.
- The pH now
- The healthy window, pH 7.35 to 7.45
- The pH over the last 30 seconds
- The moment you made a change
- Load blood, press Add 5 mM acid, then press Record. Look for: the pH falls from 7.40 to 7.30, then creeps back as the kidneys restore bicarbonate.
- Load pure water, press Add 5 mM acid, then press Record. Look for: the pH falls from 7.00 to 2.30 and stays there.
- Load fast breathing, wait a few seconds, then press Record. Look for: the pH rises from 7.40 to 7.69, above the healthy window, because the lungs blow off CO2.
Use your evidence
Explain the emergency response
A bioreactor grows mammalian cells that need a pH of 4.75 ± 0.10. The engineer makes 2.0 L of buffer from two solutions.
• 0.20 M acetic acid (CH3COOH, a weak acid; Ka = 1.8 × 10−5)
• 0.20 M sodium acetate (NaCH3COO, which supplies the conjugate base CH3COO−)
During fermentation the cells make 0.050 mol of lactic acid. Lactic acid gives up all of its H+.
(a) Write the net ionic equation for how the buffer neutralizes this H+. [1 mark]
(b) The acid changes the pH of the buffer by less than 0.15. In 2.0 L of pure water the same acid drops the pH from 7.0 to 1.6. Explain the difference at the particle level. [2 marks]
(c) After many days the pH suddenly crashes to 2.5. Explain what happened to the buffer. [1 mark]
- Equation (1 mark): You write CH3COO−(aq) + H+(aq) → CH3COOH(aq).
- Buffer (1 mark): You say the acetate ions (0.40 mol) bind the added H+, so free H+ barely changes.
- Pure water (1 mark): You say pure water has nothing to bind H+, so all 0.050 mol stays free.
- Capacity (1 mark): You say the acetate ions ran out, so added H+ stayed free and the pH crashed.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. Carbon dioxide dissolves in blood and reacts with water to make H+ ions and ions.
2. Breathing faster removes CO2 and shifts the equilibrium to the .
3. Removing CO2 uses up H+, so the blood becomes acidic.
4. Breathing too slowly lets CO2 build up. The equilibrium shifts and the blood becomes more acidic.
5. Breathing into a paper bag makes a person rebreathe , which shifts the equilibrium back to the right.
A person in a panic breathes fast and deep. Their hands start to tingle. A nurse tells them to breathe into a paper bag.
Use Le Chatelier’s principle to explain why this helps.
Start with what fast breathing does to the CO2 in the blood. Then follow the shift. Write your answer first, then compare it with the model answer.
Fast breathing removes CO2 from the blood. CO2 is a reactant in CO2 + H2O ⇌ H+ + HCO3−, so removing it shifts the equilibrium left. The shift uses up H+. The blood becomes less acidic, and that causes the tingling.
Breathing into a bag makes the person rebreathe their own CO2. The CO2 in the blood rises and the equilibrium shifts right. The H+ comes back, and the blood returns toward its normal acidity.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Marcus holds his breath underwater for 60 seconds. His cells keep making CO2, and he cannot breathe it out. Carbon dioxide in his blood is in equilibrium:
(a) As the CO2 builds up, say which way the equilibrium shifts. Explain using the forward and reverse rates. [2 marks]
(b) Say what happens to how acidic Marcus’s blood is, and explain why. [1 mark]
(c) After he surfaces, Marcus breathes fast and deep for a minute. Say which way the equilibrium shifts now, and explain why. [1 mark]
- Direction (1 mark): You say the equilibrium shifts right, toward H+ and HCO3−.
- Rates (1 mark): You say more CO2 raises the forward rate above the reverse rate.
- Acidity (1 mark): You say the blood becomes more acidic, because more H+ forms.
- Fast breathing (1 mark): You say the equilibrium shifts left, because breathing out removes CO2.
Patients with end-stage kidney failure cannot remove acid or rebuild the bicarbonate in their blood. Without treatment, the acid build-up is quickly fatal. A hemodialysis machine works as an artificial kidney. It passes blood across a thin membrane that is bathed in a fluid rich in bicarbonate (32 to 35 mM). Bicarbonate moves into the blood and acid wastes move out. The treatment restores the balance three times a week.
Adaptive Competency Practice: HS-PS1-6
Test your understanding of equilibria in the body one question at a time. The adaptive engine covers the carbonic acid-bicarbonate blood buffer, neutralizing acid, breathing and acid-base balance, and acidosis or alkalosis shifts.