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Every minute, your body produces acid. Your cells release carbon dioxide as they burn glucose. Your muscles make lactic acid when you sprint. Digesting protein generates sulfuric and phosphoric acids. Yet the pH of your blood stays locked between about 7.35 and 7.45, a range narrower than most lab buffers achieve.
That precision matters. Enzymes, the proteins that run your metabolism, only keep their shape and work properly within a narrow pH window. Blood pH below about 6.8 or above about 7.8 is generally incompatible with life. So how does the body do it?
The answer is a team: chemical buffers that respond in seconds, the lungs that respond in minutes, and the kidneys that respond over hours to days.
The main buffer: carbonic acid and hydrogencarbonate
Carbon dioxide dissolves in blood and reacts with water to form carbonic acid, which dissociates into hydrogencarbonate (bicarbonate) and hydrogen ions:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The first step is slow on its own, but red blood cells contain an enzyme, carbonic anhydrase, that speeds it up enormously, to hundreds of thousands of reactions per second per enzyme molecule.
This system acts as a buffer because it contains both a weak acid (H₂CO₃) and its conjugate base (HCO₃⁻):
- If acid enters the blood, hydrogencarbonate mops up the extra H⁺: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O
- If base enters the blood, carbonic acid neutralises it: H₂CO₃ + OH⁻ → HCO₃⁻ + H₂O
Why a 20 : 1 ratio?
Using the Henderson–Hasselbalch equation with the effective pKa of about 6.1 at body temperature:
7.4 = 6.1 + log([HCO₃⁻] ÷ [H₂CO₃])
log(ratio) = 1.3, so ratio ≈ 20 : 1
Normal blood contains about 24 mmol/L of hydrogencarbonate and about 1.2 mmol/L of dissolved CO₂ (counted as carbonic acid).
By textbook standards this is a poor buffer. A buffer works best when pH = pKa and the ratio is close to 1 : 1. Blood is 1.3 pH units away from the pKa, with far more base than acid. So why is it so effective?
The secret: it’s an open system
In a beaker, a buffer is a closed system. As you add acid, HA builds up and the ratio shifts.
In the body, the carbonic acid produced when H⁺ is buffered doesn’t build up. It turns into CO₂, which the lungs breathe out. The system is open: one of the buffer components can be adjusted continuously.
- If blood becomes too acidic, sensors in the brainstem and major arteries detect it, and you breathe faster and deeper. More CO₂ is exhaled, H₂CO₃ falls, and pH rises back towards normal. This happens within minutes.
- If blood becomes too alkaline, breathing slows, CO₂ builds up, and pH falls back.
This is why you pant after a sprint. Part of the drive to breathe hard is getting rid of the extra CO₂ from buffering lactic acid.
The kidneys: the long-term controller
The lungs control CO₂; the kidneys control hydrogencarbonate and excrete excess acid. Over hours to days, the kidneys can:
- reabsorb hydrogencarbonate from urine back into the blood, or excrete more of it
- make new hydrogencarbonate
- excrete H⁺ in urine, buffered by phosphate and by ammonia made from the amino acid glutamine (forming NH₄⁺)
Urine pH can vary from about 4.5 to 8, which shows how much acid or base the kidneys can dump when needed.
Other buffers in the body
Proteins. Haemoglobin in red blood cells is a powerful buffer. Its histidine amino acids have side chains with pKa values near 6–7, ideal for picking up or releasing protons at blood pH. Deoxygenated haemoglobin is a better proton acceptor than oxygenated haemoglobin, so it takes up H⁺ exactly where CO₂ is being produced, in the tissues. Plasma proteins such as albumin also help.
Phosphate. The H₂PO₄⁻/HPO₄²⁻ pair has pKa 7.2, very close to body pH, so it’s a textbook-ideal buffer. Its concentration in blood is low, so it contributes little there, but it’s important inside cells and in urine.
| Buffer | Where it matters most | Speed |
|---|---|---|
| Carbonic acid / hydrogencarbonate | blood plasma, extracellular fluid | seconds (plus lungs in minutes) |
| Haemoglobin | red blood cells | seconds |
| Plasma proteins | blood plasma | seconds |
| Phosphate | inside cells, urine | seconds |
| Kidneys | whole body | hours to days |
When the system fails: acidosis and alkalosis
Doctors classify disturbances by direction (acid or alkaline) and by cause (respiratory, involving CO₂, or metabolic, involving everything else).
Respiratory acidosis (pH below 7.35, high CO₂)
Caused by breathing that’s too slow or ineffective, so CO₂ builds up: severe asthma, chronic lung disease, drug overdoses that suppress breathing. The kidneys compensate over days by keeping more hydrogencarbonate.
Respiratory alkalosis (pH above 7.45, low CO₂)
Caused by breathing too fast, blowing off too much CO₂: anxiety and panic attacks, high altitude, fever. Symptoms can include tingling fingers and light-headedness. Breathing slowly helps restore CO₂.
Metabolic acidosis (pH below 7.35, low HCO₃⁻)
Caused by acids building up or hydrogencarbonate being lost: uncontrolled diabetes (ketoacidosis, from ketone acids), severe exercise or shock (lactic acidosis), kidney failure, severe diarrhoea. The lungs compensate with deep, rapid breathing, sometimes called Kussmaul breathing.
Metabolic alkalosis (pH above 7.45, high HCO₃⁻)
Caused by losing acid or gaining base: prolonged vomiting (losing stomach acid), excessive antacid use, some diuretics. The lungs compensate by slowing breathing, though only to a limited extent.
| Condition | pH | Primary change | Compensation |
|---|---|---|---|
| Respiratory acidosis | ↓ | CO₂ ↑ | kidneys retain HCO₃⁻ |
| Respiratory alkalosis | ↑ | CO₂ ↓ | kidneys excrete HCO₃⁻ |
| Metabolic acidosis | ↓ | HCO₃⁻ ↓ | faster breathing lowers CO₂ |
| Metabolic alkalosis | ↑ | HCO₃⁻ ↑ | slower breathing raises CO₂ |
A worked example from medicine
A patient’s blood has [HCO₃⁻] = 12 mmol/L and dissolved CO₂ = 1.2 mmol/L. Using pKa = 6.1, estimate the blood pH.
pH = 6.1 + log(12 ÷ 1.2) = 6.1 + log(10) = 6.1 + 1.0 = 7.1
That’s a serious acidosis. The hydrogencarbonate is half its normal value, which points to a metabolic cause, such as diabetic ketoacidosis. Doctors measure exactly these quantities in an arterial blood gas test.
Why pH 7.4 and not 7.0?
At 37 °C, the ionic product of water is larger than at 25 °C, so neutral pH is about 6.8, not 7.0. Blood at 7.4 is therefore slightly alkaline relative to neutral at body temperature. See Kw and neutral pH. This mild alkalinity helps many enzymes and favours the loading and unloading of oxygen by haemoglobin in the right places.
Common misconceptions
- “Diet can change your blood pH.” Food can change urine pH, but the buffer system, lungs and kidneys keep blood pH in its narrow range in healthy people. Claims that “alkaline diets” or “alkaline water” change blood pH aren’t supported by the chemistry.
- “The body has one buffer.” There are several, working at different speeds.
- “Buffers remove acid permanently.” Chemical buffers only hold it temporarily. The lungs and kidneys remove it for good.
Key takeaways
- Blood pH is held at 7.35–7.45, mainly by the H₂CO₃/HCO₃⁻ buffer at a ratio of about 1 : 20.
- The system works so well because it’s open: the lungs adjust CO₂ in minutes, and the kidneys adjust HCO₃⁻ and excrete acid over days.
- Haemoglobin, plasma proteins and phosphate provide extra buffering.
- Acidosis and alkalosis can be respiratory or metabolic, and the body compensates with the other system.
- For the chemistry of buffers in general, see buffers explained.
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