pH is a measure of how acidic or alkaline (basic) a solution is. The scale runs from 0 (very acidic) to 14 (very alkaline), with 7 being neutral. Your blood is slightly alkaline at pH 7.35-7.45.

Why does this matter so much? Because proteins only work at the right pH. Your enzymes, antibodies, hemoglobin, and cell membrane proteins all have very specific shapes that depend on pH. Even a small change disrupts their function.

What happens when pH changes:

• pH below 7.35 (acidosis): Depresses brain function, causes confusion, can lead to coma
• pH above 7.45 (alkalosis): Makes nerves hyperexcitable, causes muscle spasms, tingling
• pH below 6.8 or above 7.8: Fatal!

Your body produces acids constantly through metabolism. Carbon dioxide forms carbonic acid. Exercise produces lactic acid. Fat breakdown produces ketones. Your body must neutralize and eliminate these acids while maintaining tight pH control.

The Bohr Effect: pH affects how readily hemoglobin releases oxygen. Lower pH (more acid) causes hemoglobin to release oxygen more easily — perfect for tissues that are actively metabolizing and producing acid!

Your body maintains pH through three lines of defense, each acting at different speeds:

1. Buffer Systems (Immediate, seconds)

Buffers are like chemical sponges that absorb excess acid or base. The main buffers are:

• Bicarbonate buffer: The most important in blood. HCO3- (bicarbonate) absorbs H+ (acid) to form H2CO3 (carbonic acid), which breaks down to CO2 and water.
• Protein buffer: Hemoglobin and plasma proteins can accept or donate H+.
• Phosphate buffer: Important in kidneys and inside cells.

2. Respiratory Regulation (Minutes to hours)

Your lungs can rapidly change blood pH by controlling CO2 elimination. Remember: CO2 + H2O = H2CO3 = H+ + HCO3-. More CO2 means more acid!

• Breathe faster: Remove more CO2, decrease acid, raise pH
• Breathe slower: Retain more CO2, increase acid, lower pH

3. Renal Regulation (Hours to days)

Your kidneys are the most powerful pH regulators because they can:

• Excrete hydrogen ions (H+) in urine
• Reabsorb and generate new bicarbonate (HCO3-)
• Permanently correct pH imbalances

There are four primary acid-base disorders. Understanding them is crucial for clinical practice:

Respiratory Acidosis — Too much CO2

• Cause: Lung disease, opioid overdose, weak breathing muscles
• Mechanism: CO2 retention → increased H+ → low pH
• Signs: Confusion, headache, flushed skin

Respiratory Alkalosis — Too little CO2

• Cause: Hyperventilation (anxiety, panic attack, high altitude)
• Mechanism: Excessive CO2 loss → decreased H+ → high pH
• Signs: Lightheadedness, tingling, muscle spasms

Metabolic Acidosis — Too much acid or too little bicarbonate

• Cause: Diabetes (ketoacidosis), kidney failure, severe diarrhea, lactic acidosis
• Mechanism: Increased metabolic acids OR loss of bicarbonate
• Signs: Deep, rapid breathing (Kussmaul respiration) — the lungs trying to compensate!

Metabolic Alkalosis — Too much bicarbonate or too little acid

• Cause: Vomiting, diuretics, excess antacid use
• Mechanism: Loss of stomach acid OR excess bicarbonate
• Signs: Muscle weakness, irregular heartbeat

When one system causes an imbalance, the other systems try to compensate. This is why arterial blood gas (ABG) analysis is so useful — you can see both the primary problem and the body's response.

How compensation works:

• Respiratory problem: Kidneys compensate by retaining or excreting bicarbonate (slow, takes days)
• Metabolic problem: Lungs compensate by changing ventilation (fast, takes hours)

Example: In metabolic acidosis, the lungs try to help by blowing off CO2 through deep, rapid breathing (Kussmaul respiration). This reduces the amount of carbonic acid, partially correcting the low pH.

Interpreting ABGs:

• Look at pH first — is it acidosis or alkalosis?
• Look at CO2 — is it a respiratory problem?
• Look at HCO3- — is it a metabolic problem?
• Is compensation occurring? (The opposite value moves in the same direction)