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Human Physiology · Renal system

Acid–Base Balance and Micturition

The lungs and kidneys together hold blood pH in a narrow range, and reading a blood gas is a core clinical skill. The second half of this page explains how the bladder stores and empties urine, and why pelvic floor training is a first-line treatment for incontinence.

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6Tables
10Questions

Part 5 · Renal System · Chapter 24 of 47

Learning objectives

By the end of this page you should be able to:

  • Define pH and explain the bicarbonate buffer system.
  • Describe how the lungs and kidneys regulate acid–base balance.
  • Identify the four primary acid–base disorders and their compensation.
  • Interpret an arterial blood gas using a stepwise approach.
  • Describe the neural control of bladder storage and voiding.
  • Summarise the evidence for pelvic floor muscle training.

Why it matters

Respiratory physiotherapists read blood gases daily, and changes in breathing pattern are both a cause and a correction of acid–base disorders.

Urinary incontinence is common, under-reported and very treatable. Pelvic floor rehabilitation is one of the best-supported physiotherapy interventions, and bladder dysfunction is central to neurological rehabilitation.

Acids, bases and pH

pH is the negative logarithm of hydrogen ion concentration. Small changes in pH mean large changes in hydrogen ions: a fall from 7.4 to 7.1 doubles them. Enzymes, nerve conduction and heart rhythm all depend on pH staying close to 7.4.

The body produces carbon dioxide continuously, which forms carbonic acid, and smaller amounts of fixed acids from metabolism. Buffers absorb the shock; lungs and kidneys remove the excess.

Figure 1 · Acid–base as a balance

The bicarbonate to carbon dioxide ratio that sets blood pH A balance with bicarbonate, 24 millimoles per litre and set by the kidneys, on one side and carbon dioxide, 40 millimetres of mercury and set by the lungs, on the other. Their ratio of 20 to 1 gives a pH of 7.40. BicarbonateHCO3⁻ 24 mmol/Lset by the kidneysCarbon dioxidePCO2 40 mmHgset by the lungspH = 6.1 + log [ HCO3⁻ ÷ (0.03 × PCO2) ]24 ÷ (0.03 × 40) = 20 : 1 → pH 7.40Keep the ratio, keep the pHNormal arterial pH 7.35–7.45
Two organs, one ratio. Lungs adjust CO2 within minutes; kidneys adjust bicarbonate over days. Disorders shift one side; compensation shifts the other [1].
DefenceHow it worksSpeed
BuffersBicarbonate, haemoglobin, proteins and phosphate bind or release H+Seconds
LungsChange breathing to adjust CO2Minutes
KidneysExcrete H+, reabsorb and make new bicarbonateHours to days

The kidneys reabsorb nearly all filtered bicarbonate in the proximal tubule. They also make new bicarbonate by excreting acid bound to phosphate and ammonia. This is how kidney failure causes a metabolic acidosis.

Acid–base disorders

Figure 2 · The four primary disorders

Respiratory and metabolic acidosis and alkalosis with typical causes Four boxes. Respiratory acidosis: low pH with high carbon dioxide, from hypoventilation. Respiratory alkalosis: high pH with low carbon dioxide, from hyperventilation. Metabolic acidosis: low pH with low bicarbonate, as in ketoacidosis or lactic acidosis. Metabolic alkalosis: high pH with high bicarbonate, as in vomiting. Respiratory acidosispH ↓ PCO2 ↑hypoventilation: COPD,opioids, weaknessRespiratory alkalosispH ↑ PCO2 ↓hyperventilation: anxiety,pain, high altitudeMetabolic acidosispH ↓ HCO3⁻ ↓ketoacidosis, lactic acid,kidney failure, diarrhoeaMetabolic alkalosispH ↑ HCO3⁻ ↑vomiting, diuretics,low potassiumCompensation moves the other value in the same direction as the primary change [1]
Name the primary problem first. Then ask whether the other system has compensated, and whether more than one disorder is present [1].

Berend and colleagues describe a physiological approach: identify the primary disorder from pH and the value that explains it, then check whether compensation is appropriate [1]. Respiratory compensation for metabolic problems is fast; renal compensation for respiratory problems takes days [1].

Compensation rarely brings pH fully back to normal. A normal pH with clearly abnormal PCO2 and bicarbonate suggests two disorders at once [1].

Figure 3 · Reading a blood gas

Four-step approach to interpreting arterial blood gas results A four-step flow: look at pH, then carbon dioxide to see if the cause is respiratory, then bicarbonate to see if the cause is metabolic, then check compensation. An example with pH 7.29, carbon dioxide 62 and bicarbonate 29 is interpreted as partly compensated respiratory acidosis. 1. Look at pHbelow 7.35 acid; above 7.45 alkaline2. Look at PCO2does it explain the pH? = respiratory3. Look at HCO3⁻does it explain the pH? = metabolic4. Check compensationis the other value moving to correct pH?ExamplepH 7.29PCO2 62 mmHgHCO3⁻ 29 mmol/LRespiratory acidosis,partly compensated(e.g. COPD exacerbation)
A systematic routine prevents errors. Berend and colleagues describe a physiological stepwise approach [1]. Oxygen is assessed separately.
ValueNormal range
pH7.35–7.45
PaCO235–45 mmHg (4.7–6.0 kPa)
HCO3−22–26 mmol/L
Base excess−2 to +2 mmol/L
PaO2 (breathing air)80–100 mmHg (10.7–13.3 kPa)

Practical skill · what physiotherapy can change

Acute respiratory acidosis may improve when airway clearance, positioning or ventilatory support increase alveolar ventilation. Hyperventilation with tingling and light-headedness reflects respiratory alkalosis; slow, relaxed breathing helps. Metabolic disorders need medical treatment of the cause.

Exercise and acid–base

During heavy exercise, muscle produces lactate and hydrogen ions. Bicarbonate buffers them, generating extra carbon dioxide, and breathing rises disproportionately. This is the basis of the ventilatory threshold used in exercise testing (Chapter 47).

Acid build-up in muscle contributes to fatigue and the burning sensation of hard effort. It clears within minutes of stopping.

Micturition

The bladder has two jobs: store urine at low pressure without leaking, and empty completely when convenient. The detrusor muscle in its wall contracts to empty it. An internal sphincter of smooth muscle and an external sphincter of skeletal muscle, supported by the pelvic floor, keep it closed.

Figure 4 · Control of the bladder

Neural control of bladder storage and voiding The cortex and pontine micturition centre control the sacral cord segments S2 to S4. Parasympathetic nerves contract the detrusor muscle of the bladder; the pudendal nerve controls the external sphincter and pelvic floor. Stretch signals from the filling bladder travel back to the cord and brain. Cortexvoluntary delay or startPontine micturition centrecoordinates voidingSacral cord S2–S4parasympathetic: detrusorpudendal: external sphincterBladderdetrusor muscleinternal sphincter (smooth)external sphincter and pelvic floor (skeletal)stretch signalsas bladder fillsStorage: detrusor relaxed, sphincters closedVoiding: detrusor contracts, sphincters relax
A reflex under voluntary command. Lesions above the sacral cord tend to cause an overactive bladder; damage to the sacral segments or nerves tends to cause a floppy, retaining bladder.
PhaseDetrusorSphincters and pelvic floorMain nerves
StorageRelaxed (sympathetic)ContractedHypogastric (T10–L2), pudendal (S2–S4)
VoidingContracts (parasympathetic)RelaxedPelvic splanchnic (S2–S4)

As the bladder fills, stretch receptors signal the sacral cord and brain. The first urge appears at about 150–250 mL, and capacity is about 400–600 mL. The cortex delays voiding until a suitable time; the pontine micturition centre then coordinates detrusor contraction with sphincter relaxation.

LesionTypical bladderExample
Above the ponsUrgency, frequency; coordination preservedStroke, dementia
Spinal cord above S2Overactive detrusor; detrusor and sphincter may contract togetherSpinal cord injury, multiple sclerosis
Sacral cord or nervesUnderactive, retaining bladderCauda equina syndrome

Red flag

New urinary retention or loss of bladder sensation with saddle numbness and back pain suggests cauda equina syndrome. This needs emergency referral.

Incontinence and pelvic floor training

TypeMechanismTypical trigger
StressWeak sphincter and pelvic floor supportCough, sneeze, lifting, running
UrgencyOveractive detrusorSudden strong urge
MixedBothBoth
OverflowRetention with leakageObstruction, underactive detrusor

Dumoulin and colleagues reviewed 31 trials involving 1,817 women [2]. Women with stress incontinence were about eight times more likely to report cure with pelvic floor muscle training than without treatment [2].

Figure 5 · Pelvic floor muscle training works

Cure and improvement rates with pelvic floor muscle training in women with stress incontinence Paired bars. Self-reported cure was 56 per cent with pelvic floor muscle training and 6 per cent without treatment. Cure or improvement was 74 per cent and 11 per cent. 6%No treatment56%Pelvic floor trainingSelf-reported cure (stress incontinence)11%No treatment74%Pelvic floor trainingCure or improvement (stress incontinence)
First-line conservative treatment. Women with stress incontinence were about eight times more likely to report cure with training [2].

Training also reduced leakage episodes and improved quality of life, and adverse events were rare and minor [2]. The review supports including it in first-line conservative care [2].

Practical skill · teaching a pelvic floor contraction

Explain the anatomy, then ask for a squeeze and lift as if stopping wind, without holding the breath or squeezing the buttocks. Confirm the contraction by appropriate assessment. Build strength with sustained holds and endurance, and teach a pre-contraction before coughing (‘the knack’).

Clinical relevance for the physiotherapist

  • Blood gases. Use a stepwise approach to identify primary disorder and compensation [1].
  • Respiratory acidosis. Improving ventilation can correct it; worsening values need urgent review.
  • Hyperventilation. Breathing retraining corrects symptoms of respiratory alkalosis.
  • Incontinence. Pelvic floor muscle training is first-line for stress incontinence [2].
  • Neurological patients. Bladder patterns follow lesion level; coordinate with continence teams.

Assessment and application

FindingInterpretationAction
pH 7.48, PaCO2 28 mmHg, tingling fingersRespiratory alkalosis [1]Calm, slow breathing; check for pain or anxiety
pH 7.25, PaCO2 70 mmHg, drowsyAcute respiratory acidosis [1]Urgent medical review; ventilatory support
Leaks urine when running or coughingStress incontinencePelvic floor muscle training [2]
Back pain with new retention and saddle numbnessPossible cauda equina syndromeEmergency referral

Common mistakes

  • Naming the disorder from CO2 or bicarbonate before looking at pH [1].
  • Expecting compensation to fully normalise pH [1].
  • Treating all incontinence as one condition.
  • Teaching pelvic floor exercises without checking the patient can contract correctly [2].
  • Missing cauda equina red flags in back pain.

Exam preparation

Long answer

Describe the regulation of acid–base balance by the kidney. Add a note on respiratory acidosis.

  • Buffers and the Henderson–Hasselbalch relation (Figure 1).
  • Renal bicarbonate reabsorption and generation; acid excretion.
  • Four primary disorders and compensation (Figure 2) [1].
  • Blood gas interpretation (Figure 3).
  • Respiratory acidosis: causes and correction.

Short notes

  • Micturition reflex.
  • Neurogenic bladder.
  • Metabolic acidosis [1].
  • Pelvic floor muscle training [2].

Viva and practical

  • “Interpret pH 7.29, PCO2 62, HCO3− 29” — partly compensated respiratory acidosis [1].
  • “Which nerves control voiding?” — pelvic splanchnic, S2–S4.
  • “What is first-line for stress incontinence?” — pelvic floor muscle training [2].
  • OSCE: teach a correct pelvic floor contraction.

Current evidence

Exam and traditional teaching

Acid–base is taught as formula memorisation; incontinence is seen as an inevitable part of ageing or childbirth.

What the evidence adds

A physiological stepwise approach makes blood gases interpretable at the bedside [1].

Pelvic floor muscle training cures or improves stress incontinence in most women who complete it [2].

What the student should understand

Breathing changes CO2, and physiotherapy changes breathing. The pelvic floor is a trainable muscle like any other.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. Normal arterial pH lies between:
  1. (A) 6.8 and 7.0
  2. (B) 7.35 and 7.45
  3. (C) 7.6 and 7.8
  4. (D) 8.0 and 8.2

Answer: (B) Values outside 7.35–7.45 indicate acidaemia or alkalaemia; a stepwise physiological approach starts with pH [1].

Q2. The pH of blood is determined mainly by the ratio of:
  1. (A) Sodium to chloride
  2. (B) Bicarbonate to carbon dioxide
  3. (C) Glucose to insulin
  4. (D) Haemoglobin to oxygen

Answer: (B) The bicarbonate/CO2 relation underlies the physiological approach to acid–base [1].

Q3. A patient has pH 7.29, PCO2 62 mmHg and HCO3− 29 mmol/L. The primary disorder is:
  1. (A) Metabolic acidosis
  2. (B) Respiratory acidosis
  3. (C) Metabolic alkalosis
  4. (D) Respiratory alkalosis

Answer: (B) Low pH with high CO2 is respiratory acidosis; the raised bicarbonate is partial compensation [1].

Q4. Anxiety-driven hyperventilation produces:
  1. (A) Respiratory acidosis
  2. (B) Respiratory alkalosis
  3. (C) Metabolic acidosis
  4. (D) No change

Answer: (B) Low CO2 raises pH: respiratory alkalosis [1].

Q5. Diabetic ketoacidosis is an example of:
  1. (A) Metabolic acidosis
  2. (B) Respiratory alkalosis
  3. (C) Metabolic alkalosis
  4. (D) Respiratory acidosis

Answer: (A) Added acid consumes bicarbonate, lowering pH [1].

Q6. Compensation for a metabolic acidosis is mainly by:
  1. (A) Breathing more to lower CO2
  2. (B) Breathing less
  3. (C) Drinking water
  4. (D) Sweating

Answer: (A) Respiratory compensation lowers PCO2 to restore the ratio [1].

Q7. In women with stress urinary incontinence, pelvic floor muscle training compared with no treatment produced self-reported cure in about:
  1. (A) 6% vs 6%
  2. (B) 56% vs 6%
  3. (C) 100% vs 0%
  4. (D) 20% vs 56%

Answer: (B) Cure was reported by 56% with training versus 6% without [2].

Q8. The quality of evidence for participant-perceived cure in stress incontinence was rated:
  1. (A) Very low
  2. (B) Low
  3. (C) High
  4. (D) Not graded

Answer: (C) This outcome was rated high-quality evidence [2].

Q9. Adverse events from pelvic floor muscle training were:
  1. (A) Common and serious
  2. (B) Rare and minor
  3. (C) Never recorded
  4. (D) Fatal

Answer: (B) Adverse events were rare and minor [2].

Q10. Pelvic floor muscle training reduced leakage episodes in women with stress incontinence by about:
  1. (A) One per day
  2. (B) Ten per day
  3. (C) None
  4. (D) Fifty per day

Answer: (A) Leakage fell by about 1.23 episodes per 24 hours [2].

Quick review

Key takeaways

  • pH 7.35–7.45; set by HCO3−/CO2 ratio of about 20:1.
  • Lungs adjust CO2 in minutes; kidneys adjust bicarbonate over days.
  • Four disorders: respiratory or metabolic, acidosis or alkalosis [1].
  • Read pH first, then CO2, bicarbonate, compensation [1].
  • Voiding: parasympathetic S2–S4; storage: sympathetic and pudendal.
  • Pelvic floor training: 56% vs 6% cure in stress incontinence [2].

Further reading

SourceWhy it earns its place
Berend K et al. — NEJM, 2014Practical physiological approach to acid–base [1]
Dumoulin C et al. — Cochrane Review, 2018Pelvic floor muscle training evidence [2]
Bø K et al. — Evidence-Based Physical Therapy for the Pelvic FloorClinical practice

References

  1. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. New England Journal of Medicine. 2014;371(15):1434–1445. doi:10.1056/NEJMra1003327 PMID 25295502 Review
  2. Dumoulin C, Cacciari LP, Hay-Smith EJC. Pelvic floor muscle training versus no treatment, or inactive control treatments, for urinary incontinence in women. Cochrane Database of Systematic Reviews. 2018;10(10):CD005654. doi:10.1002/14651858.CD005654.pub4 PMID 30288727 Cochrane review

Related topics

Reviewed by the Physiotherapist India Team. · Human Physiology contents