Evidence-based physiotherapy and rehabilitation resources
Editorial & review policy
Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Physiology · Renal system
Every day your kidneys filter about 180 litres of plasma and return almost all of it. This page explains how the kidney is supplied with blood, how filtration works and is controlled, how kidney function is measured, and what this means for exercise and muscle injury.
Part 5 · Renal System · Chapter 21 of 47
By the end of this page you should be able to:
Kidney disease is common among the patients physiotherapists treat: people with diabetes, hypertension, heart failure and older adults. Many are deconditioned and benefit from exercise.
Physiotherapists also need to recognise exertional muscle breakdown, which can damage the kidneys after unaccustomed intense exercise, crush injury or prolonged lying after a fall.
| Function | Examples |
|---|---|
| Excretion | Urea, creatinine, uric acid, drugs |
| Water and salt balance | Control of blood volume and blood pressure |
| Electrolyte balance | Potassium, calcium, phosphate |
| Acid–base balance | Excrete acid, regenerate bicarbonate |
| Hormones | Erythropoietin (red cells), renin, active vitamin D |
Figure 1 · The nephron
The kidneys receive about 1.1 litres of blood per minute, roughly a fifth of cardiac output at rest, though they make up less than 1% of body weight. Each nephron begins at a glomerulus, a tuft of capillaries between two arterioles: the afferent arteriole in and the efferent arteriole out.
Having resistance on both sides of the glomerulus keeps glomerular capillary pressure high, which suits filtration. After the efferent arteriole, blood flows through a second capillary network around the tubules.
| Layer | Structure | What it stops |
|---|---|---|
| Endothelium | Fenestrated capillary cells | Blood cells |
| Basement membrane | Negatively charged meshwork | Large and negatively charged proteins |
| Podocytes | Interlocking foot processes with slit diaphragms | Medium-sized proteins such as albumin |
Water, glucose, salts, urea and other small molecules pass freely. Albumin is largely held back by size and charge. Protein in the urine is therefore an early sign of glomerular damage, as in diabetes and hypertension.
Figure 2 · The forces that drive filtration
Filtration follows the same Starling forces met in Chapter 14, but the glomerulus is unusual. Its capillary pressure is high, its barrier is very permeable, and its surface area is large. A net pressure of only about 10 mmHg produces a filtration rate of about 125 mL/min.
That is about 180 litres a day. Since plasma volume is about 3 litres, the whole plasma is filtered roughly 60 times a day. About a fifth of the plasma reaching the glomerulus is filtered; this is the filtration fraction.
| Change | Effect on glomerular pressure | Effect on GFR |
|---|---|---|
| Afferent arteriole constricts | Falls | Falls |
| Afferent arteriole dilates | Rises | Rises |
| Efferent arteriole constricts (moderate) | Rises | Rises |
| Low plasma protein | Less opposing oncotic pressure | Rises |
| Ureter blocked | Capsule pressure rises | Falls |
Figure 3 · Renal autoregulation
Two local mechanisms keep GFR steady. The myogenic response constricts the afferent arteriole when pressure stretches it. In tubuloglomerular feedback, cells of the macula densa sense salt delivery to the distal tubule and adjust afferent tone.
Sympathetic activity, for example in heavy exercise or haemorrhage, constricts renal arterioles and lowers renal blood flow. Angiotensin II constricts the efferent arteriole more than the afferent, which helps hold GFR when blood pressure is low.
Exercise, dehydration and anti-inflammatory drugs
Prostaglandins help keep the afferent arteriole open when renal blood flow is reduced. Non-steroidal anti-inflammatory drugs block them. Combining these drugs with dehydration and prolonged hard exercise increases the risk of kidney injury.
The clearance of a substance is the volume of plasma completely cleared of it per minute. A substance that is freely filtered and neither reabsorbed nor secreted, such as inulin, has a clearance equal to GFR.
In practice, GFR is estimated from blood creatinine, a waste product of muscle. Levey and colleagues developed the CKD-EPI equation, which estimated GFR more accurately than the earlier MDRD equation, especially at higher values [1].
Figure 4 · Categories of kidney function
The equation’s developers noted limited data for older people and some population groups [1]. Because creatinine comes from muscle, estimates can be misleading in people with very large or very small muscle mass, such as amputees, bodybuilders or frail older adults. Equations have since been revised; use the one your laboratory reports.
Rhabdomyolysis is the breakdown of skeletal muscle with release of its contents into the blood. Myoglobin damages the kidney tubules, especially when the patient is dehydrated and the urine is acidic [2]. Potassium release can cause dangerous heart rhythms [2].
| Cause [2] | Example relevant to physiotherapy |
|---|---|
| Unaccustomed intense exercise | First high-intensity class, eccentric overload, exercise in heat |
| Crush or prolonged immobility | Long lie after a fall, prolonged surgery |
| Drugs and toxins | Statins, alcohol, some recreational drugs |
| Heat illness | Exertional heat stroke |
| Seizures and electrolyte disorders | Low potassium or phosphate |
Red flags after exercise
Muscle pain and swelling out of proportion to the session, marked weakness and dark, tea-coloured urine suggest rhabdomyolysis [2]. Stop exercise, encourage fluids if allowed, and arrange urgent medical assessment, including creatine kinase and kidney function.
People with chronic kidney disease are often very deconditioned. Heiwe and Jacobson reviewed 45 trials in 1,863 adults with CKD or kidney transplants [3].
Figure 5 · Exercise in chronic kidney disease
Regular exercise improved aerobic and walking capacity, lowered resting blood pressure and heart rate, and improved quality of life [3]. Programmes included aerobic, resistance, mixed and yoga-based exercise, both supervised and home-based [3].
Practical skill · exercising patients on dialysis
Protect the arm with an arteriovenous fistula: no blood pressure cuff, tight straps or heavy loading on it. Exercise on non-dialysis days or early in a dialysis session, monitor blood pressure, and adjust for fluid shifts and fatigue.
| Situation | Concern | Action |
|---|---|---|
| Dark urine and severe thigh pain two days after a first spin class | Rhabdomyolysis [2] | Stop exercise; urgent medical review |
| eGFR 38 in a frail 85-year-old | Category G3b, but creatinine may underestimate impairment | Exercise with monitoring; review medications with team |
| Patient on haemodialysis, breathless and tired | Deconditioning, anaemia, fluid shifts | Graded aerobic and resistance training [3] |
| Older patient found on the floor after a night | Crush-related muscle breakdown [2] | Medical check before mobilising |
Define glomerular filtration rate. Describe the factors that determine and regulate it.
Kidney patients are advised to rest; GFR is described mainly through inulin clearance.
eGFR equations such as CKD-EPI estimate function from creatinine with useful accuracy [1].
Regular exercise benefits adults with CKD [3], while sudden intense exercise can cause rhabdomyolysis [2].
Protect the kidney from sudden insults, but do not protect it from exercise. Graded training is part of kidney care.
10 questions on this page. Tap one to see the answer and the reasoning.
Answer: (B) The equation regresses measured GFR on standardised creatinine, sex, age and other variables [1].
Answer: (B) CKD-EPI showed less bias and greater accuracy, particularly at higher GFR [1].
Answer: (B) 84.1% of estimates fell within 30% of measured GFR [1].
Answer: (A) Muscle breakdown releases myoglobin that damages the kidney [2].
Answer: (B) Potassium released from damaged muscle can cause hyperkalaemia [2].
Answer: (B) Myoglobinuria darkens the urine [2].
Answer: (B) Aerobic capacity improved, SMD about 0.56 [3].
Answer: (B) Systolic pressure fell by about 6.08 mmHg [3].
Answer: (B) Forty-five RCTs with 1,863 participants [3].
Answer: (B) The equations assume typical creatinine generation; they had limited data in some groups, such as elderly people [1].
Key takeaways
| Source | Why it earns its place |
|---|---|
| Levey AS et al. — Ann Intern Med, 2009 | The CKD-EPI equation [1] |
| Bosch X et al. — NEJM, 2009 | Rhabdomyolysis explained [2] |
| Heiwe S, Jacobson SH — Cochrane Review, 2011 | Exercise in CKD [3] |
| Eaton DC, Pooler JP — Vander’s Renal Physiology | Clear account of renal physiology |
Reviewed by the Physiotherapist India Team. · Human Physiology contents