GLOSSARY

Renal blood flow, or RBF, is the amount of blood passing through the kidneys each minute. In a healthy adult it is about 1.1–1.2 litres per minute (about 20–25 percent of cardiac output), one of the highest blood flows per gram of tissue in the body.

Distribution of Blood Flow

About 90 percent of kidney blood flow goes to the cortex and only about 10 percent to the medulla. The low medullary flow is needed to preserve the osmotic gradient, but makes the medulla vulnerable to low oxygen.

Renal Plasma Flow

Renal plasma flow (RPF) is about 600–650 mL per minute and is measured by para-aminohippurate (PAH) clearance.

Control

• Autoregulation: through the myogenic mechanism and tubuloglomerular feedback, RBF stays roughly constant at blood pressures of about 80–180 mmHg.
• Sympathetic: reduces kidney blood flow during stress, exercise and low blood pressure.
• Hormones: angiotensin II, endothelin and adrenaline (decrease); prostaglandins, nitric oxide, dopamine and ANP (increase).
• Diet: a high-protein meal increases RBF and GFR.

Relevance to Sport

• Fall with intensity: RBF falls by about 30 percent in moderate exercise and by up to about 75 percent in maximal exercise; heat and dehydration intensify this fall.
• Practical example: the combination of long exercise, heat, dehydration and NSAIDs is the most dangerous situation for the kidney; for pain relief in an ultramarathon, paracetamol at an appropriate dose is a better choice than ibuprofen.