Équilibre acido-basique

Normal arterial blood pH is 7.4, with venous blood and interstitial fluid at 7.35, and intracellular fluid averaging 7.0. Maintaining this narrow pH range is crucial because changes in hydrogen ion concentration can disrupt cell membrane stability, alter protein structures, and affect enzyme activities. Even small deviations impair critical body functions.

A drop in arterial blood pH below 7.35 leads to acidosis, a dangerous condition that impairs the central nervous system, causing coma, and weakens cardiac contractions. Severe acidosis can be fatal, leading to low blood pressure and potential circulatory collapse. Acidosis with pH below seven poses immediate life-threatening risks.

Chemical buffers resist pH changes by binding hydrogen ions during acidity and releasing them during alkalinity. This mechanism maintains stability in body fluids by preventing rapid pH fluctuations. Buffer systems work continuously to neutralize excess acids or bases, protecting cellular function and enzyme activity.

Respiratory compensation changes the rate and depth of breathing to control carbon dioxide and carbonic acid concentrations in the blood. By adjusting ventilation, the lungs can increase or decrease acid levels, helping restore normal pH. This rapid response mechanism works alongside other systems to maintain acid-base equilibrium.

Renal regulation involves the kidneys modulating the secretion and reabsorption rates of hydrogen and bicarbonate ions in response to blood pH changes. The kidneys adjust these ion levels to correct acidosis or alkalosis, providing a slower but more sustained compensation mechanism than respiratory responses.

Alkalosis occurs when arterial blood pH rises above 7.45, the opposite of acidosis. While acidosis results from excess hydrogen ions, alkalosis results from insufficient hydrogen ions or excess bicarbonate. Both conditions affect all body systems, particularly the nervous and cardiovascular systems, and require compensation mechanisms to restore normal pH.

Buffer systems provide immediate pH resistance, respiratory compensation adjusts carbon dioxide levels within minutes, and renal regulation modulates ion secretion over hours to days. These three mechanisms work synergistically to keep body pH within narrow limits, with each system compensating when others cannot fully restore balance.