Physiology · MU Sofia

67. Regulation of acid-base balance. Buffer systems. Respiratory and renal control of pH.

Covers how the body keeps blood pH near 7.4, starting with buffers: the bicarbonate/CO2, phosphate and protein systems, plus intracellular buffers and how H+ crosses cell membranes. It then describes respiratory control through central and peripheral chemoreceptors, and renal control through bicarbonate reabsorption, titratable acid and NH4+ excretion, and compensation for respiratory disturbances. Includes a bicarbonate shift explanation and a short list of recap questions.

Inside: 4 sections · 8 labelled pictures · 21 flashcards · 7 min

Topic 66 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 4 sections, about 974 words (7 min of reading), with 21 flashcards.

In this note

  1. Regulation of acidbase balance. Buffer systems. Respiratory and renal control of pH.
  2. Buffer Systems
  3. Respiratory Control of pH
  4. Renal Control of pH

Regulation of acidbase balance. Buffer systems. Respiratory and renal control of pH.

The body produces large amounts of carbon dioxide (CO2) (volatile acid) and fixed acids (nonvolatile acids) daily. However, the pH of body fluids is normally maintained at 7.4. The mechanisms that maintain a constant pH include buffering, respiratory compensation, and renal compensation.

Buffer Systems

Buffers are the first line of defense in protecting the pH. A buffer is a mixture of a weak acid and its conjugate base. The most effective physiologic buffers have a pK near 7.4. Buffers resist changes in pH: H+ can be added to or removed from a buffered solution, but the pH of that solution will change only minimally.

  • Extracellular buffers include bicarbonate/CO2 (the most important) and phosphate.
  • The bicarbonate/CO2 buffer system is more effective than the phosphate buffer system because:-
  1. (1) bicarbonate is present in much higher concentration
  2. (2) the acid form of the buffer is CO2, which is volatile and can be expired by the lungs.
  • Intracellular buffers include organic phosphates and proteins (e.g., deoxyhemoglobin).

Bicarbonate Buffer System

The bicarbonate buffer system is the primary buffer system in the ECF. This system involves the following reactions:-

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

where:

  • CO2 is carbon dioxide
  • H2O is water
  • H2CO3 is carbonic acid
  • H+ is hydrogen ion
  • HCO3- is bicarbonate ion

This system is particularly effective because CO2, the acid form of the buffer, can be expired by the lungs. When a strong acid like HCl is added to the ECF, it reacts with HCO3- to form H2CO3, which is a weak acid. H2CO3 then dissociates into CO2 and H2O, which are expired by the lungs. This helps to minimize changes in blood pH.

Bicarbonate (HCO3⁻)

  • Role in Blood: The most abundant form (85–90%) in the blood, bicarbonate acts as a pH buffer. It is formed from carbonic acid (H2CO3) dissociating into HCO3⁻ and H⁺.
  • Hamburger’s Shift: When HCO3⁻ is produced in erythrocytes, it exits the cell, and chloride ions (Cl⁻) enter to maintain electrochemical balance. This exchange is known as the Hamburger’s phenomenon.
  • Binding with Hemoglobin: The H⁺ from dissociated H2CO3 binds to hemoglobin (Hb), forming HHb.
  • In the Lungs:
  • HCO3⁻ reenters erythrocytes (exchanging with Cl⁻), reacts with H⁺ from hemoglobin, and forms H2CO3.
  • H2CO3 breaks down into H2O and CO2, which is expelled by the lungs.

Phosphate Buffer System

The phosphate buffer system is the second most important buffer system in the ECF. This system uses the following reactions:

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