Human Anatomy & Physiology - Fluid and Electrolyte Balance: pH Balance
Human Anatomy & Physiology
Chapter 26 - Fluid and Electrolyte Balance
Part 2: pH Balance
pH Balance Overview
pH balance is crucial for maintaining homeostasis within the body.
Normal arterial blood pH is approximately 7.4, which can be affected by:
Acids and bases entering and exiting the bloodstream.
Output regulated through the kidneys and respiration.
Buffers that help manage changes in pH.
Factors Affecting pH Balance
Fixed Acids: Produced from metabolic processes and require regulation.
Volatile Acids: Primarily represented by carbonic acid which can be converted into CO2 and eliminated via respiration.
Sources of Excess H+
Excess hydrogen ions (H+) can arise from:
Dietary sources or metabolic activities.
Loss of bicarbonate ions (HCO3-) often due to diarrhea.
The kidneys manage excess H+ by:
Secreting H+ into urine (filtrate).
Producing bicarbonate ions (HCO3-) to be reabsorbed into the bloodstream.
Alkaline Conditions
Alkaline conditions are characterized by low H+ concentrations.
More common in certain diets (e.g., vegetarian) and excessive intake of antacids.
Vomiting can also elevate pH.
The kidneys react by:
Reabsorbing H+ ions into blood.
Producing HCO3- which is secreted into the urine (filtrate).
Role of Respiration in pH Balance
Respiration helps manage pH by eliminating excess CO2:
As CO2 levels rise, H+ levels increase, causing pH to drop (become more acidic).
This change stimulates increased respiratory activity to expel CO2 and restore pH balance.
Buffering Systems
Protein Buffers:
Found in plasma and intracellular fluid (ICF).
Account for approximately 75% of the body's buffering capacity.
Amino groups in proteins act as weak bases, while carboxylic groups function as weak acids.
Phosphate Buffering:
Predominantly present in ICF.
Composed of:
HPO4²- (weak base)
H2PO4- (weak acid)
Bicarbonate Buffering:
Found primarily in plasma.
Consists of:
HCO3- (weak base)
H2CO3 (weak acid)
Acid-Base Imbalance Conditions
Acidosis (acidemia): When blood pH falls below 7.35.
Alkalosis (alkalemia): When blood pH rises above 7.45.
pH levels beyond 7.0 or under 7.7 can be life-threatening.
A buffering capacity exceeding limits can lead to the above conditions.
Types of Acid-Base Imbalances
Respiratory Acidosis:
Caused by hypoventilation which leads to decreased airflow and impaired alveolar gas exchange.
Reaction: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (shifts towards more acid).
Respiratory Alkalosis:
Results from hyperventilation, often triggered by anxiety, hypoxia, or overdose of aspirin.
Diagnosed clinically by low blood CO2 and high pH.
Reaction: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (shifts toward less acid).
Metabolic Acidosis:
Associated with increased lactic acid, ketoacidosis, or acetic acidosis.
Can be aggravated by reduced kidney function and loss of HCO3- due to diarrhea.
Metabolic Alkalosis:
Often results from loss of H+ ions through vomiting, increased urine production due to diuretics, or excessive consumption of antacids.
Compensation Mechanisms
Respiratory Compensation:
In response to metabolic acidosis:
Increased rate and depth of breathing help lower blood CO2 and carbonic acid levels.
Limits exist for respiratory compensatory responses.
For metabolic alkalosis:
Slow shallow breathing allows CO2 to accumulate in the blood, attempting to restore pH balance.
Renal Compensation:
In cases of acidosis:
Increased secretion of H+ into filtrate and enhanced production/reabsorption of HCO3- into blood.
In cases of alkalosis:
Increased reabsorption of H+ ions and increased secretion of HCO3- into filtrate.