Acid-Base Balance, Homeostasis, and Digestive System: Study Guide

Acid-Base Balance in the Human Body

Normal Blood pH and Disorders

The pH of blood is tightly regulated to maintain proper physiological function. Deviations from the normal range can result in serious health consequences.

• Normal Blood pH: The normal arterial blood pH is approximately 7.35–7.45.

• Acidosis: A condition where blood pH falls below 7.35. Common causes:

  • Respiratory acidosis (e.g., hypoventilation, chronic obstructive pulmonary disease)

  • Metabolic acidosis (e.g., diabetic ketoacidosis, lactic acidosis, renal failure, severe diarrhea)

• Alkalosis: A condition where blood pH rises above 7.45. Common causes:

  • Respiratory alkalosis (e.g., hyperventilation, anxiety, high altitude)

  • Metabolic alkalosis (e.g., vomiting, excessive antacid use, diuretic therapy)

Bicarbonate Buffer System

The bicarbonate buffer system is the primary buffer system in the blood, helping to maintain pH homeostasis.

• Definition: A chemical system that minimizes changes in pH by reversible conversion between carbonic acid (H2CO3) and bicarbonate ion (HCO3-).

• Key Reaction: $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

• Formation of Carbonic Acid: Carbon dioxide (CO2) produced by cellular metabolism dissolves in blood and reacts with water to form carbonic acid, catalyzed by the enzyme carbonic anhydrase.

Feedback Loops and Organ Systems in pH Regulation

Blood pH is regulated by negative feedback mechanisms involving the respiratory and renal systems.

• High Blood pH (Alkalosis):

  • Detected by chemoreceptors in the brain and arteries.

  • Respiratory compensation: Decreased ventilation (hypoventilation) increases CO2, shifting the buffer equation to produce more H+ and lower pH.

  • Renal compensation: Kidneys excrete more bicarbonate and retain hydrogen ions.

• Low Blood pH (Acidosis):

  • Detected by chemoreceptors.

  • Respiratory compensation: Increased ventilation (hyperventilation) removes CO2, shifting the buffer equation to consume H+ and raise pH.

  • Renal compensation: Kidneys retain bicarbonate and excrete hydrogen ions.

• Le Chatelier's Principle: Changes in CO2 or H+ concentration shift the equilibrium of the buffer system, altering pH.

Example: During exercise, increased CO2 production shifts the reaction to the right, increasing H+ and lowering pH (acidosis). Increased ventilation removes CO2, shifting the reaction left and raising pH.

Homeostasis and Acid-Base Balance

Definition and Application

Homeostasis is the maintenance of a stable internal environment despite external changes. The body uses feedback mechanisms to regulate variables such as temperature, pH, and glucose levels.

• Homeostasis Model: Involves sensors (receptors), a control center (e.g., brain), and effectors (e.g., lungs, kidneys).

• Application to Acidosis: When blood pH drops, sensors detect the change, the control center initiates responses (increased breathing rate, renal adjustments), and effectors restore pH to normal.

Example: In metabolic acidosis, the body compensates by increasing respiratory rate to expel CO2 and by the kidneys excreting more hydrogen ions.

Digestive System Overview

Main Organs and Their Functions

The digestive system is responsible for the breakdown and absorption of nutrients, as well as the elimination of waste.

• Mouth: Mechanical breakdown of food; saliva begins carbohydrate digestion.

• Pharynx and Esophagus: Transport food to the stomach via swallowing and peristalsis.

• Stomach: Secretes acid and enzymes; churns food to form chyme; begins protein digestion.

• Small Intestine: Major site of digestion and absorption; receives bile and pancreatic enzymes.

• Liver: Produces bile for fat emulsification; processes nutrients absorbed from the intestine.

• Gallbladder: Stores and concentrates bile, releasing it into the small intestine.

• Pancreas: Secretes digestive enzymes and bicarbonate into the small intestine; regulates blood glucose.

• Large Intestine (Colon): Absorbs water and electrolytes; forms and stores feces.

• Rectum and Anus: Control the elimination of feces from the body.

Example: After eating, food is digested in the stomach and small intestine, nutrients are absorbed, and waste is eliminated through the large intestine.