Blood Volume and Pressure Responses
A decrease in blood volume and blood pressure results in:
Release of renin from kidneys.
Activation of the angiotensin-aldosterone system.
Increased thirst and fluid retention.
Urine Color and Vasopressin Levels
Pale yellow, large volume urine infers low vasopressin levels.
Low levels indicate less water reabsorption in kidneys, diluting urine.
Nephron Loop
Descending Limb:
Water exits the tubule due to high osmolarity in the medulla, concentrating the urine.
Ascending Limb:
Sodium, potassium, and chloride are reabsorbed, making urine less concentrated.
Aldosterone Production and Target
Produced in the adrenal cortex.
Targets principal cells in the distal nephron for sodium and potassium balance.
Water Gain vs. Loss
Gain:
Food and drink: 2.2 L/day.
Metabolism: 0.3 L/day.
Loss:
Skin: 0.9 L/day.
Lungs: 0.3 L/day.
Urine: 1.5 L/day.
Feces: 0.1 L/day.
Total Intake: 2.2 L/day.
Total Output: 2.5 L/day (balance = 0).
Hormone Regulation
Aldosterone:
Regulates potassium levels, acting on principal cells of the nephron.
Involved in both reabsorption and secretion.
Action Mechanism of Aldosterone:
Combines with cytoplasmic receptor in the principal cell.
Initiates transcription for new channels and pumps.
Increases sodium reabsorption and potassium secretion in distal nephron.
Effects of K+ Concentration on Neurons
Decrease in plasma K+:
Increases resting membrane potential, leading to reduced excitability.
Increase in plasma K+:
Decrease in resting membrane potential, leading to greater excitability.
Thirst Mechanism
Osmoreceptors in hypothalamus signal thirst when osmolarity rises above 280 mOsM, triggering drinking response.
Salt Appetite Mechanism
Craving for salty foods when Na+ concentrations drop.
Centers for appetite linked to aldosterone.
pH and Hydrogen Ion Concentration
Normal pH range in the body: 7.38 - 7.42.
Effects of pH Changes
Acidosis: Neurons less excitable; CNS depression may occur.
Alkalosis: Neurons become hyper excitable; symptoms include muscle twitches and paralysis.
Three Mechanisms:
Buffers: HCO3 in extracellular fluid, proteins, and hemoglobin.
Ventilation: Adjusting CO2 levels via breathing.
Renal: Directly excreting H+ or changing HCO3- levels.
Classification: Acidosis or Alkalosis, Metabolic or Respiratory based on hydrogen ions/CO2 levels.
Compensation Mechanisms:
Renal and respiratory mechanisms can adjust pH but may not correct the underlying problem.
Common Disturbances:
Respiratory Acidosis (hypoventilation) vs. Respiratory Alkalosis (hyperventilation).
Case Study in Acid-Base State:
Example: Patient with asthma attack showing:
HCO3- = 30 mEq/L, PCO2 = 70 mmHg, pH = 7.24 indicating respiratory acidosis.
Review Schedule: Regularly review material and complete problem sets before exam dates.