lecture 23 physiology

Lecture Information

• Course: BIO 1220 Human Physiology

• Lecture: 23, Urinary System

• Instructor: Dr. Suzanne Gray, UPEI - Winter 2025

1. Body Fluids

a. Body Water Content

• Water content varies by:

  • Age: Body water decreases with age.

  • Sex: Males generally have higher body water percentage than females.

• Typical Body Water Percentages:

  • 50% in older adults

  • 70% in infants

  • 45% in elderly

b. Fluid Compartments

• Types of Fluids:

  • Intracellular Fluid: inside cells

  • Extracellular Fluid: outside cells

    • Interstitium: fluid in tissue spaces

    • Plasma: fluid portion of blood

• Composition:

  • Total volume: 25 L as intracellular fluid (ICF) and 15 L as extracellular fluid (ECF).

  • 80% of ECF is interstitial fluid, the remaining 20% includes lymph, cerebrospinal fluid, and gastrointestinal secretions.

c. Composition of Body Fluid

• Solvents and Solutes:

  • Water is the universal solvent; solutes classify as electrolytes and nonelectrolytes.

  • Electrolytes: charged particles (e.g., ions) that dissociate in solution.

  • Nonelectrolytes: don't dissociate in solution (e.g., glucose, lipids).

d. Fluid Movement Among Compartments

• Water moves between compartments based on osmotic gradients.

• Movement occurs via:

  • Capillary walls

  • Cell membranes

• Mechanisms of Exchange:

  • Osmotic pressures regulate fluid movement into and out of capillaries and cells.

2. The Urinary System

a. Overview of the System

• Components include the kidneys, ureters, bladder, and urethra.

• Kidneys filter blood and produce urine, while ureters transport urine to the bladder, where it is stored before excretion.

b. Kidney Structure and Function

• Nephron: The functional unit of the kidney; over 1 million per kidney.

  • Comprises glomerulus, Bowman’s capsule, proximal convoluted tubule, etc.

• Blood Filtering:

  • ~1200 ml of blood passes through the glomeruli each minute (650 ml as plasma).

  • ~125 ml filtered into Bowman’s capsule.

c. Processes of Kidney Function

• Filtration:

  • Unselectively filters plasma components into Bowman’s capsule.

• Reabsorption:

  • Selective reabsorption occurs mainly in the proximal convoluted tubule (PCT).

• Secretion:

  • Removes excess waste from blood into urine.

3. Water Balance

a. Regulation of Water Intake

• Controlled by thirst mechanism in response to osmotic conditions in the extracellular fluid (ECF).

b. Regulation of Water Output

• Excretion via urine must equal intake to maintain hydration.

• Minimum urine output to remove metabolic wastes is 500 ml/day.

c. Antidiuretic Hormone (ADH)

• Increases water reabsorption in kidneys, concentrating urine.

d. Disorders of Water Balance

• Dehydration: excess water loss relative to intake; causes include vomiting, diarrhea.

• Hypotonic Hydration: overhydration; can disrupt neuron function.

• Edema: accumulation of fluid in interstitial spaces due to increased capillary pressure or permeability.

4. Electrolyte Balance

a. Sodium Balance

• Sodium is the main component influencing osmotic pressure in ECF.

• Sodium reabsorption is mostly managed in the proximal tubule and loop of Henle.

b. Potassium Balance

• Critical for neural and muscle function; slight changes can be life-threatening.

• Majority of potassium is reabsorbed in the PCT and loop of Henle but is primarily regulated in the collecting duct.

c. Calcium Balance

• Calcium is essential for bones, clotting, and neuromuscular function.

• Regulated by parathyroid hormone affecting reabsorption and mobilization from bones.

5. Renal Acid-Base Balance

a. Regulation of pH

• The kidneys regulate acidity by controlling H+ and bicarbonate (HCO3-) levels via filtration and secretion processes.

b. Secretion Processes

• Blood acidity is counteracted by excreting H+, adjusting HCO3- levels, and utilizing ammonia to absorb H+.

Conclusion

• Key topics covered in lecture include the structure and function of the urinary system, body fluid composition, water and electrolyte balance, and renal regulation of acid-base homeostasis.