lecture 23 and 24 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.