Urinary System & Fluid Balance — Study Notes

Urinary System & Fluid Balance

How the Kidneys Promote Homeostasis

The kidneys are essential organs that maintain the body's internal environment by filtering blood, removing waste, and regulating fluid and electrolyte balance. They also play a key role in hormone production and blood pressure regulation.

• Filtration: Removes waste and toxins from the blood.

• Regulation: Maintains fluid, electrolyte, and pH balance; regulates blood volume and pressure.

• Hormone Production: Produces erythropoietin (EPO), calcitriol (active vitamin D), and renin.

• Perfusion: Kidneys receive approximately 20% of cardiac output for filtration.

Summary: Kidneys filter blood, regulate fluid and electrolytes, and produce hormones.

Where Urine Comes From

Urine formation occurs through a series of steps in the nephron, the functional unit of the kidney.

• Filtration: Occurs in the glomerulus and Bowman's capsule.

• Reabsorption: Takes place in the proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT).

• Secretion: Additional wastes are secreted into the tubule.

• Excretion: Final urine is collected in the collecting duct and exits via the bladder and urethra.

Process: Filtration → Reabsorption → Secretion → Excretion

Renin–Angiotensin–Aldosterone System (RAAS)

The RAAS is a hormone system that regulates blood pressure and fluid balance.

• Triggered by: Low blood pressure or low sodium.

• Sequence: Renin (from kidney) → Angiotensinogen (from liver) → Angiotensin I → ACE (from lungs) → Angiotensin II → Aldosterone (from adrenal cortex), ADH, thirst.

• Effect: Raises blood pressure and increases sodium and water reabsorption.

Bicarbonate & Blood pH Regulation

The kidneys help maintain acid-base balance by regulating bicarbonate (HCO3-) and hydrogen ion (H+) excretion.

• Normal blood pH: 7.35–7.45

• Acidosis: Blood pH < 7.35; kidneys excrete H+ and reabsorb HCO3-.

• Alkalosis: Blood pH > 7.45; kidneys excrete HCO3- and retain H+.

• Type A cells: Secrete H+ (combat acidosis).

• Type B cells: Secrete HCO3- (combat alkalosis).

Specialized Kidney Cells

• Granular (juxtaglomerular) cells: Release renin in response to low blood pressure.

• Macula densa: Detects sodium chloride concentration in the distal tubule.

• Intercalated cells: Regulate acid-base balance (Type A and B cells).

Osmolarity & Fluid Movement

Osmolarity refers to the concentration of solutes in a solution, affecting water movement across cell membranes.

• Hypertonic: Water exits cells, causing them to shrink.

• Hypotonic: Water enters cells, causing them to swell.

• Isotonic: No net water movement; cell size remains unchanged.

Fluid Movement Between Compartments

Water moves between intracellular and extracellular compartments based on osmotic gradients and membrane permeability.

• ICF (Intracellular Fluid): Fluid inside cells; about two-thirds of total body water.

• ECF (Extracellular Fluid): Fluid outside cells; includes plasma and interstitial fluid.

• High Na+: More water outside cells (ECF).

• High K+: More water inside cells (ICF).

Hormones Controlling Fluid Balance

• Aldosterone: Increases sodium and water reabsorption, raising blood pressure.

• ADH (Antidiuretic Hormone): Increases water reabsorption in the kidneys, reducing urine output.

• ANP (Atrial Natriuretic Peptide): Inhibits sodium reabsorption, lowering blood pressure.

• Renin: Activates the RAAS pathway to increase blood pressure.

Fluid Intake & Output

• Intake: Water, food, metabolic water.

• Output: Urine, feces, sweat, exhalation.

• Obligatory loss: Minimum urine output required to remove wastes.

• Facultative loss: Variable urine output depending on hydration status.

IV Solution Types

Type	Description	Effect
0.9% NaCl (Normal Saline)	Isotonic	No fluid shift
Pure Water	Hypotonic	Cells swell; risk of lysis
Hypertonic Saline	Hypertonic	Cells shrink; water leaves cells

Common Fluid Imbalances

• Dehydration: Water loss exceeds intake; plasma becomes hypertonic, cells shrink.

• Overhydration: Excess water intake; plasma becomes hypotonic, cells swell.

• Edema: Excess fluid in interstitial spaces; may result from heart, liver, or kidney failure.

Age & Fluid Regulation

• Infants: Higher fluid turnover, increased risk of dehydration.

• Elderly: Lower fluid reserves, reduced kidney function, increased risk of dehydration.

Nephron Flow Diagram

Pathway of filtrate through the nephron:

Glomerulus → Bowman's Capsule → PCT → Loop of Henle → DCT → Collecting Duct → Ureter

RAAS Pathway Diagram

Sequence of events in the RAAS:

BP ↓ → Renin → Angiotensinogen → Angiotensin I → ACE → Angiotensin II → TPR (vasoconstriction), Aldosterone, Thirst

Tonicity Diagram

Hypotonic	Isotonic	Hypertonic
Cells swell	No change	Cells shrink

Additional info: These notes cover key aspects of the urinary system and fluid balance, including nephron function, hormonal regulation, and clinical applications such as IV fluids and fluid imbalances. The content is suitable for exam preparation in a college-level Anatomy & Physiology course.