13.2 Urinary Physiology

13.2 Urinary Physiology

Quick Answer: Kidneys form urine by filtration, reabsorption, and secretion. GFR describes how fast glomeruli filter plasma. ADH saves water; aldosterone saves sodium (and water follows). Kidneys adjust H⁺ and bicarbonate for acid–base balance. Micturition empties the bladder under reflex plus voluntary sphincter control. Anatomy gave you nephrons and conduits; physiology explains how blood becomes urine and how volume, osmolarity, and pH stay livable for NEX.

Every day the kidneys filter a huge volume of plasma yet excrete only about 1–2 L of urine because most filtered water and solutes are reclaimed. Nursing entrance items ask what filtration vs reabsorption means, how ADH changes urine volume, or what happens when the bladder fills. Start with the three urine-formation processes, then hormones, acid–base, and voiding.

Three Processes of Urine Formation

Urine composition conceptually:

Urine = filtered − reabsorbed + secreted

If a substance is freely filtered and completely reabsorbed (e.g., glucose under normal conditions), little appears in final urine. If it is filtered and not reabsorbed (e.g., much of creatinine), it is excreted. Secretion can raise urine content of a substance above what filtration alone would produce.

Filtration Details (Intro Depth)

Filtration depends on blood pressure in glomerular capillaries and the permeability of the filtration membrane. The filtrate resembles plasma without proteins. Anything that sharply drops renal perfusion or damages the filter can change what enters the tubule—clinical motivation without needing Starling-equation physics.

Reabsorption and Secretion Highlights

GFR: Glomerular Filtration Rate Concept

Glomerular filtration rate (GFR) is the volume of filtrate formed by both kidneys per unit time (often expressed in mL/min). It is an intro index of how well glomeruli are filtering.

You do not need to calculate GFR from clearance formulas for NEX Science, but you must know GFR = rate of filtrate formation and that it depends on adequate renal blood flow and intact glomeruli. Autoregulation keeps GFR relatively stable across ordinary blood-pressure ranges; extreme hypotension can drop GFR and urine output.

ADH and Aldosterone: Water and Sodium

Two hormones dominate intro fluid–electrolyte physiology at the kidney (you met ADH in endocrine physiology; here attach the renal effect).

Antidiuretic Hormone (ADH, Vasopressin)

Antidiuretic literally means “against diuresis”—ADH reduces urine water loss. Without effective ADH action, large volumes of dilute urine are lost (diabetes insipidus concept at awareness level).

Aldosterone

Exam trap: ADH is not “the sodium hormone,” and aldosterone is not primarily the collecting-duct water-pore hormone—though both can reduce urine volume through related mechanisms.

Kidneys and Acid–Base Balance (Brief)

Blood pH must stay near 7.35–7.45. Lungs adjust CO₂ quickly; kidneys adjust acid and base more slowly but powerfully.

In acidosis, kidneys increase H⁺ secretion and bicarbonate conservation/generation. In alkalosis, they excrete more bicarbonate. Intro exams rarely demand full titration curves—own the idea that kidneys regulate H⁺ and HCO₃⁻ to support pH.

Micturition Overview

Micturition (urination) is emptying of the urinary bladder.

In infants, the reflex empties the bladder automatically. In continent adults, awareness of fullness plus voluntary external-sphincter control permits timed voiding. After spinal injury, patterns of retention or reflex incontinence can appear depending on lesion level—clinical anchor for why anatomy of sphincters matters.

Ureteral peristalsis still delivers urine from kidney to bladder between voids; storage and elimination are bladder–urethra physiology.

Integrated Fluid Story

1. Glomeruli filter plasma → filtrate enters tubules.
2. PCT reclaims bulk of water, Na⁺, glucose, amino acids.
3. Loop and distal segments fine-tune concentration and ion balance.
4. ADH sets collecting-duct water permeability; aldosterone sets distal Na⁺ salvage.
5. Secretion adds H⁺, K⁺, and selected wastes/drugs.
6. Final urine reaches the bladder; micturition empties it under reflex + voluntary control.

Clinical and Nursing Anchors

• Oliguria / anuria may reflect low GFR or obstruction; interpret with volume status.
• Daily weights and I&O track water balance influenced by ADH and aldosterone.
• Diuretics often target reabsorption steps (clinical pharmacology later).
• ABGs and electrolytes link to renal H⁺/HCO₃⁻ and K⁺ handling.
• Catheterization and bladder scans assess storage/emptying when micturition fails.
• UTI risk rises with stasis—incomplete emptying is a physiologic risk factor.

Exam Traps

• Filtration ≠ reabsorption ≠ secretion — learn directions relative to blood vs tubule.
• GFR is filtrate formation rate, not “how much urine you pee today” (urine is after reabsorption/secretion).
• ADH → water; aldosterone → Na⁺ (water follows Na⁺).
• Glucose in urine suggests filtered load exceeded reabsorptive capacity, not that glucose is normally excreted.
• External sphincter is voluntary; internal is involuntary.
• Kidneys adjust H⁺/HCO₃⁻ — lungs adjust CO₂; do not swap those primary tools.

Study Map for NEX

1. Define filtration, reabsorption, and secretion with an arrow each (blood ↔ tubule).
2. State what GFR measures in one sentence.
3. Fill a two-column ADH vs aldosterone table from memory.
4. Explain one renal response that helps correct acidosis.
5. Narrate micturition: stretch → detrusor → sphincters → voluntary override.

Renal control of volume and composition sets the internal environment that immune and reproductive systems also depend on—next, immune physiology explains how the body defends that environment.