Intake, elimination & reproduction · Topic 17 of 18

Urinary System

Kidney, Nephron & Urine Formation

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A&P · urinary system

Kidney, Nephron & Urine Formation

Nephrons filter blood, reabsorb what the body needs and secrete extras; the urine then flows from kidney to ureter, bladder and urethra.

01 · The pathKidney → ureter → bladder → urethra

02 · The unitNephron = functional unit

03 · Three stepsFilter → reabsorb → secrete

What it is

Nephron = the kidney's functional unit (about 1 million per kidney) must know

Loop of Henle reabsorbs water + salt so the kidney can concentrate urine

DCT + collecting duct fine-tune urine under hormone control

Urine = water + urea, creatinine, salts; normally no glucose or protein

Kidneys also make EPO (red cells) and activate vitamin D (calcitriol)

Parts & jobs

Urine path: kidney → ureter → bladder → urethra must know

Bowman's capsule → PCT → loop of Henle → DCT → collecting duct must know

Kidney: outer cortex, inner medulla (pyramids), renal pelvis → ureter

Afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries

Bladder: detrusor muscle; internal sphincter involuntary, external voluntary

Making urine

Urine = filtration → reabsorption → secretion

Filtration: pressure pushes water + small solutes out; cells and proteins stay

PCT reabsorbs most: ~⅔ of water + Na⁺, nearly all glucose + amino acids

Secretion: blood → tubule (H⁺, K⁺, some drugs and wastes)

Know by heart

~180 L filtered a day → only 1–2 L of urine (~99% reabsorbed)

Urethra: about 4 cm in females vs about 20 cm in males

Reabsorption or secretion?

Commonly confused

	Reabsorption	Secretion
Direction	Tubule → blood	Blood → tubule
Effect	Keeps the substance in the body	Adds the substance to the urine
Examples	Water, Na⁺, glucose, amino acids	H⁺, K⁺, some drugs and wastes
Where it happens most	Proximal convoluted tubule	Along the tubule; fine-tuned in DCT and collecting duct

More detail

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Descending limb loses water; ascending limb pumps out salt, holds water

GFR ≈ 125 mL/min (males), about 105 mL/min (females)

Minimum urine output to clear wastes ≈ 500 mL a day

Kidneys are retroperitoneal (left ≈ T12–L3); the right one sits lower

Ureters move urine to the bladder by peristalsis

Bladder stretch receptors trigger the urination reflex

Pearl

Filter a lot, take most of it back, add a little: about 180 L filtered a day leaves as only 1–2 L of urine.

Sources · OpenStax Anatomy & Physiology 2e §25.1 Physical Characteristics of Urine · OpenStax Anatomy & Physiology 2e §25.2 Gross Anatomy of Urine Transport · OpenStax Anatomy & Physiology 2e §25.3 Gross Anatomy of the Kidney · OpenStax Anatomy & Physiology 2e §25.4 Microscopic Anatomy of the Kidney · OpenStax Anatomy & Physiology 2e §25.5 Physiology of Urine Formation · OpenStax Anatomy & Physiology 2e §25.6 Tubular Reabsorption · OpenStax Anatomy & Physiology 2e §25.10 The Urinary System and Homeostasis · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 4 (urinary system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Fluid, Electrolyte & Blood-Pressure Regulation

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A&P · urinary system · fluid balance

Fluid, Electrolyte & Blood-Pressure Regulation

Kidneys set blood volume and pressure by how much water and sodium they keep, steered by ADH, aldosterone, renin-angiotensin and ANP.

01 · ADHADH = keep water

02 · AldosteroneAldosterone = keep Na⁺, lose K⁺

03 · Renin-angiotensinLow BP → renin → angiotensin II → BP up

What it is

Kidneys set blood volume and BP by how much water + Na⁺ they keep must know

Concentrated blood or low blood volume → thirst + ADH release

Less ADH → fewer water channels → lots of dilute urine

Alcohol blocks ADH release → more urine (a diuretic)

Angiotensin II: constricts vessels (fast), aldosterone + thirst (slower)

ANP (from stretched atria) → lose Na⁺ + water → BP down

Kidneys balance pH: secrete H⁺ and keep bicarbonate (HCO₃⁻)

Renin-angiotensin-aldosterone system

Low BP → renin → angiotensin II → vessels constrict + aldosterone → BP up

Angiotensinogen → (renin) → angiotensin I → (ACE) → angiotensin II

Know by heart

Na⁺ = main cation outside cells; K⁺ = main cation inside cells

Water in ≈ water out ≈ 2,500 mL a day; urine is the biggest loss

The rules

ADH → collecting ducts keep water → less, more concentrated urine must know

Aldosterone → keep Na⁺ (water follows), excrete K⁺ must know

ADH or aldosterone?

Commonly confused

	ADH (vasopressin)	Aldosterone
Released by	Posterior pituitary (made in hypothalamus)	Adrenal cortex
Main trigger	Blood too concentrated; low blood volume	Angiotensin II; high blood K⁺
Acts on	Collecting ducts (water channels)	DCT and collecting ducts
What the kidney keeps	Water only	Na⁺, with water following; K⁺ is excreted
Result	Less, more concentrated urine	More blood volume; more K⁺ in urine

Mix-ups to avoid

ADH saves water alone; aldosterone saves salt and water comes along

More detail

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Renin (kidney JG cells) + angiotensinogen (liver) + ACE (lungs)

Juxtaglomerular apparatus: macula densa senses Na⁺; JG cells release renin

High blood K⁺ also triggers aldosterone release

PTH: kidneys keep Ca²⁺, lose phosphate, activate vitamin D

Coffee, tea and alcohol are common diuretics

High levels of ADH constrict vessels — hence the name vasopressin

Pearl

ADH holds on to water; aldosterone holds on to salt, and water follows the salt.

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