SOMAPL1J
The Urinary System
OBJ 2751 — Components & Functions
5 questions
OBJ 2752 — Kidney Structure
8 questions
OBJ 2753 — Blood Flow
8 questions
OBJ 2754 — Nephron & Urine Formation
14 questions
OBJ 2755 — Filtration & GFR
10 questions
OBJ 2756 — Tubular Fluid Changes
12 questions
OBJ 2757 — Ureters, Bladder & Urethra
7 questions
OBJ 2758 — Micturition
5 questions
OBJ 2759 — Water & Electrolyte Distribution
7 questions
OBJ 2760 — Fluid & Electrolyte Regulation
7 questions
OBJ 2761 — Buffer Systems
7 questions
OBJ 2762 — Acid-Base Threats
5 questions
OBJ 2763 — Aging
3 questions
OBJ 2764 — System Integration
2 questions
★ Final Score — SOMAPL1J
The Urinary System
Martini, Ober, Bartholomew — Essentials of Anatomy & Physiology (Pearson, 2013) · Chapter 18
Word Roots & Meanings
| Root / Prefix | Meaning | Example Term |
|---|---|---|
| calyx | a cup of flowers | minor calyx — cup-shaped drain receiving urine from a renal papilla |
| detrudere | to push down | detrusor muscle — smooth muscle of the bladder wall that compresses the bladder |
| fenestra | a window | fenestrated capillaries — glomerular capillaries with endothelial pores |
| glomus | a ball | glomerulus — ball-shaped capillary network in the renal corpuscle |
| gonion | angle | trigone — triangular area on the floor of the urinary bladder |
| juxta | near | juxtaglomerular complex — endocrine structure near the glomerulus |
| micturire | to urinate | micturition — the process of urination |
| nephros | kidney | nephron — the functional unit of the kidney |
| papillae | small, nipple-shaped projections | renal papillae — tips of renal pyramids that discharge urine |
| podon | foot | podocyte — cell with foot-like processes covering glomerular capillaries |
| rectus | straight | vasa recta — straight capillaries paralleling the nephron loop |
| ren | kidney | renal artery — artery supplying the kidney |
| retro- | behind | retroperitoneal — behind the peritoneum (kidney position) |
| vasa | vessels | vasa recta — straight vessels in the renal medulla |
Four Components
| Component | Function |
|---|---|
| Kidneys (paired) | Produce urine — a fluid containing water, ions, and small soluble compounds. Perform all excretory functions. |
| Ureters (paired) | Transport urine from the renal pelvis to the urinary bladder via peristalsis. |
| Urinary bladder | Muscular sac for temporary storage of urine prior to elimination. |
| Urethra | Conducts urine from the bladder to the exterior. In males, also transports semen. |
Three Primary Functions
Additional Homeostatic Functions
- Regulating blood volume and blood pressure by adjusting water lost in urine, and releasing erythropoietin and renin
- Regulating plasma ion concentrations (sodium, potassium, chloride, calcium via calcitriol synthesis)
- Stabilizing blood pH by controlling loss of hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻)
- Conserving nutrients — preventing excretion of glucose and amino acids while excreting nitrogenous wastes (urea and uric acid)
Position and External Features
The kidneys lie on either side of the vertebral column between the last thoracic and third lumbar vertebrae. The right kidney sits slightly lower than the left. Both are retroperitoneal — behind the peritoneum, between the dorsal body wall muscles and the peritoneal lining.
Position is maintained by: (1) overlying peritoneum, (2) contact with adjacent organs, (3) supporting connective tissues — fibrous capsule, cushion of adipose tissue, and dense fibrous outer layer with collagen fibers. Damage to these supports = floating kidney — dangerous because ureters or blood vessels may twist.
A typical kidney is ~10 cm long, ~5.5 cm wide, ~3 cm thick, ~150 g. The hilum is the medial indentation where the ureter exits, and the renal artery/nerve enter and renal vein exits.
Internal Anatomy
Renal Cortex (outer)
Covers the medulla. Contains the renal corpuscles and convoluted tubules of nephrons. Renal columns extend inward between pyramids.
Renal Medulla (inner)
Contains 6–18 conical renal pyramids. Tips = renal papillae projecting into the renal sinus.
Urine Drainage Pathway (within kidney)
A renal lobe = one renal pyramid + overlying cortex + adjacent renal column tissue.
Each kidney has roughly 1.25 million nephrons with a combined tubular length of ~145 km (85 miles).
Blood Supply Overview
The kidneys receive 20–25% of total cardiac output — about 1200 mL/min for organs weighing less than 300 g combined.
Arterial Pathway
Venous Pathway (mirrors arterial)
Peritubular capillaries/vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein → inferior vena cava.
Two Types of Nephrons
Cortical Nephrons (~85%)
Located mostly within the cortex. Shorter nephron loops. Peritubular capillaries only — no vasa recta.
Juxtamedullary Nephrons (~15%)
Located near the medulla. Long nephron loops extending deep into medulla. Peritubular capillaries connect to vasa recta (long, straight capillaries paralleling the loop). Enable production of concentrated urine.
Nephron = Functional Unit of the Kidney
Each nephron has two main parts: (1) a renal corpuscle and (2) a 50-mm-long renal tubule.
The Renal Corpuscle
Consists of the glomerular (Bowman's) capsule surrounding the glomerulus (capillary network). Blood arrives via the afferent arteriole and departs via the efferent arteriole. Blood pressure forces fluid and dissolved solutes out of glomerular capillaries into the capsular space — this is filtration. The resulting protein-free solution is called filtrate.
The Filtration Membrane
Three layers that determine what passes into the capsular space:
The filtration membrane blocks blood cells and most plasma proteins but permits water, metabolic wastes, ions, glucose, fatty acids, amino acids, and vitamins.
Segments of the Renal Tubule
| Segment | Location | Primary Function |
|---|---|---|
| Proximal convoluted tubule (PCT) | Cortex | Reabsorbs 60–70% of filtrate: all organic nutrients (glucose, amino acids), plasma proteins, and ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃⁻, etc.). Some H⁺ secretion. |
| Nephron loop (loop of Henle) | Extends into medulla | Descending limb: permeable to water, not solutes — water exits by osmosis. Ascending limb: impermeable to water AND solutes — actively pumps Na⁺ and Cl⁻ out. Creates medullary concentration gradient. |
| Distal convoluted tubule (DCT) | Cortex | Active secretion of ions, acids, drugs, toxins. Selective reabsorption of Na⁺ (aldosterone-controlled) and water (ADH-controlled). |
The Collecting System
DCT empties into collecting ducts → papillary ducts → minor calyx. The collecting system makes final adjustments: reabsorbing water, and reabsorbing or secreting sodium, potassium, hydrogen, and bicarbonate ions.
The Juxtaglomerular Complex
Located where the DCT contacts the afferent arteriole. Consists of: (1) macula densa — tall epithelial cells at DCT start that monitor tubular fluid, and (2) juxtaglomerular cells — smooth muscle cells in the afferent arteriole wall. Secretes renin (enzyme for blood pressure regulation) and erythropoietin (hormone stimulating RBC production).
Three Nephron Processes
Filtration Pressure
Filtration pressure is the net force promoting filtration at the glomerulus. It is very low — approximately 10 mm Hg. Filtration pressure is higher than in other capillary beds because the efferent arteriole is slightly smaller in diameter than the afferent arteriole. This creates back-pressure that elevates glomerular capillary blood pressure.
Glomerular Filtration Rate (GFR)
GFR = the amount of filtrate produced per minute, averaging 125 mL/min. This means ~20% of blood delivered to the kidneys enters the capsular spaces. Daily filtrate production: ~180 liters (48 gallons) — 70 times total plasma volume. Over 99% is reabsorbed; only 700–2000 mL exits as urine.
Three Levels of GFR Regulation
| Level | Mechanism | Details |
|---|---|---|
| Autoregulation (local) | Automatic diameter changes in afferent/efferent arterioles | Low blood flow: dilate afferent + constrict efferent → maintains glomerular pressure. High BP: constrict afferent → reduces flow. |
| Hormonal | Renin-angiotensin system, ADH, aldosterone, ANP | Long-term adjustments to blood pressure and volume. Renin released when glomerular pressure is low. |
| Autonomic (sympathetic) | Sympathetic vasoconstriction of afferent arterioles | Decreases GFR and shifts blood away from kidneys. During crisis (hemorrhage, heart attack), overrides autoregulation. |
The Renin-Angiotensin System
Activated when glomerular pressures remain low (decreased blood volume, falling systemic pressure, renal artery blockage):
ANP (atrial natriuretic peptide) opposes the renin-angiotensin system: released when blood volume/pressure is too high. Decreases sodium reabsorption, dilates glomerular capillaries, inhibits renin/aldosterone/ADH → more sodium and water lost in urine → lower blood volume and pressure.
Three Metabolic Waste Products Excreted in Urine
| Waste | Source | Daily Production |
|---|---|---|
| Urea (most abundant) | Amino acid breakdown | ~21 g/day |
| Creatinine | Creatine phosphate breakdown in skeletal muscle | ~1.8 g/day |
| Uric acid | RNA recycling/breakdown | ~480 mg/day |
What Happens at Each Segment
| Segment | Filtrate Status | Key Events |
|---|---|---|
| Renal corpuscle | Protein-free plasma | Filtration produces filtrate. Same composition as plasma minus proteins. |
| PCT | 60–70% of volume reabsorbed | All glucose, amino acids, plasma proteins reabsorbed. Na⁺, K⁺, Ca²⁺, and other ions reabsorbed. Water follows by osmosis. Urea IGNORED — concentration rises. |
| Descending limb | Water continues leaving | Permeable to water, not solutes. Water exits by osmosis into hypertonic medullary interstitium. Volume drops further. |
| Ascending limb | Na⁺/Cl⁻ removed | Impermeable to water AND solutes. Actively pumps Na⁺ and Cl⁻ out. Tubular fluid becomes dilute (~1/3 plasma concentration). Creates medullary concentration gradient. |
| DCT | ~80% water, ~85% solutes gone | Active secretion of ions, acids, drugs, toxins. Na⁺ reabsorbed (aldosterone). Water reabsorbed (ADH). K⁺/H⁺ secreted in exchange for Na⁺. |
| Collecting duct | Final adjustments | ADH determines water permeability: high ADH = concentrated urine; no ADH = dilute urine. Aldosterone adjusts Na⁺/K⁺ balance. |
ADH and Urine Concentration
No ADH
DCT and collecting duct are impermeable to water. Large volume of dilute urine produced. Water cannot be reclaimed.
High ADH
DCT and collecting duct become highly permeable to water. Water reabsorbed by osmosis into concentrated medullary interstitium. Small volume of concentrated urine (up to 4–5x body fluid concentration).
Normal Urine Properties
| Characteristic | Normal Range |
|---|---|
| pH | 4.5–8 (average 6.0) |
| Specific gravity | 1.003–1.030 |
| Osmolarity | 855–1335 mOsm/L |
| Volume | 700–2000 mL/day |
| Water content | 93–97% |
| Bacterial content | None (sterile) |
The Ureters
Paired muscular tubes, ~30 cm (12 in.) long. Begin at the renal pelvis, end at the posterior wall of the urinary bladder without entering the peritoneal cavity. Ureteral openings in the bladder are slit-like (not round) — prevents backflow when bladder contracts.
Wall layers: inner transitional epithelium, middle longitudinal and circular smooth muscle, outer connective tissue. Peristaltic contractions every ~30 seconds sweep urine toward the bladder.
The Urinary Bladder
Hollow, muscular organ in the pelvic cavity. Only the superior surface is covered by peritoneum. Held in position by umbilical ligaments and connective tissue bands. Can contain up to ~1 liter of urine.
Males: between rectum and pubic symphysis. Females: inferior to uterus, anterior to vagina.
| Feature | Description |
|---|---|
| Trigone | Triangular area bounded by two ureteral openings and the urethral entrance. Urethral entrance at the apex (lowest point). |
| Neck | Area surrounding the urethral entrance. Contains the internal urethral sphincter. |
| Detrusor muscle | Three layers of smooth muscle (inner/outer longitudinal, middle circular). Contraction compresses bladder, expels urine into urethra. |
| Transitional epithelium | Lines the bladder; continuous with renal pelvis and ureters. Can tolerate considerable stretching. |
The Urethra
Extends from bladder neck to the exterior of the body.
Male Urethra
18–20 cm (7–8 in.). Extends to the external urethral orifice at the tip of the penis. Also transports semen.
Female Urethra
2.5–3.0 cm (~1 in.). Opens in the vestibule anterior to the vagina. Short length makes women more susceptible to UTIs.
Two Sphincters
| Sphincter | Muscle Type | Control |
|---|---|---|
| Internal urethral sphincter | Smooth muscle | Involuntary — provides automatic control over discharge |
| External urethral sphincter | Skeletal muscle | Voluntary — conscious control over urination |
The Micturition Reflex
Key Volumes
- Urge to urinate begins at ~200 mL
- At >500 mL, the micturition reflex may force open the internal sphincter, followed by reflexive relaxation of the external sphincter — urination occurs despite voluntary opposition
- After normal micturition, <10 mL remains in the bladder
Body Water Content
Adult Males
~60% of total body weight is water (greater muscle mass — muscle is 75% water).
Adult Females
~50% of total body weight is water (relatively more adipose tissue — adipose is only 10% water).
Two Fluid Compartments
Intracellular Fluid (ICF) — ~2/3 of body water
Cytosol. Dominant ions: potassium (K⁺), magnesium, phosphate, and negatively charged proteins.
Extracellular Fluid (ECF) — ~1/3 of body water
Interstitial fluid + plasma + minor components (lymph, CSF, synovial fluid, serous fluids, aqueous humor). Dominant ions: sodium (Na⁺), chloride, bicarbonate.
>90% of the osmotic concentration of the ECF results from sodium salts (primarily NaCl and NaHCO₃). Changes in ECF osmolarity almost always reflect changes in Na⁺ concentration.
Three Interrelated Balances
- Fluid balance — water gained daily = water lost daily. Maintained by creating ion concentration gradients that are eliminated by osmosis.
- Electrolyte balance — no net gain or loss of any ion. Balances absorption (digestive tract) with excretion (kidneys).
- Acid-base balance — H⁺ production = H⁺ loss. Maintaining body fluid pH within normal limits.
Daily Water Balance
| Input (~2500 mL) | Output (~2500 mL) |
|---|---|
| Water in food: ~1000 mL (40%) | Urination: ~1200 mL |
| Liquid consumption: ~1200 mL (48%) | Skin evaporation: ~750 mL |
| Metabolic water: ~300 mL (12%) | Lung evaporation: ~400 mL |
| Feces: ~150 mL |
Fluid Shifts
A fluid shift is water movement between the ECF and ICF in response to changes in osmotic concentration (osmolarity). Reaches equilibrium within minutes to hours.
- ECF becomes hypertonic (more concentrated) → water moves from ICF into ECF (cells shrink)
- ECF becomes hypotonic (more dilute) → water moves from ECF into cells (cells swell)
The ICF (larger volume) acts as a water reserve — prevents large changes in ECF osmolarity by distributing the change across both compartments.
Sodium Balance
Most common electrolyte balance problems involve sodium. Eating a heavily salted meal does NOT significantly raise Na⁺ concentration — osmosis brings water along from the digestive tract, diluting the sodium. But the ECF volume increases, which is why high-salt diets raise blood pressure.
Kidneys regulate Na⁺ loss: aldosterone stimulates Na⁺ reabsorption (decreases loss); ANP increases Na⁺ excretion.
Potassium Balance
~98% of body potassium is in the ICF. ECF K⁺ is normally low. Problems with K⁺ balance are less common but significantly more dangerous than Na⁺ imbalances (K⁺ directly affects cardiac function). When ECF K⁺ rises, aldosterone increases → K⁺ secreted into urine at the DCT. When ECF K⁺ falls, aldosterone decreases → K⁺ conserved.
Three Major Buffer Systems
| Buffer System | Primary Location | Mechanism |
|---|---|---|
| Protein buffer systems | Both ECF and ICF | Amino acid side groups accept H⁺ when pH drops (amino group acts as weak base) or release H⁺ when pH rises (carboxyl group acts as weak acid). Plasma proteins and hemoglobin in RBCs are major contributors. |
| Carbonic acid–bicarbonate buffer | Primarily ECF | CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Carbonic acid (weak acid) and bicarbonate ion (weak base). Neutralizes metabolic acids. The bicarbonate reserve provides a large supply of HCO₃⁻ for this buffering. |
| Phosphate buffer system | Primarily ICF | H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻. Less important in ECF because HCO₃⁻ far exceeds phosphate concentration there. But phosphate concentration is high inside cells. |
CO₂ and pH: The Inverse Relationship
When CO₂ levels RISE: more carbonic acid forms → more H⁺ released → pH FALLS (more acidic).
When CO₂ levels FALL: carbonic acid dissociates into CO₂ + H₂O → H⁺ removed from solution → pH RISES.
PCO₂ is the most important factor affecting pH in body tissues.
Respiratory Compensation
A change in respiratory rate that stabilizes ECF pH. Increasing breathing → more CO₂ exhaled → lower PCO₂ → pH rises. Decreasing breathing → CO₂ retained → higher PCO₂ → pH falls. Provides rapid pH adjustment.
Renal Compensation
Kidneys adjust rates of H⁺ secretion and HCO₃⁻ reabsorption. If pH too low (acidosis): secrete more H⁺, reabsorb more HCO₃⁻. If pH too high (alkalosis): secrete less H⁺, allow more HCO₃⁻ to be lost in urine. Slower than respiratory compensation but more thorough.
Normal pH Range
ECF pH = 7.35–7.45. Below 7.35 = acidosis. Above 7.45 = alkalosis. Survival range: 6.8–7.7. Severe acidosis (pH < 7.0): CNS deterioration → coma, cardiac failure, circulatory collapse, death.
Four Acid-Base Disorders
| Disorder | pH | Cause | Treatment |
|---|---|---|---|
| Respiratory acidosis (MOST COMMON) | <7.35 | Hypoventilation — CO₂ accumulates because it cannot be exhaled fast enough. Even minutes of hypoventilation can drop pH to 7.0. | Improve ventilation (bronchodilation, mechanical assistance) |
| Metabolic acidosis (2nd most common) | <7.35 | Production of excess metabolic acids (lactic acid, ketone bodies) OR impaired H⁺ excretion at the kidneys (severe kidney damage). | Gradual bicarbonate administration + correct primary cause |
| Respiratory alkalosis (uncommon) | >7.45 | Hyperventilation — excessive CO₂ loss (hypocapnia). Usually self-correcting as reduced PCO₂ removes chemoreceptor stimulation. | Reduce respiratory rate, allow PCO₂ to rise |
| Metabolic alkalosis (rare in severe form) | >7.45 | Prolonged vomiting — stomach generates replacement HCl, releasing HCO₃⁻ into blood each time. Bicarbonate accumulates, driving pH up. | If pH >7.55: ammonium chloride administration |
Age-Related Changes
| Change | Details |
|---|---|
| Decline in nephrons | Total number drops 30–40% between ages 25 and 85. |
| Reduced GFR | Fewer glomeruli, cumulative filtration damage, reduced renal blood flow. Also reduces ability to regulate pH through renal compensation. |
| Reduced ADH/aldosterone sensitivity | Distal nephron and collecting system become less responsive. Result: reduced water and sodium reabsorption, increased potassium loss in urine. |
| Micturition problems | Sphincter muscles lose tone → incontinence (slow leakage). CNS problems (stroke, Alzheimer's) → loss of voluntary control. Males: prostate enlargement compresses urethra → urinary retention. |
| Decreased total body water | Ages 40–60: males 55%, females 47%. After 60: males ~50%, females ~45%. Less dilution of wastes, toxins, and administered drugs. |
| Net mineral loss | After age 60, as muscle and skeletal mass decrease. Can be partially prevented by exercise and increased dietary mineral intake. |
| Increased systemic disorders | More disorders affecting major systems, many with impact on fluid, electrolyte, and/or acid-base balance. |
The urinary system provides the same fundamental service to every other body system: excreting waste products and maintaining normal body fluid pH and ion composition. Without this, no organ system can function.
| System | What It Does for the Urinary System | What the Urinary System Does for It |
|---|---|---|
| Integumentary | Sweat glands eliminate water/solutes; epidermis prevents excess fluid loss; produces vitamin D₃ for calcitriol | Eliminates nitrogenous wastes; maintains fluid/electrolyte/acid-base balance of blood nourishing skin |
| Skeletal | Axial skeleton protects kidneys/ureters; pelvis protects bladder and proximal urethra | Conserves calcium and phosphate for bone growth |
| Muscular | Sphincter muscles control urination; trunk muscles protect urinary organs | Removes protein metabolism wastes; regulates Ca²⁺ and phosphate concentrations |
| Nervous | Adjusts renal blood pressure; monitors bladder distension; controls micturition reflex | Eliminates nitrogenous wastes; maintains blood composition critical for neural function |
| Endocrine | Aldosterone and ADH adjust rates of fluid and electrolyte reabsorption | Kidney cells release renin (BP drops) and erythropoietin (O₂ levels fall); calcitriol production |
| Cardiovascular | Delivers blood to glomerular capillaries for filtration; accepts reabsorbed fluids and solutes | Releases renin for BP regulation; releases EPO for RBC production |
| Lymphatic | Provides adaptive (specific) immune defense against UTIs | Eliminates toxins and wastes; acid pH of urine provides innate defense against infection |
| Respiratory | Assists in pH regulation by eliminating CO₂ | Assists in CO₂ elimination; provides bicarbonate buffers for pH regulation |
| Digestive | Absorbs water for excretion; absorbs ions for normal fluid concentrations; liver removes bilirubin | Excretes digestive toxins; excretes bilirubin and nitrogenous wastes from liver; calcitriol aids Ca²⁺/phosphate absorption |
The Excretory System
The urinary system is the major component of an anatomically diverse excretory system that also includes:
- Integumentary system — water, electrolytes, and small amounts of urea in perspiration
- Respiratory system — CO₂ elimination; small amounts of acetone and water vapor
- Digestive system — liver excretes metabolic wastes in bile; variable water loss in feces