Marieb Human Anatomy & Physiology Chapter 25: The Urinary System

Introduction to the Urinary System

• Functions of the Kidneys:

  • Regulation of Total Body Water Volume and Solute Concentration: Ensures the body maintains a balance of fluids and dissolved substances.

  • Regulation of Ion Concentrations: Specifically monitors and adjusts ions in the extracellular fluid ($ECF$).

  • Long-term Acid-Base Balance: Ensures the pH of the body remains within physiological limits over the long term.

  • Excretion: Removal of metabolic wastes, toxins, and drugs from the bloodstream.

  • Hormone Production:

    • Erythropoietin: A hormone that regulates red blood cell ($RBC$) production.

    • Renin: An enzyme required to synthesize Angiotensin, a hormone involved in blood pressure ($BP$) regulation.

  • Activation of Vitamin D: Forms calcitriol, a hormone involved in regularizing $ECF$ calcium levels.

  • Gluconeogenesis: Assists the liver in producing glucose from non-carbohydrate sources during periods of prolonged fasting.

• Components of the Urinary System:

  • Kidneys: The primary functional organs of the system.

  • Ureters: Tubes that transport urine from the kidneys to the urinary bladder.

  • Urinary Bladder: A temporary storage reservoir for urine.

  • Urethra: A tube that transports urine out of the body.

External Anatomy and Location of the Kidneys

• Location:

  • The kidneys are retroperitoneal, meaning they are located behind the peritoneum.

  • Positioned in the superior lumbar region between the levels of $T_{12}$ and $L_{3}$.

  • Right Kidney: Crowded by the liver and sits slightly lower than the left kidney.

  • Protection: The rib cage provides some protection to the upper parts of the kidneys.

  • Adrenal (Suprarenal) Gland: Sits atop each kidney.

• External Features:

  • Shape: Bean-shaped, similar in size to a large bar of soap.

  • Convex Lateral Surface: The outer curved edge.

  • Concave Medial Surface: Contains the renal hilum, a deep cleft where the ureters, blood vessels, lymphatics, and nerves enter or exit the kidney.

• Clinical Homeostatic Imbalance 25.1:

  • Perirenal Fat: Provides cushioning; if the rib cage does not protect the lower parts, they are susceptible to blunt trauma (especially the right kidney).

  • Renal Artery Vulnerability: Susceptible to injury from rapid deceleration (e.g., car crashes), leading to lacerations (tears) or thrombosis (blood clot).

  • Hematuria: Blood in the urine, an important sign of renal trauma.

Internal Gross Anatomy of the Kidney

• Three Distinct Regions:

  • Renal Cortex: The light-colored, superficial region with a granular appearance.

  • Renal Medulla: Darker, reddish-brown region deep to the cortex.

    • Medullary (Renal) Pyramids: Cone-shaped structures. The broad base faces the cortex, while the papilla (tip) points internally.

    • Renal Columns: Inward extensions of cortical tissue that separate the pyramids.

    • Lobe: Composed of a medullary pyramid and its surrounding cortical tissue; there are approximately $8$ lobes per kidney.

  • Renal Pelvis: A funnel-shaped tube continuous with the ureter.

    • Minor Calyces: Cup-shaped areas that enclose and collect urine from a papilla.

    • Major Calyces: Branches of the renal pelvis that receive urine from the minor calyces.

• Urine Flow Path:

  • Renal pyramid $\rightarrow$ minor calyx $\rightarrow$ major calyx $\rightarrow$ renal pelvis $\rightarrow$ ureter $\rightarrow$ urinary bladder.

  • Urine path is facilitated by smooth muscle contractions in the walls of the calyces, pelvis, and ureter.

• Clinical Homeostatic Imbalance 25.2 (Pyelonephritis):

  • Infection or inflammation of the kidney.

  • Females: Usually caused by fecal bacteria spreading to the urethra.

  • Can result from blood-borne bacteria from other sites.

  • Signs: Severe cases involve kidney swelling, abscess formation, and pus filling the renal pelvis. Treated with antibiotics.

Blood and Nerve Supply

• Blood Supply:

  • Kidneys receive approximately one-fourth ($1200\,ml$) of the entire cardiac output each minute.

  • Arterial Blood Path: Abdominal aorta $\rightarrow$ renal artery $\rightarrow$ segmental artery $\rightarrow$ interlobar artery $\rightarrow$ arcuate artery $\rightarrow$ cortical radiate artery.

  • The cortex receives over $90\%$ of the blood.

  • Venous Blood Path: Cortical radiate vein $\rightarrow$ arcuate vein $\rightarrow$ interlobar vein $\rightarrow$ renal vein (Note: there are no segmental veins).

• Nerve Supply:

  • Provided by the renal plexus.

  • Consists of sympathetic (vasomotor) fibers that control blood flow via arteriole constriction/dilation and influence urine formation.

Nephrons: The Functional Units

• Overview:

  • Nephrons are the structural and functional units that filter blood and form urine.

  • There are approximately $1\,million$ nephrons per kidney.

• Renal Corpuscle:

  • Glomerulus: A tuft of fenestrated capillaries (fenestrated endothelium). Extremely porous, allowing for efficient formation of filtrate (plasma-derived fluid).

  • Glomerular Capsule (Bowman’s Capsule):

    • Parietal Layer: Simple squamous epithelium (structural).

    • Visceral Layer: Contains highly modified branching epithelial cells called podocytes. These have branches terminating in foot processes attached to the basement membrane.

    • Filtration Slits: Spaces between foot processes that allow water and small solutes to pass into the capsular space.

• Renal Tubule:

  • Approximately $3\,cm$ ($1.2\,in$) long, made of simple epithelia.

  • Proximal Convoluted Tubule (PCT):

    • Confined to the cortex.

    • Cuboidal epithelium with large mitochondria.

    • Dense microvilli forming a brush border to increase surface area for reabsorption and secretion.

  • Nephron Loop (Loop of Henle):

    • Descending Limb: Proximal part continuous with PCT. The distal part (descending thin limb) consists of simple squamous epithelium.

    • Ascending Limb: Proximal portion may be thin; the rest is the thick ascending limb, consisting of cuboidal or short-columnar cells.

  • Distal Convoluted Tubule (DCT):

    • Confined to the cortex.

    • Simple cuboidal epithelium, thinner than PCT, with sparse microvilli.

• Collecting Duct:

  • Receives filtrate from many nephrons.

  • Fused ducts deliver urine through papillae into minor calyces.

  • Principal Cells: Maintain water and $Na^{+}$ balance.

  • Intercalated Cells: Help maintain the acid-base balance of the blood.

• Classes of Nephrons:

  • Cortical Nephrons ($85\%$): Located almost entirely in the cortex with short nephron loops.

  • Juxtamedullary Nephrons: Originate close to the cortex-medulla junction with long nephron loops extending deep into the medulla. Essential for producing concentrated urine.

Nephron Capillary Beds

• Glomerulus:

  • Arranged in parallel for filtration.

  • Fed by the afferent arteriole (from cortical radiate arteries) and drained by the efferent arteriole.

  • Arterioles are high-resistance vessels maintaining high glomerular $BP$.

• Peritubular Capillaries:

  • Low-pressure, porous capillaries arising from efferent arterioles in cortical nephrons.

  • Surround renal tubules to pick up reabsorbed water and solutes.

• Vasa Recta:

  • Long, thin-walled vessels parallel to loops of juxtamedullary nephrons.

  • Involved in forming concentrated urine.

The Juxtaglomerular Complex (JGC)

• Functions: Regulates the rate of filtrate formation and systemic blood pressure.

• Cell Populations:

  • Macula Densa: Tall, packed cells in the distal ascending limb. Act as chemoreceptors monitoring the $NaCl$ content of filtrate.

  • Granular Cells (Juxtaglomerular/JG cells): Smooth muscle cells in the afferent arteriole. Act as mechanoreceptors monitoring $BP$. Contain secretory granules with the enzyme renin.

  • Extraglomerular Mesangial Cells: Interconnected by gap junctions; pass signals between macula densa and granular cells.

Overview of Urine Formation

• Numerical Data:

  • Kidneys process $180\,L$ of filtrate per day while forming only $1.5\,L$ of urine.

  • Filtrate matches plasma composition but lacks plasma proteins.

  • Kidneys consume $20\%$ of the body's oxygen supply at rest.

  • Entire plasma volume is filtered $60$ times a day.

• The Three Processes:

  1. Glomerular Filtration: Produces cell-free and protein-free filtrate.

  2. Tubular Reabsorption: Moves substances from filtrate back into the blood.

  3. Tubular Secretion: Moves substances from blood into the filtrate.

Step 1: Glomerular Filtration

• Mechanism: Passive process where hydrostatic pressure forces fluids/solutes through a membrane.

• The Filtration Membrane:

  1. Fenestrated Endothelium: Excludes blood cells.

  2. Basement Membrane: Fused basal laminae; negatively charged glycoproteins repel large anions (plasma proteins).

  3. Foot Processes of Podocytes: Filtration slits with slit diaphragms restrict remaining proteins.

• Permeability: Allows molecules smaller than $3\,nm$ (water, glucose, amino acids, nitrogenous wastes) to pass.

• Pressures Affecting Filtration:

  • Outward Pressure: Hydrostatic pressure in glomerular capillaries ($HP_{g}$) = $55\,mmHg$ (High glomerular $BP$).

  • Inward Pressures:

    • Hydrostatic pressure in capsular space ($HP_{c}$) = $15\,mmHg$.

    • Colloid osmotic pressure in glomerular capillaries ($OP_{g}$) = $30\,mmHg$.

  • Net Filtration Pressure (NFP):

    • $NFP = HP_{g} - (HP_{c} + OP_{g})$

    • $NFP = 55\,mmHg - (15\,mmHg + 30\,mmHg) = 10\,mmHg$.

• Glomerular Filtration Rate (GFR):

  • Volume of filtrate formed per minute by both kidneys.

  • Proportional to $NFP$, total surface area (fine-tuned by mesangial cells), and membrane permeability.

• Regulation of GFR:

  • Intrinsic Controls (Renal Autoregulation): Used when MAP is between $80$ and $180\,mmHg$.

    • Myogenic Mechanism: Vascular smooth muscle contracts when stretched (high $BP$) to restrict flow; relaxes when $BP$ falls.

    • Tubuloglomerular Feedback: Macula densa responds to high $NaCl$ (due to high GFR) by releasing vasoconstrictors to slow GFR.

  • Extrinsic Controls: Override intrinsic controls if MAP is far outside normal ranges (<80 or >180\,mmHg).

    • Sympathetic Nervous System: Norepinephrine causes systemic vasoconstriction. Constricting afferent arterioles decreases GFR to save blood volume.

    • Renin-Angiotensin-Aldosterone Mechanism: Low $BP$ triggers granular cells to release renin via SNS activation, macula densa signaling (low ATP/NaCl), or reduced stretch of granular cells.

• Anuria: Abnormally low urinary output (less than $50\,ml/day$), indicating $BP$ is too low for filtration.

Step 2: Tubular Reabsorption

• Routes of Reabsorption:

  • Transcellular: Across the apical membrane, through the cytosol, across the basolateral membrane, into interstitial fluid ($IF$), and into the capillary.

  • Paracellular: Between tubule cells (limited by tight junctions, but "leaky" in the PCT for water and ions like $Ca^{2+}$, $Mg^{2+}$, and $K^{+}$).

• Sodium ($Na^{+}$) Reabsorption:

  • $80\%$ of ATP used for active transport is for $Na^{+}$.

  • Basolateral Membrane: $Na^{+}-K^{+}$ ATPase pumps $Na^{+}$ into $IF$.

  • Apical Membrane: $Na^{+}$ enters via secondary active transport or facilitated diffusion.

• Reabsorption of Other Substances:

  • Nutrients: Glucose, amino acids, vitamins use secondary active transport (cotransport with $Na^{+}$).

  • Water: Osmotic gradient created by $Na^{+}$ causes water to follow via aquaporins.

    • Obligatory: In the PCT (aquaporins always present).

    • Facultative: In the collecting duct, regulated by ADH.

  • Solutes: Urea, ions, and lipid-soluble drugs follow water down concentration gradients.

• Transport Maximum ($T_{m}$):

  • Reflects the number of transport proteins available. When saturated, excess solute is excreted (e.g., glycosuria in diabetes mellitus).

• Regional Reabsorptive Capabilities:

  • PCT: Reabsorbs all glucose/amino acids, $65\%$ of $Na^{+}$ and water, and half of the urea.

  • Nephron Loop:

    • Descending limb: Permeable to water, not solutes.

    • Ascending limb: Permeable to solutes ($Na^{+}$, $Cl^{-}$, $K^{+}$), not water.

  • DCT and Collecting Duct: Fine-tuning by hormones:

    • ADH: Increases water reabsorption.

    • Aldosterone: Increases $Na^{+}$ reabsorption (and water follows) while decreasing $K^{+}$ levels.

    • Atrial Natriuretic Peptide (ANP): Inhibits $Na^{+}$ reabsorption to lower $BP$.

    • Parathyroid Hormone (PTH): Increases $Ca^{2+}$ reabsorption at the DCT.

Step 3: Tubular Secretion

• Importance:

  • Disposing of protein-bound drugs/metabolites.

  • Eliminating urea and uric acid reabsorbed passively.

  • Ridding the body of excess $K^{+}$.

  • Controlling blood pH by altering $H^{+}$ or $HCO_{3}^{-}$ secretion.

Osmotic Gradient and Urine Concentration

• Numerical Reference: Normal blood osmolality is $300\,mOsm$.

• Countercurrent Multiplier: Filtrate flow in ascending and descending limbs of long nephron loops creates the medullary osmotic gradient ($300\,mOsm$ at cortex to $1200\,mOsm$ at the papilla).

• Countercurrent Exchanger: Vasa recta preserve the gradient by removing reabsorbed water and prevent rapid removal of solutes.

• Urine Concentration Scenarios:

  • Overhydrated: Decrease in ADH; urine osmolality falls as low as $50\,mOsm$ (dilute urine).

  • Dehydrated: Maximal ADH release; urine osmolality reaches $1200\,mOsm$ (concentrated urine).

  • Minimum Urine Output: Required to excrete solutes is $0.5\,L/day$.

Diuretics

• Alcohol: Inhibits ADH release.

• Loop Diuretics (e.g., Furosemide): Inhibit $Na^{+}$ reabsorption in the thick ascending limb, diminishing the osmotic gradient.

• Osmotic Diuretics: Not reabsorbed, so water follows them (e.g., high glucose in diabetes).

Urine Characteristics and Composition

• Chemical Composition: $95\%$ water, $5\%$ solutes.

  • Wastes: Urea (largest solute component), uric acid, creatinine.

  • Normal Ions: $Na^{+}$, $K^{+}$, $PO_{4}^{3-}$, $SO_{4}^{2-}$.

• Physical Characteristics:

  • Color: Clear to deep yellow (due to urochrome).

  • pH: Slightly acidic (average $6.0$; range $4.5$ to $8.0$).

  • Specific Gravity: $1.001$ to $1.035$.

• Abnormal Constituents:

  • Glycosuria: Glucose (diabetes mellitus).

  • Proteinuria: Proteins (heart failure, GN, severe hypertension).

  • Ketonuria: Ketone bodies (starvation, untreated diabetes).

  • Hematuria: Erythrocytes (trauma, stones, infection).

Other Urinary Organs

• Ureters: Tubes entering the bladder at the posterior wall. Distal ends close when bladder pressure rises to prevent backflow.

• Clinical (Renal Calculi): Kidney stones made of calcium, magnesium, or uric acid salts. Treated with lithotripsy.

• Urinary Bladder:

  • Trigone: Triangular area between ureter and urethra openings (common site of infection).

  • Detrusor: Three layers of smooth muscle.

  • Capacity: Normal full bladder holds $500\,ml$ ($1\,pint$); max capacity is $1000\,ml$.

• Urethra:

  • Internal Urethral Sphincter: Involuntary smooth muscle.

  • External Urethral Sphincter: Voluntary skeletal muscle.

  • Female: Short ($3-4\,cm$).

  • Male: Long ($20\,cm$) with three sections: Prostatic, Intermediate (Membranous), and Spongy.

Questions & Discussion

• Q: What is the cause of chronic renal disease?

• A: Defined as GFR < 60\,ml/min for at least three months, often caused by diabetes mellitus ($44\%$) and hypertension ($28\%$).

• Q: What is uremia?

• A: "Urine in blood," occurs during renal failure (GFR < 15\,ml/min) leading to accumulate of toxins, fatigue, and mental changes.

• Q: What causes urinary incontinence?

• A: Stress incontinence (coughing/laughing) from weakened pelvic muscles or overflow incontinence when the bladder overfills.