The Urinary System Quiz

The Urinary System

• The urinary system's functions include removing waste products, regulating blood volume and pressure, and maintaining electrolyte balance.
• The urinary system consists of six organs: two kidneys, two ureters, a urinary bladder, and a urethra.
• Kidneys filter blood plasma, separating waste from useful chemicals and return useful substances to the blood.
• Kidneys regulate blood volume and pressure by eliminating or conserving water.
• Kidneys regulate the osmolarity of body fluids. (Working with lungs) Maintaining the acid-base balance of body fluids.
• Kidneys perform gluconeogenesis from amino acids (during extreme starvation).
• Kidneys secrete renin to control blood pressure and electrolyte balance.
• Kidneys secrete erythropoietin to produce red blood cells.
• Kidneys are the final step in synthesizing calcitriol which contributes to calcium homeostasis (vitamin D).

Waste Products & Kidney Function

• Metabolism produces toxic waste products.
• The urinary system is the primary method for removing waste.
• Kidney functions include regulating blood volume and pressure, red blood cell count, blood gases, blood pH, and electrolyte/acid balance.
• Urologists treat urinary and reproductive disorders, especially in males.

Anatomy of Kidney

• Kidneys are positioned against the posterior abdominal wall (T12-L3).
• The size of a kidney is similar to a bar of soap.
• The lateral surface of a kidney is convex, the medial surface is concave and has a slit called the hilum.
• The hilum permits blood vessels, nerves, lymphatics, and the ureter to pass through.
• Kidneys are protected by connective tissues.
• Perirenal fat capsule cushions and holds kidneys in place.
• Fibrous capsule protects against trauma and infection.

Gross Anatomy of Kidneys

• The kidney is comprised of a Renal cortex and Renal medulla.
• The renal papilla is part of the medulla.
• The renal pelvis, major calyx, minor calyx, renal column, renal pyramid, ureter are parts of the kidneys anatomy.

Anatomy of Kidney (subdivision)

• Renal parenchyma—glandular tissue forming urine.
• Two zones of renal parenchyma:
  • Outer renal cortex
  • Inner renal medulla
• Renal columns—extensions of the cortex that project into the medulla.
• Renal pyramids—6–10 with broad bases facing cortex, and renal papilla (pointy part) facing inward.

Anatomy of Kidney

• Minor calyx—cup/tube that nestles the papilla of each pyramid, collects urine.
• Major calyces—formed by convergence of 2 or 3 minor calyces.
• Renal pelvis—formed by the convergence of 2 or 3 major calyces.
• Ureter—tubular continuation of pelvis, drains urine to the urinary bladder.

Renal Circulation

• Kidneys account for only 0.4% of body weight but receive approximately 21% of cardiac output (renal fraction).
• Filtration of waste from the blood occurs constantly.

The Nephron

• Each kidney has about 1.2 million nephrons.
• Each nephron has two parts:
  • Renal corpuscle—filters blood plasma into filtrate.
  • Renal tubule—long, coiled tube that converts filtrate into urine.
• Renal corpuscle—glomerulus and a capsule enclosing it (glomerular or Bowman's capsule).
  • Outer layer of capsule: simple squamous epithelium.
  • Inner layer of capsule: podocytes that wrap around glomerular capillaries.
• Note: afferent arteriole is larger than efferent arteriole.

Renal Tubule

• Renal tubule—a duct leading away from the glomerular capsule and ends at the tip of medullary pyramid.
• Divided into four regions:
  • Proximal convoluted tubule
  • Nephron loop (loop of Henle)
  • Distal convoluted tubule
  • Collecting duct

Renal Tubule (Subdivisions)

• Proximal convoluted tubule (PCT)—starts at glomerular capsule; longest and most coiled region.

• Nephron loop (loop of Henle)—U-shaped portion—descending limb and ascending limb.

  • Thick segments—initial part of descending limb and part or all of the ascending limb; active transport of salts, many mitochondria.
  • Thin segment—forms lower part of descending limb; cells are very permeable to water

• Distal convoluted tubule (DCT)—starts shortly after ascending limb; shorter and less coiled than PCT; end of a single nephron.

• Collecting duct—receives fluid from DCTs of several nephrons; numerous collecting ducts converge toward the tip of medullary pyramid.

• Flow of fluid from glomerulus to where urine leaves body:

  • glomerular capsule → proximal convoluted tubule → nephron loop → distal convoluted tubule → collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra

Renal Tubule (Types of Nephrons)

• Two types of nephrons exist: Cortical (80%) – short nephron loops and Juxtamedullary( 15%) – long nephron loops

Filtration Pressure

• Blood hydrostatic pressure (BHP) is much higher in glomerular capillaries (60 mm Hg) compared to other capillaries (10-15 mm Hg).
• Hydrostatic pressure in capsular space (18 mm Hg) is elevated due to high filtration rate and fluid accumulation in the capsule.
• Colloid osmotic pressure (COP) of blood is approximately the same as elsewhere (32 mm Hg).
• Net filtration pressure is determined by the differences in BHP, hydrostatic pressure in capsule space, and COP (10 mm Hg).

Glomerular Filtration Rate (GFR)

• Glomerular filtration rate (GFR)—amount of filtrate formed per minute by both kidneys combined.
• Total daily filtrate equals approximately 50-60 times the amount of blood in the body.
• 99% of filtrate is reabsorbed; only 1-2 liters of urine excreted daily.

Regulation of Glomerular Filtration

• GFR—controlled by adjusting glomerular blood pressure, moment-to-moment.
• GFR control is achieved by three homeostatic mechanisms:
  • Renal autoregulation
  • Sympathetic control
  • Hormonal control

Renal Autoregulation

• Renal autoregulation—kidneys adjust blood flow and GFR without external control (nervous or hormonal).
• Allows control of GFR despite changes in systemic arterial blood pressure.
• Monitored by juxtamedullary apparatus.

Sympathetic Control of GFR

• Sympathetic nerve fibers innervate renal blood vessels.
• Sympathetic nervous system & adrenal epinephrine constricts the afferent arterioles during strenuous exercise or acute conditions (e.g., circulatory shock).
• Reduces GFR and urine output.
• Redirects blood from kidneys to heart, brain, and skeletal muscles.

Renin-Angiotensin-Aldosterone Mechanism

• Renin is secreted by juxtaglomerular cells if blood pressure drops dramatically.

• Renin converts angiotensinogen (blood protein) to angiotensin I.

• In the lungs and kidneys, angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II.

• Angiotensin II is the active hormone that works to return blood volume and blood pressure towards normal.

Falling BP & Angiotensin II

• Angiotensin II plays a crucial role in raising blood pressure throughout the body by constricting efferent arterioles and reducing pressure in peritubular capillaries which enhances the reabsorption of NaCl and water.
• Stimulates the adrenal cortex to help regulate Na+ and water retention and secretion in DCT and collecting duct.
• Promotes the pituitary secretion of ADH for more water reabsorption.
• Raises thirst and drives water intake.

Urine Formation: Tubular Reabsorption and Secretion

• Converting glomerular filtrate to urine involves tubular reabsorption (back into blood) and secretion (from blood).

Proximal Convoluted Tubule (PCT)

• PCT reabsorbs about 65% of the glomerular filtrate, plus it secretes additional waste products into the tubular fluid for disposal in the urine.
• Microvilli and long length enhance absorption in the PCT.
• Abundant mitochondria provide ATP for active transport.
• PCT activities together account for roughly 6% of the body's resting ATP and calorie requirement.

Sodium Reabsorption

• Sodium reabsorption is crucial for other processes.

• Creates an osmotic and electrical gradient to drive the reabsorption of water and other solutes.

• Most abundant cation in the filtrate and high concentration favors its diffusion into the PCT cells.

• Two types of transport proteins in apical cell surfaces handle sodium uptake:

  • Symports simultaneously bind Na+ and another solute (glucose, amino acids, or lactate) without ATP.
  • Na+-H+ antiport pulls Na+ into the cell and pumps out H+ into tubular fluid, requiring no ATP.

• Sodium doesn't build up in the PCT due to Na+-K+ pumps on the basal surface.

• Na+ is picked by peritubular capillaries and returned to the blood.

• Negative chloride ions follow the positive sodium ions by electrical attraction.

• Apical cell membrane antiports can swap Cl- for HCO3- if required.

Reabsorption in the PCT: Other Electrolytes

• K+, Mg, and PO4 ions diffuse with water.
• Some calcium is reabsorbed through the paracellular route.
• Glucose is cotransported with Na+ by sodium-glucose transport (SGLT) proteins.
• Urea diffuses through the tubule epithelium with water.
• Kidneys remove approximately half of the urea from the blood.
• Creatinine is not reabsorbed.

Water Reabsorption

• Kidneys reduce the 180 L of glomerular filtrate to 1–2 L of urine daily.
• Two-thirds of the water in the filtrate is reabsorbed in the PCT.
• Reabsorption of all the salt and organic solutes makes the tubule cells and tissue fluid hypertonic, driving water reabsorption through channels called aquaporins via osmosis.
• Water is reabsorbed at a constant rate in the PCT called obligatory water reabsorption.

Transport Maximum

• There's a limit to the amount of solute for renal tubules that can be reabsorbed.
• Limited by the number of transport proteins in the plasma membrane If transporters are occupied to the maximum—excess solutes appear in the urine

Tubular Secretion

• Adding additional waste/substance to filtrate.

  • Waste Removal: Urea, uric acid, unneeded paracrines, and some creatinine are secreted into the tubule. Blood is cleared of toxins and drugs due to this action; thus, often prescriptions must be taken 3–4 times per day.
  • Acid-base balance: Secretion of H+ and bicarbonate ions help regulate body fluid pH.

Function of Nephron Loop

• Primary function of the nephron loop is creating a salinity gradient in the renal medulla which allows the collecting duct to concentrate urine and conserve water.
• Thick segment—reabsorbs 25% of Na+, K+, and Cl–. NaCl remains in the renal medulla tissue. H2O does not follow due to impermeable thick segment. Tubular fluid is very dilute as it enters the DCT.

DCT and Collecting Duct

• Fluid arriving in the DCT still contains about 20% of the water and 7% of the salts from the glomerular filtrate.
• If all this material passes as urine it would total 36 Liters per day.
• DCT and collecting duct reabsorb variable amounts of water and salt; these processes are regulated by hormones (aldosterone, ADH, and ANP)

DCT and Collecting Duct (Aldosterone)

• Aldosterone—the "salt-retaining" hormone.
• Steroid secreted by the adrenal cortex when blood Na+ concentration falls or when K+ concentration rises; or a drop in blood pressure.
• Causes more Na+ reabsorption and K+ secretion.
• Water and chloride follow Na+, resulting in the body retaining NaCl and water to help maintain blood volume and pressure.

DCT and Collecting Duct (ADH)

• Antidiuretic hormone (ADH)—secreted by posterior lobe of pituitary in response to dehydration and rising blood osmolarity (stimulated by hypothalamus).
• Makes collecting duct more permeable to water by adding aquaporins.
• More water in tubular fluid is reabsorbed.

DCT and Collecting Duct (ANP)

• Atrial natriuretic peptide (ANP)—secreted by atrial myocardium of the heart in response to high blood pressure.
• Increases excretion of salt and thus water in urine, reducing blood volume and blood pressure.
  • Dilates afferent arterioles, constricts efferent arterioles (improving GFR).
  • Inhibits renin, aldosterone, and ADH secretion

Summary of Tubular Reabsorption and Secretion

• PCT reabsorbs 65% of glomerular filtrate and returns it to peritubular capillaries.
• Nephron loop reabsorbs another 25% of filtrate.
• DCT reabsorbs Na+, Cl–, and water—hormonal control.
• Tubules extract drugs, wastes, and some solutes from blood and secrete them into tubular fluid.
• Collecting ducts conserve water.

Urine Formation: Water Conservation

• The kidneys eliminate metabolic waste, but also prevent excessive water loss.
• As the kidney returns water to tissue fluid and bloodstream, the fluid remaining in the renal tubules becomes more concentrated.

Collecting Duct Urine

• As collecting ducts pass through the medulla, they concentrate urine up to four times.
• Medullary portion of collecting ducts is more permeable to water than to NaCl.
• As urine flows through increasingly salty medulla, water leaves by osmosis to concentrate urine.

Countercurrent Multiplier

• Kidney's ability to concentrate urine depends on the salinity gradient in the renal medulla (4X as salty as in the cortex). Nephron loop acts as countercurrent multiplier that continually recaptures salt and returns it to renal medulla. Fluid flows in opposite directions in adjacent tubules causing the countercurrent effect.

Countercurrent Multiplier

• Fluid flowing downward in descending limb (thin segment)—very permeable to water but not to NaCl. Water passes to vasa recta/peritubular capillaries, concentrating tubular fluid at bottom of loop.
• Fluid flowing upward (ascending limb, mostly thick segment)—impermeable to water, reabsorbs Na+, K+, and Cl– into ECF. Maintains high osmolarity of renal medulla.
• Recycling of urea—lower collecting ducts are permeable to urea; some urea diffuses out into the medulla, contributing to osmolarity.

Maintenance of Osmolarity in Renal Medulla

• Maintaining a high osmolarity in renal medulla is assisted by vasa recta which helps preserve the concentration gradient through counter-current exchange. Active transport plays a crucial role transporting and maintaining optimal osmolarity, through membrane channels which support passive transport of chemicals, water, and salts.

Control of Water Loss

• Urine concentration depends on body's hydration state.
• Water diuresis—drinking large volumes of water produces a large volume of hypotonic urine.
• Producing hypertonic urine—dehydration causes less urine, more concentrated; high blood osmolarity stimulates posterior pituitary to release ADH (even if not dehydrated); more water is reabsorbed by the collecting duct.

Urine Volume

• Normal urine volume for average adult: 1–2 L/day.
• Polyuria—urine output exceeding 2 L/day.
• Oliguria—urine output less than 500 mL/day.
• Anuria—urine output of 0–100 mL/day.
• Low output (from kidney disease, dehydration, circulatory shock, or prostate enlargement). If urine output falls below 400 mL/day, the body is unable to safely maintain low waste concentration in plasma.

Composition & Properties of Urine

• Urinalysis—examining physical and chemical properties of urine.
  • Appearance: clear to deep amber yellow; cloudiness or blood may suggest infection/trauma/stones.
  • Specific gravity: Water comparison.
  • pH: Range of 4.5 to 8.2, often 6.0 (mildly acidic). Chemical Composition: 95% water; 5% solutes.
  • Abnormal findings: glucose, free hemoglobin, albumin, ketones, bile pigments

Diabetes

• Diabetes—any metabolic disorder resulting in chronic polyuria. Four types of diabetes exist:
  • Diabetes mellitus (type 1, type 2, gestational)
    • High glucose concentration in renal tubule.
    • Opposes osmotic reabsorption of water; thus produces more urine. Glycosuria—glucose in the urine.
  • Diabetes insipidus
    • ADH hyposecretion; more water passes in urine.

Diuretics

• Diuretics—any chemical that increases urine volume. Some increase GFR.
  • Caffeine dilates the afferent arteriole. Reduces tubular reabsorption of water.
  • Alcohol—inhibits ADH secretion. Acts on nephron loop; inhibits Na+-K+-Cl– symport; impairs countercurrent multiplier; collecting duct unable to reabsorb water effectively.
• Commonly used to treat hypertension and congestive heart failure, reducing the body's fluid volume and blood pressure.

Urine Storage & Elimination

• Urine is produced continually.
• Urination is episodic.
• Storage apparatus and neural controls make urination possible.

The Ureters

• Ureters—retroperitoneal muscular tubes extending from the kidney to the urinary bladder.
• About 25 cm long.
• Pass posterior to the bladder and enter from below.
• Flap of mucosa acts as a valve to the bladder.
• Lumen is very narrow, easily obstructed by kidney stones.

Urinary Bladder

• Urinary bladder—muscular sac on the floor of the pelvic cavity. Three layers exist:
  • Parietal peritoneum superiorly; fibrous adventitia in other areas.
  • Muscularis (detrusor muscle—three layers of smooth muscle).
  • Mucosa (transitional epithelium).
• Rugae—distinct wrinkles in relaxed bladder.
• Trigone—smooth-surfaced triangular area marked with openings of ureters and urethra.

Kidney Stones

• Renal calculus (kidney stone)—hard granule (calcium phosphate, calcium oxalate, uric acid, or magnesium salt called struvite).
• Usually small enough to pass unnoticed; large stones can block renal pelvis or ureter.
• Passage of large stones is excruciatingly painful and causes hematuria; can damage ureter.
  • Causes include hypercalcemia, dehydration, pH imbalances, frequent UTIs, or an enlarged prostate.
• Treatment includes using stone-dissolving drugs and/or surgery or lithotripsy (nonsurgical technique using ultrasound to break up stones)

Female Urethra

• 3-4 cm in length, bound to the anterior wall of the vagina.
• External urethral orifice lies between the vaginal orifice and clitoris.
• Internal urethral sphincter—smooth muscle under involuntary control.
• External urethral sphincter—skeletal muscle under voluntary control; where urethra passes through pelvic floor.

Male Urethra

• 18 cm long
• Three regions:
  • Prostatic urethra
  • Membranous urethra
  • Spongy (penile) urethra
• Passes through the prostate, pelvic floor and corpus spongiosum of penis.
  • Internal urethral sphincter and external urethral sphincter control urination.

Urinary Tract Infection (UTI)

• Cystitis: Infection of the urinary bladder; often in females due to a short urethra; often triggered by sexual intercourse; can cause fever and burning upon urination (may be asymptomatic); may spread up the ureter, causing pyelitis
• Pyelitis: An infection of the renal pelvis.
• Pyelonephritis: An infection reaching the cortex and nephrons; from blood-borne bacteria; fever, backache, and burning or bloody urine.
• Glomerulonephritis: Infection of the glomeruli. Common in children (often follows strep infection); fatigue, swelling of hands/feet, and high blood pressure.

Voiding Urine

• Micturition—act of urinating.
• Micturition reflex—spinal reflex partly controlling urination (involuntary), learned as children.
  • Between acts of urination: bladder filling; detrusor muscle relaxes; urethral sphincters are tightly closed.
  • Stretch of bladder (from filling) stimulates detrusor muscle contraction, leading to micturition (unless external sphincters control it).

Renal Insufficiency & Hemodialysis

• Renal insufficiency—kidneys fail to maintain homeostasis due to extensive nephron destruction.
  • Causes include hypertension, chronic kidney infections, trauma, prolonged ischemia/hypoxia, heavy metal poisoning/solvents, transfusion reactions, atherosclerosis, or glomerulonephritis.
• Nephro regeneration—nephrons can regenerate after short-term injuries; other nephrons hypertrophize to compensate.
• Survival—a person can survive with one-third of a kidney.
• Hemodialysis—procedure to artificially clear wastes from blood. Wastes leave bloodstream and enter dialysis fluid (through a semi-permeable cellophane tube), and also removing excess body water.

Test your knowledge on the urinary system's organs, functions, and the role of kidneys in maintaining overall body homeostasis. This quiz covers critical aspects like waste removal, electrolyte balance, and the production of hormones. Challenge yourself and see how well you understand the complexities of the urinary system.