Urinary System Flashcards

Urinary System Part 1 – General Characteristics

• Organs of the urinary system:
  • Kidneys
    • Filter blood and form urine
  • Ureters
    • Transport urine from kidneys to urinary bladder
  • Urinary bladder
    • Collects and stores urine
  • Urethra
    • Conveys urine from urinary bladder to outside of body
• The kidneys play a vital role in maintaining homeostasis by regulating the composition, pH, and volume of body fluids.
• Functions of the kidneys include:
  • Filter blood and remove metabolic waste from the body.
    • Kidneys transfer waste products from blood to urine.
  • Maintenance of fluid and electrolyte balance.
    • By either conversing or eliminating water and electrolytes such as sodium, potassium, and calcium ions.
  • Maintenance of acid-base balance.
    • By conserving or eliminating H+ and HCO3- ions.
  • Maintenance of blood pressure.
    • By directly controlling blood volume, and indirectly by releasing an enzyme called renin.
  • Regulation of erythropoiesis.
    • By releasing a hormone called erythropoietin.
  • Performing other metabolic functions.
    • Detoxifying blood, activating vitamin D, and making new glucose through gluconeogenesis.
• Location of the kidneys:
  • Lie on both sides of the vertebral column, in a depression on the posterior abdominal wall
    • Positioned retroperitoneally (behind the parietal peritoneum).
  • The upper and lower borders of the kidneys are generally at the levels of T12 and L3.
    • Left kidney is 1.5 to 2 cm higher than the right kidney due to the position of the liver.
  • Surrounded by fibrous renal capsule, adipose tissue (renal fat), and connective tissue (renal fascia).
• Each kidney has a convex lateral surface and a concave medial surface.
  • Renal sinus – hollow chamber in medial depression
  • Renal cortex – outer region of the kidney
  • Renal medulla – inner region; composed of renal pyramids
  • Renal columns – extensions of cortex that dip into the medulla
  • Hilum – entrance to renal sinus
  • Renal pelvis – funnel-shaped sac; superior end of the ureter
    • Major calyces – large tubes that merge to form the renal pelvis
    • Minor calyces – small tubes that merge to form major calyces
  • Renal capsule – fibrous capsule around the kidney
  • Nephrons – functional units of the kidney, each of which is a site of urine production
• Blood flow through the kidneys:
  • Major arteries of the kidney:
    • Renal artery – branches off the abdominal aorta, enters the kidney through the hilum
      • Kidneys receive around 25% of total cardiac output.
    • Segmental arteries – branch off the renal artery
    • Interlobar arteries – branch off the segmental arteries, flow between renal pyramids
    • Arcuate (arciform) arteries – branch off the interlobar arteries, arch around renal pyramids
    • Cortical radiate (interlobular) arteries – branch off the arcuate arteries
• The nephron is the functional unit of the kidney (about 1 million in each kidney) and is composed of 2 parts:
  • Renal corpuscle is the filtering unit and is in the renal cortex.
    • Glomerulus – fenestrated capillary cluster
      • Filters blood to begin urine formation
      • Arises from the afferent arteriole, drains into the efferent arteriole
    • Glomerular (Bowman’s) capsule – receives filtrate from the glomerulus
  • Renal tubule extends from the glomerular capsule to the collecting duct
    • Filtrate proceeds from Glomerular Capsule → Proximal (convoluted) tubule → Nephron loop (loop of Henle, composed of a descending and an ascending limb) → Distal (convoluted) tubule
    • Distal tubules of several nephrons empty into a collecting duct
      • Collecting duct continues through the medulla and drains through the renal papilla into a minor calyx

Urinary System Part 2 – Glomerular Filtration, and the Renin-Angiotensin System

• Urine is produced by the nephrons and collecting ducts as they remove waste from the body.
  • Three processes of urine formation:
    • Glomerular filtration:
      • Performed by specialized glomerular capillaries (first capillary bed)
      • Water and small molecules are filtered
      • Filtered fluid enters renal tubules and becomes tubular fluid (filtrate)
    • Tubular reabsorption:
      • Transfer of filtered substances from renal tubules to peritubular capillaries (second capillary bed)
      • Only reclaims useful substances, while wastes continue to become urine
    • Tubular secretion:
      • Transfer of certain substances from peritubular capillaries to renal tubules
      • Adds waste products and excess substances to forming urine
• Urine Excreted = Glomerular Filtration + Tubular Secretion – Tubular Reabsorption
• The renal corpuscle is responsible for filtering blood.
  • The renal corpuscle has two parts:
    • A glomerular capsule that is composed of two layers of epithelial cells.
      • An outer parietal layer.
      • An inner visceral layer that consists of modified epithelial cells called podocytes.
        • Podocytes wrap around glomerular capillaries with foot-like processes called pedicles.
        • Pedicles form filtration slits.
      • A capsular space is found between the parietal and visceral layers and is continuous with the lumen of the renal tubule.
    • Glomerulus – a group of fenestrated capillaries.
• Urine formation begins when the glomerular capillaries filter blood plasma through a process called glomerular filtration.
  • Glomerular capillaries are many times more permeable to small molecules than continuous capillaries due to many tiny openings called fenestrae in their walls.
    • Water, small molecules, and ions are filtered into the glomerular capsule from the glomerular capillaries.
    • Cells and large proteins are not normally filtered by the glomerular capillaries.
  • Glomerular filtrate has about the same composition as tissue fluid in other areas of the body.
    • Filtrate is mostly water with dissolved glucose, amino acids, urea, uric acid, creatine, creatinine, Na+, Cl-, K+, HCO3-, PO43-, and SO42- ions.
• The main force that filters substances through the glomerular capillary wall is the hydrostatic pressure of the blood of the glomerulus.
  • The afferent arteriole has a larger diameter than the efferent arteriole, which helps increase the hydrostatic pressure within the glomerular capillaries.
    • Resistance in the efferent arteriole increases blood pressure in the glomerulus, which favors filtration.
  • Net filtration pressure = force favoring filtration − forces opposing filtration.
    • Force favoring filtration:
      • Glomerular capillary hydrostatic pressure (blood pressure)
    • Forces opposing filtration:
      • Glomerular capillary colloid osmotic pressure (due to plasma proteins in the glomerulus)
      • Capsular hydrostatic pressure
  • As long as the net filtration pressure in the glomerulus is positive, filtration will occur.
  • Net filtration pressure components:
    • Outward force, glomerular hydrostatic pressure = +60 mm
    • Inward force of plasma colloid osmotic pressure = -32 mm
    • Inward force of capsular hydrostatic pressure = -18 mm
    • Net filtration pressure = +10 mm
• Glomerular filtration rate (GFR) is the amount of filtrate produced per minute by both kidneys.
  • GFR is the most measured index of kidney function and is directly proportional to the net filtration pressure.
  • Kidneys receive 25% of cardiac output
    • An average GFR = 125 ml/minute
    • Equivalent to 180 liters (45 gallons) per day.
    • Most of this is reabsorbed by the nephron.
  • Controlling GFR is vital to remove adequate amounts of wastes and to control the composition of body fluids.
    • An increase in GFR will increase urine output.
    • A decrease in GFR will decrease urine output.
  • GFR remains relatively constant through two mechanisms, which include:
    • Autoregulation
    • Renin-angiotensin system
• Renal autoregulation is the ability of the nephrons to adjust their own blood flow and GFR without external (nervous or hormonal) control
  • Dilation of the afferent arteriole or constriction of the efferent arteriole will increase GFR.
  • Constriction of the afferent arteriole or dilation of the efferent arteriole will decrease GFR.
  • Autoregulation can be overridden by the sympathetic ANS during significant volume loss or gain.
    • A large blood volume loss, which markedly decreases blood pressure causes vasoconstriction of afferent arterioles, decreasing GFR, which decreases urine output to conserve water.
    • A large blood volume gain, which markedly increases blood pressure causes vasodilation of afferent arterioles, increasing GFR, which increases urine output to eliminate the excess water.
• A second control of GFR is the hormone-like renin-angiotensin-aldosterone system.
  • A decrease in blood pressure will lead to a drop in GFR.
    • Receptors in the macula densa cells of the juxtaglomerular apparatus (JGA) detect this decrease in two ways:
      • Renal baroreceptors detect changes in blood pressure
      • Chemoreceptors in the detect changes in the levels of Na+, K+, and Cl-.
    • When the macula densa cells detect a decrease in blood pressure, they will stimulate the juxtaglomerular cells to secrete the enzyme renin.
• Action of aldosterone in the distal convoluted tubule:
  • Stimulates reabsorption of Na+ and stimulates the secretion of K+.
• Nephritis is inflammation of the kidney.
  • Glomerulonephritis - inflammation of the glomeruli
    • Acute Glomerulonephritis (AGN):
      • Results from abnormal immune reaction, 1 to 3 weeks after infection by beta-hemolytic Streptococcus
      • Infection does not start in the kidney
      • Antigen-antibody complexes form insoluble immune complexes, which lodge in the kidneys
      • Complexes deposit in and block glomeruli
    • Chronic Glomerulonephritis:
      • Progressive disease, involving prolonged inflammation
      • More and more nephrons are damaged, until kidneys cannot function
      • Fibrous tissue replaces glomerular membranes, disabling nephrons

Urinary System Part 3 – Tubular Reabsorption and Secretion

• Tubular reabsorption is defined as the process by which substances are transported from the glomerular filtrate (through the walls of the renal tubule) to blood in the peritubular capillaries.
  • 65% of reabsorption occurs in the PCT.
    • The Na+ is actively reabsorbed and drives the reabsorption of water and other solutes.
    • Water is reabsorbed through osmosis as it follows other solutes.
      • Substances that remain in filtrate become concentrated as water is reabsorbed.
  • Substances such as glucose that are reabsorbed with a carrier have a transport maximum.
    • Renal plasma threshold = concentration of a substance in plasma at which it begins to be excreted in the urine
  • Reabsorbed substances pass from the lumen of the renal tubule through the epithelial cells (PCT) and into the lumen of a peritubular capillary where they are returned to bloodstream.
• Tubular secretion is the process in which the renal tubule extracts chemicals from the capillary blood and secretes them into tubular fluid
  • Two purposes of tubular secretion:
    • Waste removal
      • Urea, uric acid, creatinine, and clears blood of pollutants, morphine, penicillin, aspirin, and other drugs metabolites
    • Acid-base balance
      • Secretion of hydrogen and bicarbonate ions help regulate the pH of the body fluids

Urinary System Part 4 – Regulation of Urine Concentration and Volume

• The kidneys can absorb large amounts of water independent of sodium in the distal convoluted tubules and collecting ducts.
  • The hypothalamus stimulates the posterior pituitary gland to release antidiuretic hormone (ADH, or vasopressin) in response to decreased water levels in body fluids, or a decrease in blood volume or pressure.
    • ADH stimulates the distal convoluted tubules and collecting ducts to insert specialized water channels called aquaporins.
    • Aquaporins greatly enhance the reabsorption of water, especially by the juxtaglomerular nephrons in the renal medulla.
    • The countercurrent mechanism keeps the interstitial fluid in the medulla hypertonic, which favors reabsorption.
  • An increase in ADH secretion leads to more water reabsorption and less urine.
    • The urine produced will be more concentrated.
  • A decrease in ADH secretion leads to less water reabsorption and more urine.
    • The urine produce will be less concentrated (diluted).
• A countercurrent mechanism in the nephron loops of juxtamedullary nephrons keeps medullary interstitial fluid hypertonic, so a concentrated urine can be produced when needed.
  • Ascending and descending limbs of nephron loops are parallel and near each other.
    • The thick ascending limb of the nephron loop is impermeable to H2O but actively reabsorbs Na+ and Cl- ions (and some K+ ions)
      • Results in hypertonic interstitial fluid and hypotonic tubular fluid in the ascending limb
    • In response, H2O in the descending limb leaves by osmosis since it is permeable to H2O, but not to Na+ and Cl- ions
      • Results in hypertonic tubular fluid in the descending limb
    • The mechanism is called the countercurrent multiplier because every time a cycle is completed, the NaCl concentration in the ascending limb increases
  • The vasa recta of the juxtamedullary nephrons also have a countercurrent mechanism, called a countercurrent exchanger.
    • They absorb water but keep most of the salt in the renal medulla.
• Urine composition reflects the volumes of water and solutes that the kidneys must excrete or conserve to maintain homeostasis
  • Composition of urine varies due to dietary intake and physical activity, but typically:
    • Consists of about 95% water
    • Contains metabolic waste products
      • Urea, uric acid, and creatinine
    • Contains small amounts of amino acids and varying amounts of electrolytes.
    • Under normal conditions, urine will have very little protein and no glucose.
  • Urine volume:
    • 0.6 to 2.5 L/day
      • 50 to 60 mL of urine output/hour is normal
    • Volume varies with fluid intake, sweating, body temperature, emotional state, environmental factors
• Urea and uric acid
  • Urea – a by-product of amino acid catabolism
    • Plasma concentration reflects the amount of protein in the diet
    • Enters renal tubules through glomerular filtration and undergoes both tubular reabsorption and tubular secretion
    • Up to 80% is reabsorbed; the rest is excreted in the urine
  • Uric acid – a product of nucleic acid metabolism
    • Enters the renal tubules through glomerular filtration
    • Active transport completely reabsorbs filtered uric acid
    • About 10% of uric acid enters urine through tubular secretion and is excreted
    • Excess uric acid may lead to a painful condition called gout, in which uric acid precipitates in the blood and forms crystals in joints
• The renal clearance test measures the rate at which a chemical is removed from the plasma by the kidney.
  • Renal clearance tests are used to determine GFR
    • Tests indicate kidney efficiency, glomerular damage, and progression of renal diseases
  • Renal clearance test:
    • Inulin, a plant polysaccharide, is the most accurate test but slow (3 – 4 hours).
      • Freely filtered and neither reabsorbed nor secreted by kidneys
    • Creatinine is used in clinical settings and is quick and easy.
      • Freely filtered but also secreted in small amounts.
      • Slightly overestimates GFR but provides a quick and easy estimate.

Urinary System Part 5 – Transport, Storage, and Elimination of Urine

• The ureters are tubular organs, about 25 cm long, which transports urine from the kidney to the urinary bladder.
  • Each begins as the renal pelvis in the kidney and empties into the posterior portion of the urinary bladder in the pelvic cavity.
  • The wall of the ureter consists of 3 layers:
    • Inner mucous coat (or mucosa, consists of transitional epithelium)
    • Middle muscular coat (or muscularis)
    • Outer fibrous coat (or adventitia)
  • Peristaltic waves transport urine along the ureters
  • An obstruction, such as a kidney stone (or renal calculus) in a ureter causes:
    • Strong peristaltic waves in the obstructed ureter to move the stone toward the urinary bladder
• Transitional epithelium consists of many layers of cells that change shape in response to tension.
  • Primary functions include distensibility and preventing urine from leaking back into the internal environment.
  • Locations:
    • Inner lining of the urinary bladder and linings of ureters and part of the urethra
• Kidney stones can be composed of uric acid, calcium oxalate, calcium phosphate, or magnesium phosphate.
  • Form in the collecting ducts or renal pelvis of the kidney
  • Causes severe pain, nausea and vomiting, blood in urine
  • 60% of kidney stones pass on their own; others can be shattered with lithotripsy or removed surgically
  • Causes:
    • Calcium supplements (in those with inherited tendency), excess vitamin D, urinary tract blockage, urinary tract infections.
    • The tendency to form kidney stones is inherited, especially calcium stones
• The urinary bladder is a hollow, distensible, muscular organ in the pelvic cavity that stores urine and sends it into the urethra.
  • Contacts the anterior walls of the uterus and vagina in the female and lies posteriorly against the rectum in the male.
  • Triangular trigone at the floor of the bladder contains openings at each of its three corners:
    • 2 for ureters and 1 for the urethra
  • The wall consists of 4 layers:
    • Inner mucous coat (mucosa, transitional epithelium)
    • Submucous coat (submucosa)
    • Muscular coat (muscularis)
      • Smooth muscle fibers of the muscular coat comprise the detrusor muscle.
    • Outer serous coat (serosa, upper surface only)
  • Can store up to 600 – 800 mL of urine in both sexes.
• The urethra is a tubular organ that conveys urine from the urinary bladder to the outside of the body.
  • Layers of the urethra:
    • Innermost mucous coat (mucosa)
    • Thick muscular coat (muscularis)
      • Internal urethral sphincter – smooth muscle, involuntary
      • External urethral sphincter – skeletal muscle, voluntary
    • The submucous coat (submucosa) has many mucous glands, called urethral glands
• The urethra is a tubular organ that conveys urine from the urinary bladder to the outside of the body.
  • Female urethra:
    • About 4 cm long
    • The external urethral orifice is anterior to the vaginal opening
  • Male urethra:
    • About 19.5 cm long
    • Has a dual function for both urination and reproduction
    • Has 3 sections:
      • Prostatic urethra
      • Membranous urethra
      • Spongy urethra
        • Terminates at the external urethral orifice in the penis
• Micturition (urination) is the expulsion of urine from the urinary bladder.
  • Urine leaves urinary bladder by micturition reflex:
    • Occurs through reflex contraction of the detrusor muscle and reflex relaxation of the internal urethral sphincter
    • Requires relaxation of the external urethral sphincter (voluntary)
  • Controlled by the micturition reflex center in the sacral spinal cord.