Urinary System

• Kidneys


  Organs that maintain the purity and constancy of our internal fluids, are perfect examples of homeostatic organs
• The kidneys filter gallons of fluid from the bloodstream every day, process this filtrate, and allow wastes and excess ions to leave the body in urine while returning needed substances to the blood in just the right proportions
• The lungs and the skin also play roles in excretion, but the kidneys bear the major responsibility for eliminating nitrogenous (nitrogen-containing) wastes, toxins, and drugs from the body
• Kidneys


  • Regulate blood volume
  • Maintain the proper balance between water and salts
  • Maintain the proper balance between acids and bases
  • Produce the enzyme renin to help regulate blood pressure
  • Release the hormone erythropoietin to stimulate red blood cell production
  • Convert vitamin D to its active form
• The other organs of the urinary system—the paired ureters and the single urinary bladder and urethra—provide temporary storage for urine or serve as transportation channels to carry it from the kidneys to the outside of the body
• Kidneys


  Small, dark red organs with a kidney-bean shape that lie against the dorsal body wall in a retroperitoneal position (behind the parietal peritoneum) in the superior lumbar region
• The right kidney is slightly lower than the left because it is crowded by the liver
• Kidney structure


  • Convex laterally
  • Has a medial indentation called the renal hilum
  • Several structures, including the ureters, the renal blood vessels, and nerves, enter or exit the kidney at the hilum
  • Has three protective layers: a transparent fibrous capsule, a fatty mass called the perirenal fat capsule, and the renal fascia, the most superficial layer made of dense fibrous connective tissue
• Renal cortex


  The outer, light-colored region of the kidney
• Renal medulla


  The deeper, reddish-brown area of the kidney
• Renal pyramids


  Triangular regions with a striped appearance in the renal medulla, with their broad bases facing the cortex and their apices pointing toward the inner region of the kidney
• Renal columns


  Extensions of cortex-like tissue that separate the renal pyramids
• Renal pelvis


  A flat, funnel-shaped tube lateral to the hilum, continuous with the ureter leaving the hilum
• Calyces


  Extensions of the renal pelvis that form cup-shaped "drains" enclosing the tips of the pyramids, collecting urine that drains from the pyramids
• Approximately one-quarter of the total blood supply of the body passes through the kidneys each minute
• Nephrons


  Structural and functional units of the kidneys responsible for forming urine, with over a million in each kidney
• Nephron structure


  • Consists of a renal corpuscle and a renal tubule
  • Renal corpuscle has a glomerulus (a knot of capillaries) and a glomerular capsule (Bowman's capsule)
  • Renal tubule has a proximal convoluted tubule, a nephron loop (loop of Henle), and a distal convoluted tubule
• Most nephrons are cortical nephrons located almost entirely within the cortex, while a few are juxtamedullary nephrons situated close to the cortex-medulla junction with their loops dipping into the medulla
• Nephron blood supply


  • Each nephron is associated with two capillary beds: the glomerulus and the peritubular capillary bed
  • The glomerulus is fed by the afferent arteriole and drained by the efferent arteriole, resulting in high blood pressure in the glomerular capillaries
  • The peritubular capillaries arise from the efferent arteriole and are low-pressure, porous vessels adapted for absorption
• Urine formation


  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Glomerular filtration


  A nonselective, passive process in which fluid passes from the blood into the glomerular capsule part of the renal tubule
• Filtrate is essentially blood plasma without blood proteins, as proteins and blood cells are normally too large to pass through the filtration membrane
• Tubular reabsorption


  The process of reclaiming useful substances (water, glucose, amino acids, ions) from the filtrate and returning them to the blood
• Some reabsorption is done passively (e.g., water reabsorption), while other reabsorption is active (e.g., glucose and amino acid reabsorption)
• Blood plasma without blood proteins. Both proteins and blood cells are normally too large to pass through the filtration membrane, and when either of these appears in the urine, there is usually a problem with the glomerular filters.
• As long as the systemic blood pressure is normal, filtrate will be formed. If arterial blood pressure drops too low, glomerular pressure becomes inadequate to force substances out of the blood into the tubules, and filtrate formation stops.
• Oliguria


  Abnormally low urinary output between 100 and 400 ml/day
• Anuria


  Abnormally low urinary output less than 100 ml/day
• Low urinary output indicates that glomerular blood pressure is too low to cause filtration, but anuria may also result from transfusion reactions and acute inflammation or from crushing injuries to the kidneys.
• Tubular reabsorption


  1. Needed substances (e.g. glucose and amino acids) are usually entirely removed from the filtrate

  2. Most reabsorption occurs in the proximal convoluted tubules, but the distal convoluted tubule and the collecting duct are also active
• Tubular secretion


  Substances (e.g. hydrogen and potassium ions, creatinine) move from the blood of the peritubular capillaries through the tubule cells or from the tubule cells themselves into the filtrate to be eliminated in urine
• Ureters


  • Two slender tubes each 25 to 30 cm (10 to 12 inches) long and 6 mm (¼ inch) in diameter
  • Smooth muscle layers in their walls contract to propel urine by peristalsis
  • Prevent urine from flowing back into the ureters by small valvelike folds of bladder mucosa that cover the ureter openings
• Urinary bladder


  • Smooth, collapsible, muscular sac that stores urine temporarily
  • Three layers of smooth muscle, collectively called the detrusor muscle
  • Mucosa is a special type of epithelium, transitional epithelium
  • When empty, it is collapsed, 5 to 7.5 cm (2 to 3 inches) long at most, and its walls are thick and thrown into folds
  • As urine accumulates, the bladder expands and rises superiorly in the abdominal cavity, its muscular wall stretches, and the transitional epithelial layer thins, increasing its volume and allowing the bladder to store more urine without substantially increasing its internal pressure
  • A moderately full bladder is about 12.5 cm (5 inches) long and holds about 500 ml (1 pint) of urine, but it is capable of holding more than twice that amount
• Urethra


  • Thin-walled tube that carries urine by peristalsis from the bladder to the outside of the body
  • Internal urethral sphincter: thickening of the smooth muscle at the bladder-urethra junction, an involuntary sphincter that keeps the urethra closed when urine is not being passed
  • External urethral sphincter: formed by skeletal muscle as the urethra passes through the pelvic floor, voluntarily controlled
  • In men, the urethra is approximately 20 cm (8 inches) long and has three named regions: the prostatic, membranous, and spongy (or penile)
  • In women, the urethra is about 3 to 4 cm (1½ inches) long, and its external orifice, or opening, lies anterior to the vaginal opening
• The female urinary orifice is close to the anal opening, so improper toileting habits (wiping from back to front rather than from front to back) can carry fecal bacteria into the urethra. And because the mucosa of the urethra is continuous with that of the rest of the urinary tract organs, inflammation of the urethra, or urethritis, can easily ascend the tract to cause bladder inflammation (cystitis) or even kidney inflammation (pyelonephritis, or pyelitis).
• Symptoms of urinary tract infection (UTI) include dysuria (painful urination), urinary urgency and frequency, fever, and sometimes cloudy or blood-tinged urine. When the kidneys are involved, back pain and a severe headache are common.
• Micturition (voiding)


  1. Bladder continues to collect urine until about 200 ml have accumulated

  2. Stretching of the bladder wall activates stretch receptors, transmitting impulses to the sacral region of the spinal cord and then back to the bladder via the pelvic splanchnic nerves, causing the bladder to go into reflex contractions

  3. As the contractions become stronger, stored urine is forced past the internal urethral sphincter (the smooth muscle, involuntary sphincter) into the upper part of the urethra

  4. The person will then feel the urge to void

  5. The external sphincter can be relaxed so that urine is flushed from the body

  6. If the person chooses not to void, the reflex contractions of the bladder stop within a minute or so, and urine collection continues
• Incontinence


  Inability to voluntarily control the external sphincter
• Urinary retention


  Inability of the bladder to expel its contained urine
• Urinary retention often occurs after surgery in which general anesthesia has been given, because it takes a little time for the smooth muscles to regain their activity. Another cause of urinary retention, occurring primarily in older men, is enlargement, or hyperplasia, of the prostate, which surrounds the neck of the bladder. As the prostate gland enlarges, it narrows the urethra, making it very difficult to void.