Urinary System

Definitions:

1. Micturition - voiding (urination)
2. Urology - study of male reproduction & urinary, female urinary
3. Nephrology - nephron
4. Urea - end product of protein metabolism (nitrogen excreted by kidney

System Functions:

1. control volume, composition of bld.
2. elimination of wastes
3. produce renin (BP regulation)
4. role in erythropoiesis (hormonal secretion)
5. control pH of body fluids
6. Vitamin D activation

Anatomy of Kidney

1. Kidney (Renal) - located retroperitoneal external to peritoneum of abdominal cavity (T12 - L3)
2. Renal cortex vs. medulla
3. Nephron - basic functional unit (blood urine diagram)
   1. ~1 million, mostly in cortex (others juxtamedullary
   2. filter blood (90% bld)--> urine formation
   3. tubular secretion vs. tubular reabsorption
   4. internal anatomy:
      1. hilus - medial border (blood vessel, ureters exit)
      2. medullary pyramids - renal apex (papilla)
      3. papilla enclosed by minor calyces (collect urine)
      4. minor calyces empties into major calyx (2 or 3)which empties into renal pelvis empties to ureter
      5. urine propelled by peristalsis
   5. Nephron- 2 portions:
      1. Glomerulus
         1. tuft of capillaries in cortex
         2. Bowman's capsule -- start of tubules - responsible for filtration(fenestrated epithelium)
      2. Renal tubules
         1. proximal convoluted tubule
         2. Loop of Henle - ascending/descending
         3. distal convoluted tubule
         4. empties into collecting duct or tubule (not actually part of nephron)
         5. filtration ~ 45 gal/day of blood
         6. eliminate 1000-2000 ml / day
   6. Blood vessels of kidney:
      1. Renal artery - supplies kidney receives 20% of cardiac output
      2. branch into smaller arteries which diverge into afferent arterioles (supply to each glomerulus) 1st capillary bed followed by 2nd capillary bed
      3. leave glomerulus via efferent arteriole
      4. peritubular capillaries ( & vasa recta - loop extending into medulla)
      5. small veins--> renal vein
   7. Juxtaglomerular (JG) complex:
      1. afferent arteriole- within smooth muscle cells are secretory granules (jg cells)
      2. involved w/ BP regulation, Na+ balance along with protion of distal straight tubule (macula densa)
      3. jg cells + macula densa = juxta glomerular complex or apparatus
      4. macula densa-constrict or dilate afferent arteriole (regulates filtration pressure into glomerulus)

Renal Physiology

1. Glomerular Filtration
   1. blood filter from glomerular capillaries into tubules
   2. dependent on blood pressure (hydrostatic) to force fluid / solute thru membrane
   3. nonselective (all plasma contents except proteins bld cells, large particles > 7 nm)
   4. Filtrate in Bowman's capsule does not equal urine
      1. more ions, nutrients in filtrate more waste in urine
      2. once filtrate enters collecting duct = 'urine'
   5. need glomerular pressure of > 50 mmHg to maintain filtration
2. Tubular reabsorption
   1. movement of filtrate components back into capillary blood (retention by body ~ 99%)
   2. substances reabsorbed in specific areas of the renal tubule (active or passive process)
      1. proximal tubule
         1. most reabsorption
         2. 75% H20, nutrients, ions (sodium)
      2. Descending Loop
         1. only H2O, no ions
      3. Ascending Loop
         1. Na+, Cl- (no H2O)
      4. Loop of henle (passive processes)
      5. Distal convoluted tubule
         1. hormonal control of Na+ (K+ secretion)
      6. Collecting Tubules -
         1. hormonal control of H2O reabsorption
   3. Atrial Natriuretic Factor- (decreases) Na+ reabsorption
   4. Active tubular reabsorption:
      1. requires energy , carrier molecule
         1. glucose, amino acids (proximal c. tub.)
         2. Na+, K+
         3. calcium
   5. Passive tubular reabsorption:
      1. no energy required
      2. Urea , water ( some sites)
3. Tubular Secretion
   1. opposite to reabsorption
   2. wastes: organic ions, H+, ammonia, and K+
   3. H+ secreted in proximal & distal convoluted, collecting tubule
   4. pH control (can secrete H+ into urine or K+ [as Na+ reabsorbed])
   5. Countercurrent mechanism
      1. mechanisms which raise the medullary interstitial fluid concentration(300-1200 osmol) to permit low volume, concentrated urine
      2. when H2O reabsorption in collecting tubule--> urea follows in distal portion
      3. distal convoluted, ascending loop of Henle are impermeable to urea
      4. vasa recta promote water reabsorption

Ureters

1. Paired Structures
2. Functions:
   1. urine transport from kidney to bladder - via peristalsis
   2. compress during micturition when bladder contracts - muscous layer is transitional epithelium
   3. renal calculi - kidney stone (quite painful if lodge in ureters)

Urinary Bladder

1. smooth, stretchable muscular sac
   1. urine storage
      1. ~ 600-800 milliter capacity
      2. usually feel urge ` 300 ml
2. trigone ( infection site)
3. detrussor muscle (3 layers)
4. Cystitis - inflammation of bladder (women more common, shorter urethra)

Urethra:

1. muscular tube exiting bladder
   1. females 1.5" length (urine only)
   2. males 8' length (urine & semen)
2. 3 parts:
   1. prostatic urethra
   2. membranous urethra
   3. spongy (cavernous) urethra
3. Urine release
   1. controlled via internal urethral sphincter -bladder tissue
   2. external urethral sphincter- voluntary

Micturition

1. a reflex arc :
   1. Mechanoreceptors stimulate parasympathetic via sacral nerves
   2. triggered at ~ 300 ml of urine in bladder
   3. brain can inhibit or facilitate
   4. delay 'urge' until another 300ml collects
   5. voluntary control with training
      1. incontinence- lack of voluntary control (under stress, pregnancy, aging effects)

Abnormalities of the Urinary System

1. Cystitis - inflamed bladder
2. pyelonephritis- bacteria infect renal pelvis
3. Proteinuria- plasm proteins pass into urine
4. Uremia- hi blood urea , improper excretion