OpenStax_Anatomy_Chapter_25

Urinary System

• Four main parts: kidneys, ureters, urinary bladder, urethra.
• Also called the renal system (renal means kidney).
• Basic function: remove protein wastes from the body.
• Kidneys filter blood and remove excess fluid, acid, toxins, etc., as well as protein wastes.

Kidneys and Ureters

• Kidneys use a great deal of water to flush out substances.
• Two kidneys are connected to the urinary bladder by muscular tubes called ureters.
• Urine produced by the kidneys passes to the urinary bladder using gravity, even if an individual is upside down.
• Ureters have smooth muscle in the vessel wall that rhythmically contracts (peristalsis) to squeeze urine into the urinary bladder.

Bladder

• The bladder is a storage sac; specify “urinary” bladder to differentiate from the gallbladder.
• Like the ureters, there are smooth muscles (involuntary) in the bladder wall that can contract to push urine out.
• Three layers of smooth muscle collectively called the detrusor muscle.
• The bladder collects urine from both ureters and stores it (about 500-600 mL).
• Highly distensible (stretchable-transitional epithelium) and can hold well beyond 1,000 mL, though the need to empty is felt even before 500 mL.
• The ureters do not open into the bladder at the top, but at the bottom posterior openings.
• The urethra empties through an anterior (front bottom) opening.
• The three openings create a triangle at the base, called the trigone.

Urethra

• Transports urine from the bladder outside the body.
• Two sphincters (ring-like muscles) control flow out of the urinary bladder:
  • Internal (involuntary-smooth muscle).
  • External (voluntary-skeletal muscle).

Kidney Location

• Located retroperitoneal, technically not in the abdominal cavity because they are behind the peritoneal membrane.
• Press up against the back wall/last ribs.

Kidney Structure

• Covered by a fibrous connective tissue capsule and held in place by a pad of adipose (fat) tissue.
• Renal hilum: medial entry and exit site for renal artery, renal vein, and ureter.
• Like the brain, there is an outer cortex and an inner medulla.

Nephrons

• Nephrons (microscopic) are the functional units of the kidney.
• Filter blood using the glomerulus (specialized kidney capillary).
• Wastes (but also necessary substances) are placed into the uriniferous tubules.
• Most of the necessary substances (especially water) are reabsorbed from the tubules back into the bloodstream via peritubular capillaries.

Blood Flow

• Afferent arteriole: carries blood to nephron (glomerulus).
• Efferent arteriole: carries blood away from glomerulus.
• Peritubular capillaries: the efferent arteriole surrounds the tubules and becomes a capillary bed (to absorb some of the necessary substances, especially water).

Renal Corpuscle

• The Bowman’s capsule captures fluid that gets filtered out of the glomerulus.
• The fluid is then funneled into the first part of the uriniferous tubules called the proximal convoluted (means coiled) tubule.

Glomerulus Details

• Renal corpuscle
• Glomerular capillaries are called fenestrated (leakier than most capillaries).
• Podocytes: cells that surround the glomerular capillaries.
• Have pedicels: look like interlocking fingers, create filtration slits.
• Along with fenestrations (holes), filtration slits make kidney capillaries more leaky.
• Net Outward Pressure Calculation:
  • Blood hydrostatic pressure: 55 \, \text{mm Hg}
  • Blood colloid osmotic pressure: 30 \, \text{mm Hg}
  • Capsular hydrostatic pressure: 15 \, \text{mm Hg}
  • Net outward pressure: 10 \, \text{mm Hg}

Tubules

• The diagram doesn’t show the peritubular capillaries (that would reabsorb substances), but it does show where specific substances are reabsorbed (or secreted-arrows going in) into the tubules.
• The nephron loop has a descending limb and an ascending limb that extend into the medulla of the kidney.

Tubule Location

• The glomerulus and convoluted tubules are in the cortex of the kidney.

Distal Collecting Tubule

• The distal collecting tubule (DCT) drains into the collecting duct (other nephrons also empty into a collecting duct).
• Collecting ducts from multiple nephrons (there are millions per kidney) join together, eventually exiting the kidney through the ureter.
• Urine moves from the kidney, through the ureter, to the urinary bladder where it is stored until it is eventually released out of the body via the urethra.