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 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.

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.

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).

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

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 (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.

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).

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.

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}

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.

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.