Urinary System Anatomy and Physiology: Study Guide for Nurses

When you titrate a nephrotoxic drug, watch a patient's I&O, or read a urinalysis, you are working this system. Know the structures and the three urine-forming processes cold, because they explain almost every renal finding you chart.

Functions of the Urinary System

The kidneys do six jobs:

1. Filter. Every day the kidneys filter gallons of fluid from the bloodstream.
2. Waste processing. They process that filtrate, sending wastes and excess ions out in urine while returning needed substances to the blood in the right proportions.
3. Elimination. The lungs and skin help with excretion, but the kidneys carry the major responsibility for clearing nitrogenous wastes, toxins, and drugs.
4. Regulation. They regulate blood volume and chemistry, holding the balance between water and salts and between acids and bases.
5. Other regulatory functions. By producing the enzyme renin they help regulate blood pressure, and their hormone erythropoietin drives red blood cell production in the bone marrow.
6. Conversion. Kidney cells convert vitamin D to its active form.

Anatomy of the Urinary System

The system is two kidneys, two ureters, a urinary bladder, and a urethra. The kidneys make urine. The rest store it or move it from one region to another.

The Kidneys

The kidneys keep our internal fluids constant. They are textbook homeostatic organs.

• Location. Small, dark red, kidney-bean shaped, lying against the dorsal body wall in a retroperitoneal position (beneath the parietal peritoneum) in the superior lumbar region. They extend from the T12 to the L3 vertebra, so the lower rib cage protects them.
• Positioning. The liver crowds the right kidney, so it sits slightly lower than the left.
• Size. An adult kidney is about 12 cm (5 inches) long, 6 cm (2.5 inches) wide, and 3 cm (1 inch) thick, about the size of a large bar of soap.
• Adrenal gland. An adrenal gland sits atop each kidney. It is part of the endocrine system and works as a functionally separate organ.
• Fibrous capsule. A transparent fibrous capsule encloses each kidney and gives a fresh kidney a glistening look.
• Perirenal fat capsule. A fatty mass surrounds each kidney and cushions it against blows.
• Renal fascia. The outermost capsule anchors the kidney against the trunk wall muscles.
• Renal cortex. The light-colored outer region.
• Renal medulla. The darker, reddish-brown area deep to the cortex.
• Renal pyramids. Triangular, striped regions of the medulla. Each pyramid's broad base faces the cortex; its apex points inward.
• Renal columns. Extensions of cortex-like tissue that separate the pyramids.
• Renal pelvis. A flat, basinlike cavity medial to the hilum, continuous with the ureter.
• Calyces. Cup-shaped extensions of the pelvis that enclose the pyramid tips and collect urine draining from them.
• Renal artery. Each kidney's arterial supply. It divides into segmental arteries near the hilum, and each segmental artery branches into interlobar arteries.
• Arcuate arteries. Branch from the interlobar arteries at the cortex-medulla junction and curve over the pyramids.
• Cortical radiate arteries. Branch off the arcuate arteries and run outward to supply cortical tissue.

Nephrons

Nephrons are the structural and functional units of the kidney.

• Nephrons. Each kidney holds over a million of them, and they form the urine.
• Glomerulus. A knot of capillaries, one of the two main nephron structures.
• Renal tubule. The other main structure.
• Bowman's capsule. The cup-shaped closed end of the renal tubule that surrounds the glomerulus, also called the glomerular capsule.
• Podocytes. Highly modified, octopus-like cells forming the inner layer of the capsule.
• Foot processes. Long branching extensions of the podocytes that intertwine and cling to the glomerulus.
• Collecting duct. The tubule coils, forms a hairpin loop, coils again, then drains into the collecting duct, which receives urine from many nephrons.
• Proximal convoluted tubule. The part of the tubule nearest the glomerular capsule.
• Loop of Henle. The hairpin loop following the proximal convoluted tubule.
• Distal convoluted tubule. The coiled segment after the loop of Henle, just before the collecting duct.
• Cortical nephrons. Most nephrons. They sit almost entirely in the cortex.
• Juxtamedullary nephrons. The few that sit next to the cortex-medullary junction, with loops of Henle that dip deep into the medulla.
• Afferent arteriole. The "feeder vessel," arising from a cortical radiate artery.
• Efferent arteriole. Receives blood that has passed through the glomerulus.
• Peritubular capillaries. Arise from the efferent arteriole draining the glomerulus.

Ureters

The ureters actively move urine, they do not just drain it passively.

• Size. Two slender tubes, each 25 to 30 cm (10 to 12 inches) long and 6 mm (1/4 inch) in diameter.
• Location. Each runs behind the peritoneum from the renal hilum to the posterior bladder, entering at a slight angle.
• Function. Smooth muscle in their walls propels urine to the bladder by peristalsis. Small valve-like folds of bladder mucosa flap over the ureter openings and prevent backflow.

Urinary Bladder

The urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily.

• Location. Retroperitoneal in the pelvis, just posterior to the symphysis pubis.
• Function. The detrusor muscle and transitional epithelium suit it for urine storage.
• Trigone. The smooth triangular region of the bladder base outlined by the three openings (two ureters, one urethra). Infections tend to persist here.
• Detrusor muscle. Three layers of smooth muscle in the bladder wall. Its mucosa is transitional epithelium.

Urethra

The urethra is a thin-walled tube that carries urine from the bladder to the outside.

• Internal urethral sphincter. A thickening of smooth muscle at the bladder-urethral junction. Involuntary, it keeps the urethra closed when urine is not being passed.
• External urethral sphincter. Skeletal muscle where the urethra passes through the pelvic floor. Voluntarily controlled.
• Female urethra. About 3 to 4 cm (1 1/2 inches) long, with its external orifice anterior to the vaginal opening.
• Male urethra. About 20 cm (8 inches) long, with three regions: prostatic, membranous, and spongy (penile). It opens at the tip of the penis.

Physiology of the Urinary System

Every day the kidneys filter gallons of fluid from the bloodstream. Here is how.

Urine Formation

Urine formation runs on three processes:

• Glomerular filtration. Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule.
• Tubular reabsorption. Water, glucose, amino acids, and needed ions move out of the filtrate into the tubule cells and back into capillary blood.
• Tubular secretion. Hydrogen, potassium, creatinine, and drugs move from the peritubular blood into the filtrate.

Characteristics of Urine

In 24 hours the kidneys filter some 150 to 180 liters of blood plasma through the glomeruli into the tubules.

• Daily volume. Only about 1.0 to 1.8 liters of urine come out in 24 hours.
• Components. Urine contains nitrogenous wastes and unneeded substances.
• Color. Freshly voided urine is clear and pale to deep yellow.
• Odor. Fresh urine is sterile and slightly aromatic. Left to stand, it takes on an ammonia odor as bacteria act on the solutes.
• pH. Usually slightly acidic (around 6), though metabolism and certain foods can push it more acidic or basic.
• Specific gravity. Pure water is 1.0; urine usually runs 1.001 to 1.035.
• Solutes. Normally sodium and potassium ions, urea, uric acid, creatinine, ammonia, bicarbonate ions, and other ions.

Micturition

Micturition (voiding) is emptying the bladder.

• Accumulation. The bladder collects until about 200 ml have accumulated.
• Activation. That stretch activates stretch receptors in the bladder wall.
• Transmission. Impulses run to the sacral spinal cord and back via the pelvic splanchnic nerves, throwing the bladder into reflex contractions.
• Passage. As contractions strengthen, urine is forced past the internal urethral sphincter into the upper urethra.
• External sphincter. Because the external sphincter is skeletal muscle and voluntary, the patient chooses to hold it closed or relax it and void.

Age-Related Changes in the Urinary System

Kidney function declines with age but keeps clearing waste unless disease intervenes. Filtration and excretion slow, so weigh how an older patient's drugs are handled by aging kidneys.

The bladder changes more visibly. Capacity and complete emptying both drop, so urgency and frequency are common. Urinary incontinence (UI) is never normal, so investigate it promptly, especially new onset.

Support good urinary function with adequate fluid intake, fewer bladder-irritant foods (sugar, caffeine, spicy and acidic foods), and pelvic muscle exercises.