Urinary System Functions

Functions of the Urinary System

• Kidneys filter blood and produce urine to clear waste, but also regulate water and solute levels in body fluids
• Kidney function impacts the volume/composition of body fluids, with regulation based on bodily needs
• Primary kidney functions include regulation of plasma ionic composition, plasma volume, blood pressure, plasma osmolarity and plasma hydrogen ion concentration (pH)
• Kidneys regulate plasma ionic composition by adjusting excretion rates of ions like sodium, potassium, calcium, magnesium, chloride, bicarbonate, hydrogen, and phosphates
• Plasma volume and blood pressure are regulated by controlling the rate of water excretion in urine, which directly affects total blood volume
• Kidneys regulate plasma osmolarity by varying the excretion rate of water relative to solutes
• Plasma pH is regulated in partnership with the lungs, by controlling bicarbonate and hydrogen ion concentrations
• Kidneys remove metabolic waste products (urea, uric acid) and foreign substances (food additives, drugs) from the plasma, clearing these from the body via urine
• Because water and small solutes freely exchange between plasma and interstitial fluid, the kidneys regulate the volume/composition of all bodily fluids
• Secondary kidney functions include the secretion of erythropoietin (for erythrocyte production) and renin (for angiotensin II production, which regulates salt, water balance, and blood pressure)
• Kidneys activate vitamin D3, which is important for regulating blood calcium and phosphate levels and carry out gluconeogenesis during fasting to maintain plasma glucose

Structures of the Urinary System

• The urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra
• After urine is formed by the kidneys, the ureters conduct it to the bladder
• The bladder stores urine until it is excreted through the urethra

Clinical Connections: Kidney Stones

• Kidney stones (renal lithiasis) form when minerals concentrate in the kidneys, forming crystals that attach to the renal epithelium
• More than 5% of the US population develops kidney stones, leading to 500,000 emergency room visits yearly
• Kidney stones are more common in Western cultures, with males affected four times more often than females
• Dislodged stones can enter the ureter, causing severe pain in the side, back, and groin and hospitalization is often needed for pain management
• Factors increasing kidney stone likelihood include elevated concentrations of calcium oxalate (80% of stones), uric acid, cysteine, or struvite
• Cysteine crystals result from cysteinuria, a hereditary disease that increases cysteine excretion
• Low fluid intake and diets high in protein, low in fiber, or high in vitamin C or D can promote stone formation
• Substances like citrate and magnesium, along with high fluid intake, can help prevent kidney stones
• Small kidney stones often pass without intervention, with advice to consume large volumes of water to enhance natural passing
• Larger stones can be broken down by extracorporeal shock wave lithotripsy (ESWL) and then passed
• Very large stones may require surgical removal via nephrolithotomy (from kidneys) or uteroscopy (from ureters)

Anatomy of the Kidney

• Kidneys are paired, bean-shaped organs located retroperitoneally on the rear abdominal wall above the waistline (at the 12th rib)
• Renal arteries, branching off the aorta at the renal hilus, supply blood to the kidneys
• The kidneys receive ~20% of the cardiac output providing oxygen, nutrients, and enabling removal of unneeded solutes/water
• Filtered blood returns to circulation via the renal veins, draining into the inferior vena cava
• The kidney contains a reddish-brown outer cortex and a darker, striped inner medulla
• The medulla contains renal pyramids, whose tips (papillae) drain into minor calyces via collecting ducts
• Minor calyces converge into major calyces, which drain into the renal pelvis (the initial portion of the ureter)
• Kidneys contain over a million nephrons, which are microscopic subunits that filter blood and form urine
• Nephrons feature a long, coiled renal tubule forming a hairpin loop
• During urine formation, fluid flows through the renal tubules, and its composition is modified by exchange with interstitial fluid
• Fluid from tubules drains into collecting ducts, where it is further modified, and the fluid exiting the collecting ducts is urine, flowing to the ureters

Microscopic Anatomy of the Kidney

• Each nephron is a "mini-kidney" that filters blood and forms urine
• The nephron consists of the renal corpuscle for filtering blood, and the renal tubule for modifying filtrate into urine

Renal Corpuscle

• The renal corpuscle includes Bowman’s capsule, a spherical structure, and the glomerulus, a capillary tuft
• The renal corpuscle is where blood is filtered to create tubular fluid (filtrate)
• Blood enters the glomerular capillaries via the afferent arteriole where protein free plasma filters into Bowman's capsule via glomerular filtration
• Remaining blood exits through the efferent arteriole
• The afferent and efferent arterioles' smooth muscle regulates glomerular filtration in response to paracrine and sympathetic nervous system input

Renal Tubule

• Glomerular filtrate flows from Bowman’s capsule to the proximal convoluted tubule, then the proximal straight tubule (together, the proximal tubule)
• The proximal tubule empties into the loop of Henle, which has a descending limb, a thin ascending limb, and a thick ascending limb
• The descending limb extends into the renal medulla
• The thin ascending limb extends toward the cortex
• As the tubule approaches the cortex, it widens into the thick ascending limb
• Fluid flows from the ascending limb of the loop of Henle into the distal convoluted tubule
• The distal convoluted tubule leads to the connecting tubule, which joins the nephron to the collecting duct, that empties into the minor calyces

Cortical and Juxtamedullary Nephrons

• Cortical nephrons are located almost entirely within the renal cortex while juxtamedullary nephrons have the renal corpuscle near the cortex-medulla border
• Cortical nephrons make up the vast majority and only the tip of the loop of Henle dips into the renal medulla
• Juxtamedullary nephrons make up approximately 15–20% of all nephrons, with the loop of Henle dipping deep into the renal medulla
• Both nephron types function directly in urine formation, but juxtamedullary nephrons also establish an osmotic gradient in the renal medulla to concentrate urine and conserve water

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus is located where the distal tubule contacts the afferent and efferent arterioles
• The juxtaglomerular apparatus includes the macula densa (tubule epithelial cells) and granular cells (juxtaglomerular cells) found in the walls of the afferent and efferent arterioles
• Granular cells contain renin-filled secretory granules and the juxtaglomerular apparatus regulates blood volume and pressure

Blood Supply to the Kidney

• The renal artery branches into segmental arteries, then interlobar arteries, then arcuate arteries, then interlobular arteries Blood is carried to individual nephrons via afferent arterioles, leading to the glomerular capillary beds
• Efferent arterioles arising from the glomerular capillary beds give rise to peritubular capillaries (branching from cortical nephrons' efferent arterioles, located around renal tubules) and vasa recta (branching from juxtamedullary nephrons' efferent arterioles, forming loops along the loops of Henle and collecting ducts)
• Peritubular capillaries and vasa recta drain into interlobular veins, arcuate veins, and interlobar veins, eventually draining into the renal vein