Urinary System Anatomy and Function

• Primary functions involve regulating plasma ionic composition, volume, osmolarity, and pH.
• It also removes metabolic waste and foreign substances from the body.
• Secretes erythropoietin and renin, activates vitamin D₃ to calcitriol, and performs gluconeogenesis.

Anatomy of the Urinary System

• Consists of the kidneys, ureters, bladder, and urethra.
• Kidneys form urine, ureters transport urine to the bladder, the bladder stores urine, and the urethra excretes urine.
• Kidneys are bean-shaped, retroperitoneal organs weighing approximately 115–170 g each.
• The nephron is the functional unit of the kidney, containing the renal corpuscle and renal tubules.
• The renal corpuscle includes the glomerulus and Bowman's capsule.
• Renal tubules consist of the proximal tubule, loop of Henle, distal tubule, and collecting duct.
• Cortical nephrons (85%) have a short loop of Henle.
• Juxtamedullary nephrons have a long loop of Henle and establish the medullary osmotic gradient.
• The juxtaglomerular apparatus regulates GFR and blood pressure and contains the macula densa and granular (JG) cells.
• Blood flows through the renal artery, segmental artery, interlobar artery, arcuate artery, interlobular artery, afferent arteriole, glomerulus, and efferent arteriole.
• Peritubular capillaries and vasa recta (in juxtamedullary nephrons) are also part of the blood supply.

Basic Renal Exchange Processes

• Glomerular filtration moves fluid from the glomerulus to Bowman's capsule.
• Reabsorption moves substances from the tubule into the peritubular capillaries.
• Secretion moves substances from the peritubular capillaries into the tubule.
• Excretion moves substances from the tubule out of the body.
• Glomerular filtration: 180 L/day is filtered, but only about 1.5 L/day of urine is excreted.
• Substances must cross the capillary endothelium, basement membrane, and podocyte filtration slits during filtration.
• Starling forces that favor filtration include glomerular hydrostatic pressure (60 mmHg).
• Starling forces that oppose filtration include Bowman’s capsule hydrostatic pressure (15 mmHg) and glomerular oncotic pressure (29 mmHg).

GFR Regulation

• Intrinsic mechanisms for GFR regulation include myogenic smooth muscle contraction and tubuloglomerular feedback via macula densa.
• Extrinsic mechanisms for GFR regulation: a drop in blood pressure activates baroreceptors, leading to a sympathetic response and vasoconstriction of arterioles, which decreases GFR.
• Reabsorption mostly occurs in the proximal tubule.
• Substances must cross epithelial and capillary endothelial cells during reabsorption.
• Active reabsorption uses transporters, while passive reabsorption occurs via diffusion or osmosis.
• Transport Maximum (Tm): occurs when carriers are saturated.
• Renal threshold is the plasma concentration at which a solute appears in the urine.
• Glucose is normally 100% reabsorbed.
• Tm for glucose = 375 mg/min
• Threshold for glucose = ~160–180 mg/dL
• Secretion involves the same barriers and mechanisms as reabsorption, but in reverse.
• Substances such as K⁺, H⁺, creatinine, and penicillin are secreted.

Regional Specialization

• Proximal tubule: responsible for bulk reabsorption (70% of Na⁺ and H₂O, and 100% of glucose).
• Distal tubule & collecting duct: regulated reabsorption/secretion.
• Loop of Henle: responsible for water conservation and concentrates urine.

Excretion

• Excretion rate = filtration + secretion − reabsorption
• Clearance: volume of plasma cleared per unit time
• Creatinine is used to estimate GFR (~140 mL/min).
• Micturition (Urination): controlled by the micturition reflex.
• Stretch receptors in the bladder wall send signals to the spinal cord.
• Decreased sympathetic and somatic nervous system activity leads to sphincter relaxation.
• Increased parasympathetic nervous system activity causes the detrusor muscle to contract.
• Urine is then expelled.