Urinary System Functions and Anatomy

Urinary System Functions

• The urinary system has three main functions: excretion (removing waste), elimination (discharging waste), and homeostatic regulation of blood plasma volume and solute concentration.
• Additional homeostatic functions include regulating blood volume and pressure, ion concentrations, pH, and nutrient conservation.

Urinary System Components

• The urinary system comprises the kidneys, ureters, urinary bladder, and urethra.
• Kidneys produce urine, a fluid containing water, ions, and solutes.
• Urine is expelled from the body during urination (micturition).

Kidney Structure and Vessels

• The left kidney sits slightly higher than the right.
• Both kidneys are retroperitoneal.
• A fibrous capsule surrounds each kidney.
• The hilum allows for access of the renal artery/nerve, and exit of the renal vein/ureter.
• The ureter connects to the renal pelvis, which branches into calyces surrounding renal papillae.
• Urine formation begins in nephrons.
• Kidney blood vessels include interlobar, arcuate, and cortical radiate arteries/veins.
• Blood flows between afferent/efferent arterioles and peritubular capillaries/vasa recta.
• Diffusion exchange occurs between capillaries, nephron tubules, and surrounding interstitial fluid.

Nephron Structure and Function

• The nephron is the kidney's functional unit consisting of a renal corpuscle and renal tubule.
• The renal corpuscle is composed of a glomerulus (capillary knot) surrounded by Bowman's capsule.
• Blood enters via the afferent arteriole and exits via the efferent arteriole.
• Podocytes cover glomerular capillaries.
• Nephron processes include filtration, reabsorption, and secretion.
• Filtrate travels through the proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT) before entering the collecting system.

Proximal Convoluted Tubule (PCT) Function

• The PCT actively reabsorbs nutrients, proteins, and electrolytes from filtrate.
• Reabsorbed substances enter the peritubular fluid.

Nephron Loop (Loop of Henle) Function

• The descending limb is permeable to water, allowing reabsorption.
• The ascending limb is impermeable to water, actively pumping sodium and chloride ions out.

Distal Convoluted Tubule (DCT) Function

• The DCT actively secretes ions, toxins, and drugs.
• Reabsorbs sodium ions.
• The juxtaglomerular complex is located here; it releases renin and erythropoietin.

Collecting System Function

• The collecting system (collecting and papillary ducts) makes final urine adjustments.
• Reabsorption or secretion of water or various ions.

Urine Formation

• The primary purpose of urine production is excretion and elimination of metabolic waste, including urea, creatinine, and uric acid.
• Urine production is via filtration, reabsorption, and secretion.
• Glomerular filtration moves fluids from blood into Bowman's capsule due to blood pressure.
• Glomerular Filtration Rate (GFR) is the filtrate production rate per minute; affects kidney function.
• Declining filtration pressures trigger renin release by the juxtaglomerular complex, increasing blood volume/pressure.
• The PCT reabsorbs 60-70% of filtrate volume.
• Reabsorbs nutrients, sodium, ions, and water.

Regulation of Urine Production

• Local adjustments in glomerular filtration pressures occur through changes in afferent/efferent arteriole diameter.
• Hormones like angiotensin II, aldosterone, ADH, and ANP regulate kidney function.
• Sympathetic activation constricts afferent arterioles, decreasing GFR and filtrate production.
• Adjusting regional blood circulation also affects GFR.

Urine Transport and Storage

• Ureters carry urine from renal pelvis to bladder.
• Peristaltic contractions move urine.
• Urinary bladder is a distensible sac for urine storage.
• Internal urethral sphincter is involuntary.
• External urethral sphincter is voluntary; controls urination.
• Micturition reflex is triggered by stretch receptors in bladder wall.
• Coordinating the reflex with sphincter relaxation allows urination.

Fluid, Electrolyte, and Acid-Base Balance

• Maintaining ECF and ICF volume and composition is crucial.
• Three types of homeostasis involved: fluid, electrolyte, and acid–base balance.
• ICF holds approximately 60% of total body water.
• Water intake/loss balance through diet and metabolic processes.
• Fluid shifts occur between ICF/ECF as result of osmotic equilibrium
• Electrolyte balance is important to water balance and cell functioning
• Sodium gains and loss imbalances affect balance; potassium imbalances are less common but more dangerous.
• Sodium reabsorption is stimulated by aldosterone; potassium excretion is stimulated by aldosterone.
• Maintaining blood pH (7.35–7.45): crucial role of carbonic acid and bicarbonate ions.
• Buffer systems (proteins, carbonic acid–bicarbonate, phosphates) resist pH changes.
• Respiratory compensation (lungs) and renal compensation (kidneys) adjust pH by regulating CO2 levels and hydrogen ion secretion/bicarbonate reabsorption.

Effects of Aging on Urinary System

• Aging often impacts kidney functioning.
• Loss of nephrons, reduced GFR, reduced hormone sensitivity (ADH, aldosterone), micturition reflex problems, reduced water/mineral content, and fluid/electrolyte/acid-base balance disorders may develop.

Urinary System's Role in Waste Excretion

• The urinary system is a major component of the excretory system; also includes integumentary, respiratory, and digestive systems.