The Urinary System: Anatomy and Function

• The urinary system, also known as the renal system or urinary tract, is responsible for filtering blood and eliminating waste products from the body in the form of urine.

Components of the Urinary System

• Kidneys: These are bean-shaped organs located in the abdominal cavity responsible for filtering blood and producing urine.
• Ureters: These are tubes that carry urine from the kidneys to the bladder.
• Urinary bladder: This is a muscular sac that stores urine until it is excreted.
• Urethra: This is the tube that carries urine from the bladder to the outside of the body.

Kidney Anatomy

• Renal capsule: A tough fibrous layer surrounding the kidney.
• Renal cortex: The outer region of the kidney, containing nephrons.
• Renal medulla: The inner region of the kidney, containing renal pyramids and collecting ducts.
• Renal pyramids: Cone-shaped structures within the medulla, containing tubules that transport urine.
• Renal columns: Inward extensions of the cortex that separate the renal pyramids.
• Renal papilla: The tip of the renal pyramid, where urine drains into the minor calyx.
• Minor calyx: A cup-shaped structure that collects urine from the renal papilla.
• Major calyx: Formed by the fusion of minor calyces, and drains urine into the renal pelvis.
• Renal pelvis: A funnel-shaped structure that collects urine from the major calyces and drains it into the ureter.
• Hilum: A concave fissure on the medial side of the kidney, where the renal artery, renal vein, and ureter enter and exit.

Nephron Anatomy

• The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine.
• Renal corpuscle: The initial filtering component of the nephron.
• Glomerulus: A network of capillaries within the renal corpuscle where filtration occurs.
• Bowman's capsule: A cup-shaped structure surrounding the glomerulus that collects filtrate.
• Renal tubule: A long, winding tube that modifies the filtrate to form urine.
• Proximal convoluted tubule (PCT): The first segment of the renal tubule, responsible for reabsorbing water, ions, and nutrients from the filtrate.
• Loop of Henle: A U-shaped segment of the renal tubule, consisting of a descending limb and an ascending limb, responsible for establishing a concentration gradient in the medulla.
• Descending limb: Permeable to water but not to ions, allowing water to be reabsorbed into the medulla.
• Ascending limb: Permeable to ions but not to water, allowing ions to be reabsorbed into the medulla.
• Distal convoluted tubule (DCT): The final segment of the renal tubule, responsible for further reabsorption of water and ions under hormonal control.
• Collecting duct: A tube that collects urine from multiple nephrons and transports it to the renal pelvis.

Blood Supply to the Kidneys

• Renal artery: Supplies blood to the kidney.
• Afferent arteriole: Carries blood to the glomerulus.
• Glomerular capillaries: Where filtration occurs.
• Efferent arteriole: Carries blood away from the glomerulus.
• Peritubular capillaries: Surround the renal tubules and reabsorb water and solutes from the filtrate.
• Vasa recta: Specialized peritubular capillaries that run alongside the loop of Henle and help maintain the concentration gradient in the medulla.
• Renal vein: Drains blood from the kidney.

Urine Formation

• Glomerular filtration: Blood pressure forces water and small solutes from the glomerulus into Bowman's capsule, forming filtrate.
• Tubular reabsorption: Useful substances, such as water, glucose, amino acids, and ions, are reabsorbed from the filtrate back into the blood.
• Tubular secretion: Waste products and excess ions are secreted from the blood into the filtrate.
• Concentration of urine: The loop of Henle and collecting duct work together to concentrate urine by reabsorbing water and secreting or reabsorbing solutes.

Glomerular Filtration Rate (GFR)

• GFR is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time.
• GFR is an important indicator of kidney function.
• Normal GFR is approximately 125 mL/min.
• GFR is regulated by several factors, including blood pressure, afferent and efferent arteriolar tone, and hormones.

Regulation of GFR

• Autoregulation: The kidneys can maintain a relatively constant GFR despite changes in blood pressure.
• Hormonal regulation: Hormones such as angiotensin II and atrial natriuretic peptide (ANP) can affect GFR.
• Neural regulation: Sympathetic nervous system activation can decrease GFR.

Tubular Reabsorption

• Reabsorption in the proximal convoluted tubule (PCT): About 65% of the filtered water, sodium, potassium, and chloride are reabsorbed in the PCT.
• Glucose and amino acids are completely reabsorbed in the PCT under normal conditions.
• Bicarbonate is reabsorbed in the PCT to maintain acid-base balance.
• Reabsorption in the loop of Henle: Water is reabsorbed in the descending limb, while sodium, potassium, and chloride are reabsorbed in the ascending limb.
• Reabsorption in the distal convoluted tubule (DCT) and collecting duct: Reabsorption of water and ions is regulated by hormones such as antidiuretic hormone (ADH) and aldosterone.

Tubular Secretion

• Secretion in the proximal convoluted tubule (PCT): Waste products such as creatinine, urea, and drugs are secreted into the filtrate.
• Secretion in the distal convoluted tubule (DCT) and collecting duct: Potassium and hydrogen ions are secreted to regulate electrolyte and acid-base balance.

Concentration of Urine

• The loop of Henle creates a concentration gradient in the medulla, with higher solute concentration deeper in the medulla.
• Antidiuretic hormone (ADH) increases the permeability of the collecting duct to water, allowing more water to be reabsorbed and concentrating the urine.
• In the absence of ADH, the collecting duct is impermeable to water, resulting in dilute urine.

Urine Composition

• Water: Makes up about 95% of urine.
• Electrolytes: सोडियम, पोटेशियम, क्लोराइड, कैल्शियम, मैग्नीशियम और अन्य आयन शामिल हैं।
• Waste products: Urea, creatinine, uric acid, and other metabolic waste products.
• Hormones: Small amounts of hormones may be excreted in urine.

Ureters

• The ureters are tubes that carry urine from the kidneys to the bladder.
• The ureters have smooth muscle walls that contract to propel urine towards the bladder.

Urinary Bladder

• The urinary bladder is a muscular sac that stores urine until it is excreted.
• The bladder can hold up to 500-800 mL of urine.
• The bladder wall contains smooth muscle called the detrusor muscle, which contracts to expel urine during urination.
• Internal urethral sphincter: Smooth muscle sphincter at the junction of the bladder and urethra that prevents urine from leaking out.
• External urethral sphincter: Skeletal muscle sphincter below the internal urethral sphincter that allows for voluntary control of urination.

Urethra

• The urethra is the tube that carries urine from the bladder to the outside of the body.
• The female urethra is shorter than the male urethra.
• In males, the urethra also carries semen during ejaculation.

Micturition

• Micturition, also known as urination or voiding, is the process of emptying the bladder.
• When the bladder fills with urine, stretch receptors in the bladder wall send signals to the brain, creating the urge to urinate.
• Voluntary control of the external urethral sphincter allows for urination to be initiated at will.
• During urination, the detrusor muscle contracts, the internal urethral sphincter relaxes, and the external urethral sphincter relaxes, allowing urine to flow out of the body.

Functions of the Urinary System

• Filtration of blood and removal of waste products.
• Regulation of blood volume and blood pressure.
• Regulation of electrolyte balance.
• Regulation of acid-base balance.
• Production of hormones such as erythropoietin and renin.
• Activation of vitamin D.