Urinary System

• The urinary system is responsible for maintaining the body's fluid and electrolyte balance, eliminating waste products, and regulating blood pressure, pH, and red blood cell production
• o Filtration: Removal of waste and excess substances from the blood.
• Reabsorption: Essential substances (e.g., water, glucose) are returned to the blood.
• o Secretion: The removal of additional waste from the blood into the urine.
• o Excretion: The elimination of waste as urine.
• Kidneys: Paired organs located in the lower back (retroperitoneal) that filter blood and produce urine
• Structure:

  ▪ Renal Cortex: Outer region of the kidney, contains glomeruli and renal tubules.

  ▪ Renal Medulla: Inner region of the kidney, contains renal pyramids and loops of Henle.

  ▪ Renal Pelvis: Central cavity where urine collects before moving to the ureters.
• o Nephron: The functional unit of the kidney, responsible for urine formation.
• Components of a Nephron:

  ▪ Renal Corpuscle: Includes the glomerulus (a capillary network) and Bowman’s capsule, where blood is filtered.

  ▪ Renal Tubules: Include the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting ducts. These structures reabsorb water and solutes and secrete waste.
• • Ureters: Tubes that carry urine from the kidneys to the urinary bladder.
• • Urinary Bladder: A muscular sac that stores urine until it is ready to be expelled.
• • Urethra: The duct that transports urine from the bladder to the outside of the body.
• Filtration:

  o Occurs in the glomerulus where blood pressure forces water and solutes (e.g., glucose, salts, urea) from the blood into Bowman’s capsule. This forms glomerular filtrate.

  o Large molecules (e.g., proteins, blood cells) are retained in the blood
• Reabsorption:

  o Occurs mainly in the proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT).

  o Water, glucose, amino acids, ions (Na⁺, K⁺, Cl⁻), and bicarbonate are reabsorbed into the blood.

  o The loop of Henle plays a critical role in creating a concentration gradient in the kidney that allows for water reabsorption in the collecting ducts.
• • Secretion: o In the DCT and collecting ducts, additional substances (e.g., hydrogen ions (H⁺), potassium (K⁺), ammonium (NH₄⁺), and drugs) are secreted from the blood into the filtrate.
• Excretion: o The final urine is collected in the renal pelvis and then transported via the ureters to the bladder for storage. o Urine consists of water, urea, creatinine, and other waste products.
• Antidiuretic Hormone (ADH): Released by the posterior pituitary in response to dehydration or high blood osmolarity. ADH increases water reabsorption in the collecting ducts.
• Aldosterone: Released from the adrenal cortex in response to low blood sodium levels or low blood volume. It increases sodium reabsorption in the distal convoluted tubule and collecting ducts, indirectly increasing water retention.
• Renin-Angiotensin-Aldosterone System (RAAS):

  o Renin is released by the kidneys in response to low blood pressure or sodium levels. It converts angiotensinogen to angiotensin I, which is converted to angiotensin II (in the lungs) and triggers aldosterone release, increasing sodium and water retention.
• Fluid and Electrolyte Balance: The kidneys maintain balance by regulating water, sodium, potassium, calcium, and other electrolytes.
• Acid-Base Balance: The kidneys help maintain blood pH by excreting hydrogen ions (H⁺) and reabsorbing bicarbonate (HCO₃⁻).
• Blood Pressure Regulation: The kidneys contribute to blood pressure regulation through the RAAS, ADH, and aldosterone pathways.
• Summary of Urinary Physiology

  • The kidneys filter blood to remove waste products and excess substances, producing urine.

  • Through processes like reabsorption, secretion, and filtration, the kidneys regulate the composition of the blood and urine.

  • Hormonal regulation, such as ADH, aldosterone, and the RAAS system, allows the kidneys to maintain fluid and electrolyte balance, blood pressure, and pH.

  • Disorders of the urinary system can lead to serious complications, affecting kidney function, fluid balance, and waste elimination