Urinary system pt 1

Overview of the Urinary System Structures

• The Urinary System, also known as the organs of excretion, is composed of a pair of kidneys and the urinary tract.

• Kidney Functions: The kidneys filter blood to remove metabolic waste products and modify the resulting fluid for several critical purposes:

  • Maintenance of fluid and electrolyte homeostasis.

  • Maintenance of acid-base homeostasis.

  • Maintenance of blood pressure homeostasis.

  • Influence on blood pressure.

  • Release of the hormone erythropoietin ($EPO$), which stimulates red blood cell production.

  • Detoxifying substances in the blood.

  • Activation of Vitamin D (calcitriol), which is essential for the absorption of calcium ($Ca$).

• Urinary Tract Structure: Composed of a pair of ureters, a single urinary bladder, and a single urethra.

  • Ureters: Urine exits the kidneys through these tubes located on the posterior body wall.

  • Urinary Bladder: Found on the floor of the pelvic cavity, each ureter empties into this organ where urine is stored.

  • Urethra: Allows urine to exit the body from the urinary bladder.

Anatomy of the Kidneys

• Physical Characteristics: Kidneys are bean-shaped and bean-colored.

• Location:

  • Both kidneys are retroperitoneal, meaning they are found outside and posterior to the peritoneal membrane.

  • The right kidney is positioned slightly more inferiorly than the left kidney due to the presence of the liver.

  • The left kidney is positioned between the $T12$ and $L3$ vertebrae using the vertebral column as a reference.

  • The $11th$ and $12th$ ribs provide partial protection for both kidneys.

  • The Adrenal Gland, a component of the endocrine system, is located on the superior pole of each kidney.

• External Layers (Deep to Superficial):

  1. Renal Capsule: A thin layer of dense irregular connective tissue covering the exterior of each kidney.

  2. Adipose Capsule: A fatty layer that protects the kidneys from physical trauma.

  3. Renal Fascia: A layer of dense irregular connective tissue that anchors each kidney to the peritoneum and the musculature of the posterior abdominal wall.

• Hilum: An opening on the medial surface of the kidney where the renal artery, renal vein, nerves, and ureters enter and exit.

• Internal Structure:

  • Renal Cortex: The outer region of the kidney. Approximately $90\text{--}95\%$ of all kidney blood vessels are found here.

  • Renal Medulla: The inner region of the kidney.

  • Renal Columns: Extensions of the renal cortex that pass through the renal medulla toward the renal pelvis.

  • Renal Pyramids: Cone-shaped structures within the renal medulla, separated by renal columns.

  • Renal Papilla: The slender, tapered end of each renal pyramid.

  • Drainage Pathway: Papilla $→$ Minor calyx $→$ Major calyx $→$ Renal pelvis $→$ Ureter.

  • Smooth muscle tissue within the walls of the calyces and renal pelvis contracts to propel urine toward the ureter.

Microscopic Anatomy: The Nephron

• Over one million nephrons are located within the cortex and medulla of each kidney.

• Components of the Nephron:

  • Renal Corpuscle: Responsible for filtering blood. It is found in the renal cortex.

  • Glomerulus: A group of looping fenestrated capillaries.

  • Glomerular Capsule (Bowman’s Capsule): Consists of an outer parietal layer and an inner visceral layer. The Capsular Space is the hollow region between these layers.

  • Podocytes: Cells in the visceral layer that have foot processes called pedicels, creating filtration slits.

  • Renal Tubule: Continuous with the glomerular capsule; mostly found in the cortex, but some parts dip into the medulla. It consists of:

  • Proximal Tubule (PCT): Primary site for reabsorption and some secretion. It features a brush border of microvilli to increase surface area and is composed of simple cuboidal epithelium.

  • Nephron Loop (Loop of Henle): Dips into the medulla and consists of a descending limb (simple squamous epithelium in thin parts) and an ascending limb (simple cuboidal epithelium in thick parts). It is involved in reabsorption.

  • Distal Tubule (DCT): Involved in reabsorption and secretion. It consists of simple cuboidal epithelium with very few microvilli.

• Juxtaglomerular Apparatus (JGA):

  • Composed of the macula densa (a group of cells in the distal tubule) and juxtaglomerular (JG) cells (modified smooth muscle cells in the afferent arteriole).

  • Functions to regulate blood pressure ($BP$) and the glomerular filtration rate ($GFR$).

  • If blood pressure is too low, JG cells release renin, activating the Renin-Angiotensin-Aldosterone System (RAAS) to increase blood pressure.

• Collecting System: Modified filtrate passes through the cortical collecting duct, medullary collecting duct, and finally the papillary duct. Once filtrate enters the papillary duct, it is officially known as urine.

• Types of Nephrons:

  • Cortical Nephrons ($80\%$): Renal corpuscles are in the outer renal cortex; they have short loops that barely enter the medulla.

  • Juxtamedullary Nephrons: Renal corpuscles are near the cortex-medulla boundary; they have long loops that travel deep into the medulla. These are served by the vasa recta.

Blood Supply of the Kidneys

• Arterial Pathway: Left and right renal arteries branch from the abdominal aorta.

  • 1. Renal artery $→$ 2. Segmental artery $→$ 3. Interlobar artery $→$ 4. Arcuate artery $→$ 5. Interlobular (cortical radiate) artery.

• Capillary System (Unusual system where arterioles both feed and drain capillaries):

  • 6. Afferent arteriole $→$ 7. Glomerulus $→$ 8. Efferent arteriole $→$ 9. Peritubular capillaries (which surround the renal tubule).

• Venous Pathway: Exits the kidney parallel to the arterial pathway.

  10. Interlobular veins $→$ 11. Arcuate veins $→$ 12. Interlobar vein $→$ 13. Renal vein.

  • The renal vein exits the kidney from the hilum to drain into the inferior vena cava.

Renal Physiology Processes

• Glomerular Filtration: Selectively based on size. Smaller substances exit the blood at the glomerulus to enter the capsular space as filtrate, while cells and large proteins (like albumin) stay in the blood.

• Tubular Reabsorption: Reclaiming water, glucose, amino acids, and electrolytes from the tubular fluid and returning them to the circulating blood.

• Tubular Secretion: Substances are added to the filtrate from the peritubular capillaries. This helps maintain electrolyte and acid-base homeostasis and removes toxins.