Renal System

• Kidney Diagram

  
• Kidney

  Kidney:

  - Posterior abdominal organ

  - Surrounded and held in place by fat

  
  

  
  

  Capsule/Cortex:

  - Outer layer of the kidney

  - Protects and supports the kidney structure

  
  

  Medulla:

  - Inner region

  - Contains renal pyramids, responsible for urine concentration

  
  

  Renal Pelvis:

  - Collects urine from the kidney and channels it to the ureter

  
• Microscopic structure

  Proximal Convoluted Tubule (PCT)

  - Lined with cuboidal epithelial cells

  - Cells have a brush border (microvilli) to increase surface area for reabsorption

  - Numerous mitochondria provide energy for active transport

  - Tight junctions between cells to maintain selective permeability

  - Arterioles: Afferent arteriole is larger than efferent arteriole to create pressure for filtration

  - Glomerular walls: Single cell thick with fenestrations (small gaps) to allow filtration of blood

  
  

  Loop of Henle

  - Descending limb: Thin, permeable to water, lined with simple squamous epithelium

  - Ascending limb: Thick, impermeable to water, active transport of Na+ and Cl-, lined with cuboidal to low columnar cells

  - Establishes osmotic gradient in the medulla for water reabsorption

  
  

  Distal Convoluted Tubule (DCT)

  - Lined with simple cuboidal epithelial cells

  - Fewer microvilli compared to PCT

  - Mitochondria present for active transport

  - Important for sodium, potassium, and pH regulation

  - Involved in selective reabsorption and secretion

  
  

  20% Cardiac Output

  - Kidneys receive approximately 20% of the body's cardiac output to filter blood and maintain homeostasis
• Nephron

  Basic Unit of Kidney:

  - The nephron is the basic structural and functional unit of the kidney.

  
  

  Number:

  - Each human kidney contains about one million nephrons.

  
  

  Functions:

  - Responsible for filtering blood, removing waste products, and regulating fluid and electrolyte balance.

  
  

  Components:

  - Glomerulus:

  A network of capillaries where blood filtration begins.

  
  

  - Bowman's Capsule:

  Surrounds the glomerulus and collects the filtered blood (glomerular filtrate).

  
  

  - Proximal Convoluted Tubule (PCT):

  Reabsorbs water, ions, and nutrients from the filtrate back into the bloodstream.

  
  

  - Loop of Henle:

  Establishes a concentration gradient in the medulla, aiding in water reabsorption.

  
  

  - Distal Convoluted Tubule (DCT):

  Further regulates electrolyte balance and pH.

  
  

  - Collecting Duct:

  Collects urine from multiple nephrons and transports it to the renal pelvis.

  
  

  - Blood Supply:

  Receives blood from the renal artery, with filtered blood returning to circulation via the renal vein.

  
  

  - Filtration Process:

  Blood is filtered through the glomerulus, and essential substances are reabsorbed along the tubules while waste products are excreted as urine.

  
  

  - Regulation:

  Hormones such as antidiuretic hormone (ADH) and aldosterone regulate nephron function to maintain homeostasis.

  
• Formation of urine: Filtration

  Filtration:

  - Occurs in the glomerulus of the nephron in the kidney.

  - Blood pressure forces water, ions, glucose, amino acids, and waste products (like urea) out of the blood and into Bowman's capsule.

  - Large molecules (e.g., proteins) and blood cells remain in the bloodstream.

  
• Formation of urine: Selective Reabsorption

  Selective Reabsorption:

  - Occurs mainly in the proximal convoluted tubule due to microvilli.

  - Mostly passive, but some active reabsorption e.g. glucose.

  - Essential substances like glucose, certain ions, and water are reabsorbed back into the bloodstream. E.g. H+ ions exchange to maintain blood pH.

  - 30-40% Absorbed back, 80-85% of water and salt reabsorbed.

  - Transported via passive and active transport mechanisms.

  - Larger molecules cleared from peritubular capillaries

  - Helps maintain the body's fluid and electrolyte balance.

  
• Formation of urine: Secretion

  Secretion:

  - Occurs in the distal convoluted tubule and collecting duct.

  - Additional waste products and excess ions are actively transported from the blood into the nephron.

  - Includes substances like hydrogen ions, potassium ions, and certain drugs.

  - Final adjustments are made to the urine composition before it is excreted.

  
• Water balance

  • Intake and output of fluid balanced by the kidneys

  
  

  • Anti-diuretic hormone - Increases water reabsorption

  
  

  • Aldosterone - Increases reabsorption sodium and water, and excretion of potassium

  
  

  • Osmoreceptors in the hypothalamus

  
• Ureters

  Function:

  - Transport urine from kidneys to the bladder.

  
  

  Structure:

  - Tubes about 25-30 cm long and 3-4 mm in diameter.

  
  

  Location:

  - Extend from the renal pelvis of each kidney to the bladder.

  
  

  Layers:

  - Composed of three layers - mucosa (inner lining), muscularis (smooth muscle), and adventitia (outer connective tissue).

  
  

  Peristalsis:

  - Muscular contractions propel urine towards the bladder.
• Bladder


  Function:

  - Stores urine until it is excreted from the body.

  
  

  Capacity:

  - Can hold approximately 400-600 ml of urine in adults.

  
  

  Structure:

  - Hollow, muscular organ with a flexible, stretchy wall.

  
  

  Location:

  - Situated in the pelvic cavity, behind the pubic bone.

  
• Urethra


  - Tube-like structure in the urinary system.

  
  

  - Functions to carry urine from the bladder to outside the body.

  
  

  - Different lengths in males and females:

  - In males, longer due to passing through the penis.

  - In females, shorter and opens above the vaginal opening.

  
  

  - Sphincter muscles control the opening and closing of the urethra.

  
  

  - Important in both urinary and reproductive systems.

  
• Micturition


  - The process of urination or the expulsion of urine from the urinary bladder.

  
  

  - It is a complex reflex action involving both voluntary and involuntary muscle contractions.

  
  

  - Key organs involved:

  - urinary bladder

  - ureters

  - urethra

  - & associated nervous system structures.

  
  

  - Process begins with the filling of the bladder with urine from the kidneys via the ureters.

  
  

  - Stretch receptors in the bladder wall detect the filling and send signals to the spinal cord.

  
  

  - Nervous system coordinates relaxation of the internal urethral sphincter (involuntary) and contraction of the detrusor muscle (voluntary).

  
  

  - External urethral sphincter (voluntary control) relaxes voluntarily to allow urine to pass out through the urethra.

  
  

  - Regulated by the micturition centre in the brainstem, which coordinates the entire process based on bladder filling and conscious control.