Physio 2 Renal

• Fluids comprise ~55% of the human body on average. However, when discussing "average" body fluid compartments, we should realize that variations exist, depending on age, gender, and percentage of body fat
• Body fluid compartments

  • Intracellular Fluid
  • Extracellular Fluid

  
• Components of Extracellular Fluid

  • Interstitial Fluid
  • Blood Plasma
  • Other Body Fluids including: Lymph, CSF, Synovial fluid, Aqueous humor and Vitreous body in the eyes, Endolymph and Perilymph, Pleural, Pericardial and Peritoneal fluids

  
• Fluid Balance


  Water intake = Water output
• Dilute urine


  Excretion of excess water
• Concentrated urine


  Excretion of excess electrolytes
• Sources of Body Water Gain and Loss

  • Fluids Ingested
  • From Metabolism
  • Insensible Skin
  • Insensible Lungs
  • Sweat
  • Feces
  • Urine
• Thirst Center


  Located in the hypothalamus, which governs the urge to drink

  
• Components of the Renal system

  • Kidneys (2)
  • Ureters (2)
  • Bladder (1)
  • Urethra (1)

  
• Kidneys


  Paired, reddish, bean–shaped organs located between peritoneum and posterior wall of the abdomen (retroperitonal); Spinal level: T12-L3 (partially protected by 11th and 12th ribs)

  
• Functions of the Kidneys

  • Excretion of metabolic waste products and foreign chemicals
  • Secretion, metabolism, and excretion of hormones
  • Regulation of water and electrolyte balances
  • Regulation of body fluid osmolality and electrolyte concentrations
  • Regulation of arterial pressure
  • Regulation of acid-base balance
  • Gluconeogenesis

  
• Layers of tissue surrounding the Kidneys

  • Renal Capsule
  • Adipose Capsule
  • Renal Fascia

  
• Regions of the Kidney

  • Renal Cortex
  • Renal Medulla

  
• Nephrons


  Functional units of the kidney (1 million)

  
• Types of Nephrons


  • Cortical nephrons (80-85%)
  • Juxtamedullary nephrons (15-20%)
• Parts of a Nephron

  • Renal Corpuscle
  • Renal Tubule

  
• Components of the Renal Corpuscle

  • Glomerulus
  • Glomerular/Bowman's Capsule

  
• Components of the Renal Tubule

  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting ducts

  
• Parts of the Loop of Henle

  • Descending limb
  • Ascending limb (Thin and Thick)
• Glomerular Filtration


  Water and most solutes in blood plasma move through the wall of glomerular capillaries (filtered) into the glomerular capsule and renal tubule

  
• Tubular Reabsorption


  Filtered fluid in renal tubules is reabsorbed by the tubule cells, with about 99% of the filtered water and many useful solutes being reabsorbed

  
• Tubular Secretion


  Renal tubule and duct cells secrete/remove wastes, drugs, and excess ions, into the fluid

  
• Filtration Barriers


  • Glomerular endothelial cells
  • Basal lamina
  • Filtration Slits

  
• Glomerular Blood Hydrostatic Pressure


  Blood pressure in glomerular capillaries (55 mmHg), promotes filtration by forcing water and solutes through the filtration membrane

  
• Capsular Hydrostatic Pressure


  Pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule (back pressure ~15 mmHg), opposes filtration
• Blood Colloid Osmotic Pressure

  The presence of proteins such as albumin, globulins, and fibrinogen in blood plasma (30 mmHg), also opposes filtration
• Net Filtration Pressure


  The pressure of only 10 mmHg causes a normal amount of blood plasma (minus plasma proteins) to filter from the glomerulus into the capsular space
• Glomerular Filtration Rate


  The amount of filtrate formed in all the renal corpuscles of both kidneys each minute to maintain homeostasis (Males: 125 mL/min, Females: 105 mL/min)

  
• Severe blood loss


  Decreases mean arterial blood pressure and glomerular blood hydrostatic pressure, leading to decreased filtration
• Increased systemic blood pressure

  Increases net filtration pressure and GFR, but GFR is nearly constant when mean arterial blood pressure is between 80 and 180 mmHg
• Renin-Angiotensin-Aldosterone System (RAAS)


  Kidney releases renin, which acts on angiotensinogen from the liver to form Angiotensin I, which is then converted to Angiotensin II in the lungs and liver. Angiotensin II stimulates the adrenal gland to release aldosterone, which increases blood pressure.
• Mechanisms controlling GFR


  • Renal autoregulation
  • Neural regulation
  • Hormonal regulation

  
• Renal Autoregulation of GFR

  Myogenic mechanism: Increased blood pressure stretches the walls of the afferent arteriole, triggering contraction of smooth muscle cells to narrow the arteriole's lumen, decreasing renal blood flow and GFR

  
• Renal Autoregulation of GFR

  Tubuloglomerular feedback: Increased filtrate in the distal convoluted tubule is detected by the macula densa, which signals the afferent arteriole to constrict, decreasing renal blood flow and GFR

  
• Renal Autoregulation of GFR

  1. RAAS = ↑ BV = ↑ BP

  2. Kidney = renin

  3. Liver = angiotensinogen

  4. Renin + angiotensinogen = Angiotensin I

  5. Lungs + liver = ACE

  6. ACE + Angiotensin I = Angiotensin II

  7. High Angiotensin II = Adrenal gland creates aldosterone which increases BP
• Myogenic mechanism

  • Normalizes renal blood flow and GFR within seconds after a change in blood pressure; ↑ BP, the GRF ↑ = stretching of afferent arteriole walls triggers contraction of smooth muscle cells THUS narrows the arteriole's lumen= ↓ renal blood flow and ↓ GFR
• Tubuloglomerular feedback (slow)


  • Macula densa (part of renal tubules) provides feedback to the glomerulus; ↑ GFR due to ↑ BP lead to ↑ rate of flow and ↓ reabsorption; Macula densa cells: detect the increased delivery of Na, Cl, and water and to inhibit release of nitric oxide (vasodilator)

  
• Neural Regulation of GFR

  • Sympathetic ANS release: Norepinephrine vasoconstricts (causing ↓ blood flow to glomerular capillaries and GFR); Reduces urine output; Permits greater blood flow to other body tissues

  
• Hormonal Regulation of GFR

  • Angiotensin II --> constricts (reduces/increases GFR); potent vasoconstrictor (↓ renal blood flow, ↓ GFR)
  • Atrial natriuretic peptide --> dilates (reduces/increases GFR); stretching of the atria, stimulates secretion of ANP; ↑ the capillary surface area available for filtration by causing relaxation of the glomerular mesangial cells (contractile cells). Glomerular filtration rate rises as the surface area increases

  
• pH: 7.35 - 7.45