Kidney Anatomy and Glomerular Structure

Renal blood flow: renal artery -> segmental artery -> interlobar artery -> arcuate artery -> interlobular artery -> afferent arteriole -> glomerulus -> efferent arteriole -> vasa recta/peritubular capillaries -> venous outflow.

Cross Section of Kidney

Fluid Compartments

  • 60-40-20 rule
    • 60% water
    • 40% intracellular
    • 20% extracellular
  • Plasma can be measured with radiolabeled albumin
  • Extracellular volume can be measured with inulin or mannitol
  • Osmolality = 290-310 mOsm/L
  • In: 2500 mL via water/food
  • Out: 500-1500 mL via urine, 250 mL via stool, 600 mL insensible losses (which go up 100-150 mL for each 1o C body temp rise)

Urinary output

  • Infants (<1 yr): 2.0 mL/kg/hr
  • Children and adults: 0.1-1.0 mL/kg/hr
Vd Compartment Volume Drug Types
Low Blood/plasma 3.5L Large/charged molecules; plasma protein bound
Medium ECF 14L Small hydrophilic molecules
High All tissues including fat 35L Small lipophilic molecules, especially if bound to tissue protein

Filtration

  • Prostaglandins dilate afferent arteriole (PDA)
  • ACE inhibitors constrict efferent arteriole (ACE)
  • Juxtaglomerular cells secrete renin in response to ↓ renal blood pressure and ↑ sympathetic tone (β1 ).
  • Macula densa cells sense ↓ NaCl delivery to DCT -> ↑ renin release p efferent arteriole vasoconstriction -> ↑ GFR.
Filtration

Renin-Angiotensin-Aldosterone System

Renin Angiotensin Aldosterone System
Hormone Function
Renin Secreted by JG cells in response to ↓ renal arterial pressure, ↑ renal sympathetic discharge (β1 effect), and ↓ Na+ delivery to macula densa cells
AT II Helps maintain blood volume and blood pressure. Affects baroreceptor function; limits reflex bradycardia, which would normally accompany its pressor effects.
ANP, BNP Released from atria (ANP) and ventricles (BNP) in response to ↑ volume; may act as a “check” on renin-angiotensin-aldosterone system; relaxes vascular smooth muscle via cGMP -> ↑ GFR, ↓ renin. Dilates afferent arteriole, constricts efferent arteriole, promotes natriuresis.
ADH Primarily regulates osmolarity; also responds to low blood volume states.
Aldosterone Primarily regulates ECF volume and Na+ content; responds to low blood volume states. Responds to hyperkalemia by ↑ K+ excretion.

Hormones Acting on Kidney

Hormones Acting on Kidney

Electrolyte Disturbances

ELECTROLYTE LOW SERUM CONCENTRATION HIGH SERUM CONCENTRATION
Na+ Nausea and malaise, stupor, coma, seizures  Irritability, stupor, coma
K+ U waves and fattened T waves on ECG, arrhythmias, muscle cramps, spasm, weakness Wide QRS and peaked T waves on ECG, arrhythmias, muscle weakness
Ca2+ Tetany, seizures, QT prolongation, twitching (Chvostek sign), spasm (Trousseau sign) Stones (renal), bones (pain), groans (abdominal pain), thrones (↑ urinary frequency), psychiatric overtones (anxiety, altered mental status)
Mg2+ Tetany, torsades de pointes, hypokalemia, hypocalcemia (when [Mg2+] < 1.2 mg/dL) ↓ DTRs, lethargy, bradycardia, hypotension, cardiac arrest, hypocalcemia
PO4(3−) Bone loss, osteomalacia (adults), rickets (children) Renal stones, metastatic calcifications, hypocalcemia

Acidosis and Alkalosis

Acidosis and Alkalosis

Hourly Calculation (4/2/1 rule)

  1. First 10 kg: 4 cc/kg/hr
  2. Next 10 kg: 2 cc/kg/hr
  3. Remaining kg: 1 cc/kg/hr
  4. Add 1 to 3 to get total cc/hr for maintenance fluids
  • Shortcut: weight (kg) + 40 = cc/hr

For example, a 60 kg:

  1. 10 kg x 4 cc/kg/hr = 40 cc/hr
  2. 10 kg x 2 cc/kg/hr = 20 cc/hr
  3. 40 kg x 1 cc/kg/hr = 40 cc/hr
  4. 100 cc/hr

Daily Calculation

  1. First 10 kg: 100 cc/hr
  2. Next 10 kg: 50 cc/hr
  3. Remaining kg: 20 cc/hr
  4. Add 1 to 3 to calculate total daily maintenance fluids

For example, a 60 kg:

  1. 10 kg x 100 cc/hr = 1000 cc
  2. 10 kg x 50 cc/hr = 500 cc/hr
  3. 40 kg x 20 cc/hr = 800 cc/hr
  4. 2300 cc/day OR 95 cc/hr for 24 hours
  • Maintenance requirement increases by 10% for every 1⁰C fever
  • 50% of deficit should be corrected pre-op

Intravenous Fluids

Sodium Potassium Calcium Magnesium Chloride Bicarb Osmolarity
ECF 142 4 5 3 103 27 280-310
LR 130 4 3 — 109 28 273
D5-1/2NS 77 — — — 77 — 407

Crystalloid VS Colloid

  • During time of hemorrhage, lactated ringers or 0.9% normal saline is the fluid of choice for initial resuscitation. Given excessive hemorrhage (>20%), red blood cells should be replaced to maintain oxygen carrying capacity.
Crystalloid Colloid
Solutions of simple inorganic or organic salts that distribute to varying extents throughout the body water

  • Lactated Ringer’s
  • 0.9% normal saline (N/S)
  • 5% dextrose in water (D5W)
 Suspensions of protein or other complex organic particles; cannot diffuse across capillary membranes and remain trapped in intravascular space

  • Albumin
  • PRBC
  • Platelets
  • Plasma
Crystalloid VS Colloid