Organ Systems for the MCAT

Cardiovascular

• 4 chambers: right side → lungs (pulmonary), left side → body (systemic)
• SA node pacemaker → atrial contraction → AV node → ventricular contraction
• Hemoglobin: cooperative binding, Bohr effect (low pH → right shift → O₂ release)

Respiratory

• Negative pressure breathing (diaphragm contraction)
• Gas exchange in alveoli via partial pressure gradients

Renal

• Nephron: glomerulus → PCT → Loop of Henle → DCT → Collecting duct
• ADH: increases water reabsorption
• Aldosterone: increases Na⁺ reabsorption (RAAS)

Digestive

• Stomach: HCl, pepsin (protein digestion)
• Small intestine: pancreatic enzymes + bile salts
• Absorption via villi

Endocrine

• Hypothalamus → Pituitary → Target organs
• Insulin (↓glucose) vs. Glucagon (↑glucose)
• Thyroid (T3/T4: ↑metabolism), PTH (↑Ca²⁺)

Nervous

• CNS (brain + spinal cord) + PNS (somatic + autonomic)
• Sympathetic (fight/flight) vs. Parasympathetic (rest/digest)
• Action potential: resting (−70mV) → depolarization (Na⁺ in) → repolarization (K⁺ out)