What the Circulatory System Transports (And What It Doesn't)

Circulatory System: Cargo & Exclusions

The cardiovascular system is a closed loop transporting essential substances to and from cells. It does not transport undigested food.

What Blood Carries

• Oxygen (O₂): Bound to hemoglobin in erythrocytes; delivered from lungs to tissues
• Carbon dioxide (CO₂): Dissolved, as bicarbonate (HCO₃⁻), carbamino compounds; tissues → lungs
• Nutrients: Glucose, amino acids, fatty acids, vitamins, minerals (absorbed via hepatic portal vein → liver → systemic)
• Hormones: Endocrine signals (insulin, thyroid hormone, cortisol, etc.) to target organs
• Waste products: Urea, creatinine, uric acid → kidneys; bilirubin → liver → bile
• Immune cells & factors: Leukocytes, antibodies, complement, cytokines
• Heat: Redistributes metabolic heat; thermoregulation
• Clotting factors & platelets: Hemostasis

What It Does NOT Transport

• Undigested food: Large macromolecules (intact proteins, complex carbs, fiber) cannot cross intestinal epithelium; they remain in GI lumen → feces
• Digestive enzymes: Secreted into lumen, not blood (except trace leakage)
• Fecal matter / gut microbiota: Barrier function prevents translocation (sepsis if breached)

The Digestive–Circulatory Interface

Nutrients cross the intestinal mucosa (enterocytes) → enter capillaries (water-soluble) or lacteals (fats as chylomicrons) → portal vein → liver (first-pass metabolism) → systemic circulation. This is absorption, not transport of food per se.

Clinical Relevance

• Malabsorption: Nutrients fail to enter blood → deficiencies
• Sepsis: Gut barrier failure → bacteria/toxins enter blood
• Portosystemic shunt: Nutrients/toxins bypass liver