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2.6Unit 2 — Multicellular Organisms

Transport Systems in Animals — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 2 parts Lesson 6 of 7

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Section 1 of 2Heart anatomy and circulation path

  • The human heart is a muscular double pump composed of four chambers:
    • Right atrium: Receives deoxygenated blood from the body via the vena cava.
    • Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery.
    • Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein.
    • Left ventricle: Pumps oxygenated blood to the entire body via the aorta.
  • Key anatomical adaptations:
    • The left ventricular wall is much thicker and more muscular than the right ventricle because it must pump blood under high pressure around the entire systemic body circulation.
    • Heart valves (tricuspid, bicuspid, semi-lunar) prevent the backflow of blood, ensuring one-way forward circulation.
    • Coronary arteries supply the cardiac muscle tissue directly with oxygenated blood and glucose for continuous aerobic respiration.

Section 2 of 2Blood vessels and blood components

Blood VesselWall StructureLumen SizeBlood PressureValves Present?
ArteryThick, muscular walls with extensive elastic fibresNarrow lumenHigh pressure (pumped directly from heart)No valves (except semi-lunar at heart exits)
VeinThin muscular walls with minimal elastic tissueWide lumenLow pressure (returning towards heart)Valves present (prevent backflow of slow-moving blood)
CapillaryMicroscopic, walls only one cell thickExtremely narrow (single red blood cell fits)Decreasing pressureNo valves
  • Blood components:
    • Red blood cells (erythrocytes): Biconcave disc shape increases surface area-to-volume ratio; contain haemoglobin which binds oxygen reversibly to form oxyhaemoglobin; have no nucleus to maximise haemoglobin storage.
    • White blood cells (leukocytes): Part of the immune system:
      • Phagocytes: Carry out phagocytosis by engulfing, digesting, and destroying foreign pathogens with digestive enzymes.
      • Lymphocytes: Produce specific Y-shaped antibodies that bind to foreign antigens on pathogen surfaces.

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