Transport in Humans — Study Notes (Chapter 6)

6.1 Introduction

• In multicellular organisms, each cell needs essential substances (nutrients and oxygen).
• There are billions of cells in humans, many far from the source of nutrients. Diffusion alone is inefficient for supplying cells with nutrients and removing wastes over large distances.
• A transport system is required to efficiently deliver essential substances to cells and remove waste products.
• The human circulatory system consists of:
  • Heart: a muscular pump that drives blood around the body.
  • Blood (transport medium): a fluid tissue that carries substances around the body.
  • Blood vessels: transport blood around the body; the main vessels are arteries, veins and capillaries.
• Blood has two main roles:
  • Transport: oxygen, nutrients (e.g. glucose, amino acids, fats, vitamins), hormones, carbon dioxide, urea, heat, plasma proteins, and other substances.
  • Defence: white blood cells and platelets help protect against infections and aid in clotting.
• Blood is a complex tissue containing:
  • Red blood cells (RBCs) – carry oxygen using haemoglobin.
  • Plasma – transports cells, ions, soluble food substances, hormones, carbon dioxide, urea, vitamins, plasma proteins.
  • White blood cells (WBCs) – phagocytosis and antibody formation, tissue rejection defence.
  • Platelets – clotting).

6.2 Double Circulation

• The human circulatory system is a double circulatory system: blood flows through the heart twice in one complete circuit.
• Two interconnected circuits:
  • Pulmonary circulation: carries blood between the heart and lungs.
  • Oxygenated blood moves from the lungs to the heart; deoxygenated blood moves from the heart to the lungs.
  • Gas exchange occurs in the lungs. Blood enters pulmonary circulation at low pressure, allowing ample time for oxygenation before returning to the heart.
  • Systemic circulation: carries blood between the heart and the rest of the body.
  • Oxygenated blood is pumped from the heart to the body; deoxygenated blood returns from the body to the heart.
  • Gas exchange occurs at body cells. Blood is pumped into systemic circulation at high pressure to rapidly distribute oxygen.
• Key recall: a tissue is made of similar cells working together for a function; blood is a complex tissue because it contains RBCs, WBCs and platelets suspended in plasma.

6.3 The Blood

• Blood is a fluid tissue because cells are suspended in plasma.
• Major components (as fraction of blood):
  • Plasma: 55%
  • Formed elements (cells): 45% which include RBCs, WBCs, and platelets.
• Blood fractions indicate that blood is a mixture (blood fractionation yields layers).
• Two main blood functions:
  • Transport: oxygen, nutrients (e.g. glucose, amino acids, fats, vitamins), waste products (e.g. carbon dioxide, urea), hormones, and heat.
  • Defence: protection against foreign bodies and infections.
• Plasma:
  • Pale yellow fluid; about 90% water; solvent for transported substances.
  • Functions:
  • Transport digested food (e.g., glucose) from the small intestine to body parts.
  • Remove carbon dioxide and other wastes (e.g., urea) produced by body cells.
  • Transport hormones (e.g., insulin).
• Red blood cells (RBCs):
  • Transport oxygen from lungs to other body parts.
  • Shape: circular and biconcave to increase surface area to volume for faster diffusion.
  • Contain haemoglobin (iron-containing pigment) that binds oxygen.
  • Lack a nucleus to maximize haemoglobin content and oxygen transport capacity.
  • Life span: approximately 120\;\text{days}.
  • Hb-O2 interaction: Hb + O2 ⇌ HbO2 (oxyhaemoglobin) – the bond to oxygen is reversible.
  • Colour: oxyhaemoglobin is bright red; deoxyhaemoglobin is darker; during loading, blood becomes brighter as Hb binds O2.
  • Carbon monoxide toxicity: CO can irreversibly bind to Fe2+ in haemoglobin, reducing oxygen transport and causing poisoning.
• White blood cells (WBCs):
  • Irregular shapes, colourless, with a nucleus; can move and squeeze through capillary walls.
  • More complex and fewer in number than RBCs (approx. RBC:WBC ratio ~ 700:1).
  • Life span: a few days.
  • Main functions:
  • Phagocytosis: engulf and digest foreign bodies.
  • Antibody production: defend against infections.
• Platelets:
  • Small fragments of bone marrow cells (about 2–4 μm in diameter).
  • Essential for blood clotting: form sticky plugs at wound sites and seal wounds to prevent excessive blood loss and entry of pathogens.
• Blood as a buffer system and temperature regulator:
  • Blood helps maintain pH within a narrow range: pH approximately 7.35\leq pH\leq 7.45\,.
  • Blood helps regulate body temperature due to water’s high heat capacity; normal body temperature ranges from 36.4^{\circ}\text{C} to 37.6^{\circ}\text{C}.

6.4 The Blood Vessels

• Types of vessels: Arteries, Veins, Capillaries.
• General roles:
  • Arteries carry blood away from the heart at high pressure.
  • Veins carry blood back to the heart at relatively low pressure.
  • Capillaries connect arteries to veins and facilitate material exchange with tissues.
• Extra information on vascular branching:
  • Arteries branch to form arterioles, which further divide into capillaries.
  • Capillaries connect to venules, which join to form veins.
• Structure and function relationships (Table 1 summarized):
  • Arteries: thick, elastic, muscular walls to withstand high pressure; small lumen; rapid blood flow; high pressure; no valves (except semi-lunar valves in pulmonary artery and aorta); high oxygen concentration (except in the pulmonary artery).
  • Veins: thinner, less elastic walls; larger lumen; slower blood flow; low pressure; valves to prevent backflow; generally low oxygen concentration (except in the pulmonary vein).
  • Capillaries: walls are one cell thick (no muscular wall); absent internal muscular layer; smallest lumen; slow flow; no valves; oxygen and nutrients diffuse from blood to tissues; carbon dioxide and wastes diffuse from tissues to blood.
• Capillary diffusion (between blood and tissue):
  • Capillary walls are one-cell thick and semi-permeable, allowing rapid diffusion.
  • Capillaries branch to provide a large surface area for exchange.
  • Diffusion directions:
  • Oxygen and digested nutrients (e.g., glucose) diffuse from blood to tissue fluids surrounding cells along the capillary concentration gradient.
  • Carbon dioxide and other wastes (e.g., urea) diffuse from tissue cells into tissue fluids and then into the blood to be carried away.
• Notes on cross-vascular structure (referenced figures and annex in the source):
  • Photomicrographs illustrate cross-sections and longitudinal sections of arteries, veins, arterioles, venules, and capillaries to show thickness differences and cellular composition.
  • The capillary wall is the thinnest and most permeable section, enabling efficient exchange.
  • The relative diameters of vessels differ: capillaries are smallest, arteries are larger than veins for a given external diameter, and veins have the largest lumen.
• Quick recap of vessel characteristics (as a compact reference):
  • Arteries: thick walls, small lumen, high flow speed, high pressure, no valves (except pulmonary/aortic valves), high O2 content (except in pulmonary artery).
  • Veins: thin walls, large lumen, slow flow, low pressure, valves present, low O2 content (except in pulmonary vein).
  • Capillaries: very thin walls (one cell thick), no muscular layer, smallest lumen, slow flow, no valves, site of gas/nutrient/waste exchange.

Key connections and implications

• The double-circuit arrangement (pulmonary and systemic) allows separate pressure regimes: low pressure in the lungs enables efficient gas exchange, while high pressure in systemic circulation ensures rapid distribution of O2 and nutrients to all tissues.
• The capillary network’s extensive branching and thin walls maximize exchange efficiency between blood and tissue fluids.
• The balance of blood components supports both transport and immune protection: plasma carries soluble substances; RBCs handle oxygen transport; WBCs provide defense; platelets enable rapid hemostasis.
• Practical and clinical relevance:
  • Carbon monoxide poisoning arises from high affinity of CO for Hb, reducing oxygen transport capacity.
  • Maintaining pH and temperature via blood buffers and plasma properties is essential for enzyme function and metabolic processes.
• Foundational link to other topics:
  • Understanding systemic and pulmonary gas exchange ties to respiration and cellular metabolism.
  • Blood vessel structure-function relationships relate to physiology of blood pressure and circulation control.