Transport

Overview

Blood is a specialized transport tissue that carries oxygen, nutrients, hormones, and waste products around the body. Blood vessels form a closed circulatory system with arteries, veins, and capillaries, each adapted for specific functions. Understanding blood components and vessel structures is essential for explaining circulation, gas exchange, and cardiovascular health.

Components of Blood

Blood consists of plasma (liquid) and blood cells (solid components).

1. Plasma (55% of blood volume)

• Pale yellow liquid
• Mostly water (90%) with dissolved substances
• Transports: nutrients (glucose, amino acids, fats), hormones, antibodies, urea, carbon dioxide (as hydrogen carbonate ions), heat
• Role in maintaining body temperature and pH

2. Red Blood Cells (Erythrocytes)

• **Structure**: Biconcave disc shape (flattened with depression on each side), no nucleus, contains hemoglobin
• **Adaptations**:
• Biconcave shape increases surface area for oxygen absorption
• No nucleus provides more space for hemoglobin
• Flexible membrane allows squeezing through narrow capillaries
• Contains hemoglobin protein with iron to bind oxygen
• **Function**: Transport oxygen from lungs to respiring tissues
• In lungs: Hemoglobin + Oxygen → Oxyhemoglobin (bright red)
• In tissues: Oxyhemoglobin → Hemoglobin + Oxygen (darker red)
• Lifespan: approximately 120 days, made in bone marrow

3. White Blood Cells (Leukocytes)

• **Structure**: Larger than red cells, have nucleus, can change shape
• **Types and functions**:
• **Phagocytes**: Engulf and digest pathogens (phagocytosis), non-specific immune response
• **Lymphocytes**: Produce antibodies specific to antigens on pathogens, specific immune response
• **Adaptations**: Flexible membrane allows movement out of blood vessels into tissues, can change shape to engulf bacteria
• **Function**: Fight infection and disease as part of immune system
• Much fewer than red blood cells (about 1:600 ratio)

4. Platelets (Thrombocytes)

• **Structure**: Small cell fragments, no nucleus
• **Function**: Blood clotting
• At wound site, platelets clump together
• Release enzymes that convert fibrinogen (soluble protein) to fibrin (insoluble threads)
• Fibrin forms mesh trapping red blood cells, forming clot/scab
• Prevents blood loss and pathogen entry
• Produced in bone marrow

Types of Blood Vessels

ARTERIES

Structure:

• Thick muscular walls with elastic tissue
• Small lumen (central cavity)
• No valves (except at heart)
• Layered structure: outer fibrous coat, thick muscle/elastic layer, smooth endothelium lining

Function:

• Carry blood **away from heart** to organs
• Carry blood under **high pressure** (due to heart pumping)
• Carry **oxygenated blood** (except pulmonary artery)

Adaptations:

• Thick muscle wall withstands high pressure
• Elastic tissue stretches and recoils, maintaining smooth blood flow and pressure between heartbeats (pulse)
• Smooth lining reduces friction

VEINS

Structure:

• Thin muscular walls with little elastic tissue
• Large lumen
• Contain valves
• Similar layered structure but much thinner muscle layer

Function:

• Carry blood **toward heart** from organs
• Carry blood under **low pressure**
• Carry **deoxygenated blood** (except pulmonary vein)

Adaptations:

• Large lumen reduces resistance to blood flow
• Thin walls sufficient for low pressure
• Valves prevent backflow of blood (ensure one-way flow toward heart)
• Blood moved by: surrounding muscle contraction (squeezes veins), breathing movements (pressure changes), residual pressure from heartbeat

CAPILLARIES

Structure:

• Walls one cell thick (single layer of endothelium)
• Tiny lumen (barely wide enough for red blood cells)
• No muscle or elastic tissue
• Form extensive networks in tissues

Function:

• Link arteries to veins
• Site of **exchange** between blood and tissues
• Allow passage of oxygen, nutrients, carbon dioxide, waste products

Adaptations:

• Very thin walls (one cell thick) provide short diffusion distance for efficient exchange
• Permeable walls allow substances to pass through
• Large total surface area for exchange
• Narrow lumen brings blood close to cells
• Extensive branching networks reach all cells

Blood Vessel Comparison Table

| Feature | Artery | Vein | Capillary |

|---------|--------|------|-----------|

| Direction | Away from heart | Toward heart | Connect arteries to veins |

| Pressure | High | Low | Very low |

| Lumen size | Small | Large | Very small |

| Wall thickness | Thick | Thin | One cell thick |

| Muscle/elastic | Lots | Little | None |

| Valves | No (except at heart) | Yes | No |

| Oxygenation | Usually oxygenated | Usually deoxygenated | Changes along length |

Circulation Pathways

Blood flows through two circuits:

Pulmonary Circulation:

Heart → Pulmonary artery (deoxygenated) → Lungs → Pulmonary vein (oxygenated) → Heart

Systemic Circulation:

Heart → Aorta → Arteries → Capillaries (body organs) → Veins → Vena cava → Heart

Key arteries and veins:

• **Aorta**: Main artery leaving heart to body
• **Pulmonary artery**: Carries deoxygenated blood to lungs (only artery with deoxygenated blood)
• **Pulmonary vein**: Carries oxygenated blood from lungs (only vein with oxygenated blood)
• **Vena cava**: Main vein returning blood to heart from body
• **Coronary arteries**: Supply heart muscle with oxygen and glucose

Blood Clotting Process

1. Injury damages blood vessel
2. Platelets arrive and stick to damaged area
3. Platelets release clotting factors (enzymes)
4. Clotting factors convert fibrinogen (soluble) to fibrin (insoluble)
5. Fibrin forms mesh of threads
6. Red blood cells trapped in mesh
7. Clot forms, dries to become scab
8. Prevents blood loss and pathogen entry

Importance of Transport

Blood transports:

• **To cells**: Oxygen (for respiration), glucose (for energy), amino acids (for protein synthesis), vitamins, minerals, hormones (chemical messengers), heat (for temperature regulation)
• **From cells**: Carbon dioxide (to lungs for removal), urea (to kidneys for removal), heat (from respiring tissues)

Exam Tips

• Learn structure-function relationships: thick artery walls for high pressure, thin capillary walls for exchange
• Remember exceptions: pulmonary artery carries deoxygenated blood, pulmonary vein carries oxygenated blood
• Draw clear labeled diagrams showing lumen, wall thickness, valves
• Explain adaptations using key words: surface area, diffusion distance, pressure, exchange
• Distinguish plasma (liquid) from platelets (cell fragments) - different spelling, different function
• For blood cells, link structure directly to function (e.g., no nucleus in red cells = more hemoglobin space)
• Don't confuse arteries with veins - use mnemonic "arteries away" from heart
• Practice comparing all three vessel types in a table format