Transport in Humans – Comprehensive Study Notes

Learning Outcomes

• By the end of this chapter you should be able to:
• Identify the principal blood vessels entering and leaving the heart, lungs, liver and kidneys.
• Relate structural adaptations of arteries, veins and capillaries to their respective functions (names of individual muscle layers not required).
• Explain how materials move between capillaries and tissue fluid.
• Describe the components of blood and their roles in transport and defence:
• Red blood cells – haemoglobin-based carriage of oxygen.
• Plasma – transport medium for blood cells, ions, soluble food molecules, hormones, \text{CO}_2, urea, vitamins and plasma proteins.
• White blood cells – phagocytosis, antibody production, initiation of tissue-rejection responses.
• Platelets – initiate clotting by converting fibrinogen to fibrin.
• List ABO blood groups and predict all donor/recipient compatibility permutations.
• Describe heart structure in terms of muscular contraction and valve action.
• Outline the cardiac cycle (systole and diastole). Microscopic histology and nerve names are not required.
• Explain coronary heart disease, its causes (unhealthy diet, sedentary lifestyle, smoking) and preventive measures.

Why Multicellular Organisms Need a Transport System

• Multicellular organisms contain vast numbers of cells; diffusion alone is too slow for long-distance delivery of nutrients and removal of wastes.
• A specialised transport system guarantees every cell continuous access to required substances and rapid waste disposal.
• In humans the system consists of blood, blood vessels and the heart.
• Mammals possess a double circulatory system in which blood passes through the heart twice per complete circuit.

THINKING TIME – Amoeba

• Amoebae are unicellular and only micrometres in diameter.
• Their surface-area-to-volume ratio is high; diffusion across the plasma membrane suffices for gas exchange, nutrient uptake and waste removal.
• Hence a specialised transport system is unnecessary.

Components of Blood

Plasma

• \approx 90\% water plus dissolved proteins (albumin, globulins, fibrinogen), ions, glucose, amino acids, hormones, urea, vitamins and \text{CO}_2.
• Functions as the liquid transport medium for cells and solutes.

Red Blood Cells (Erythrocytes)

• Contain the pigment haemoglobin (Hb) which reversibly binds oxygen to form oxyhaemoglobin.
• Biconcave disc shape ➔ large surface-area-to-volume ratio for rapid \text{O}2 exchange. • Lack nucleus and most organelles ➔ more internal space for haemoglobin (≈ 280\,\text{million} Hb molecules per cell). • Highly elastic membrane allows bell-shaped deformation to squeeze through narrow capillaries. • Function: transport \text{O}2 from lungs to tissues and assist \text{CO}_2 transport indirectly.

White Blood Cells (Leucocytes)

• Larger than RBCs; possess nuclei.
• Two major sub-types:
• Lymphocytes – large round nucleus, produce antibodies that:
• Recognise specific antigens on foreign particles.
• Agglutinate bacteria for easier phagocytosis.
• Neutralise bacterial toxins.
• Phagocytes – lobed nucleus, flexible cytoplasm; engulf and digest pathogens (phagocytosis).
• Overall role: immunity and defence.

Platelets (Thrombocytes)

• Cytoplasmic fragments lacking nuclei.
• Contain enzymes that convert soluble fibrinogen to insoluble fibrin threads at wound sites.
• Fibrin mesh traps blood cells forming a clot/scab, preventing blood loss and microbial entry.

Organ Transplant & Tissue Rejection

• Donor tissue may be recognised as foreign ➔ lymphocyte-mediated immune response ➔ rejection.
• Minimised by selecting genetically similar donors (ideally close relatives) and administering immunosuppressive drugs to recipient.

Blood Groups & Transfusions

• Early transfusions caused fatal agglutination of donor RBCs.
• RBC membranes possess antigens; plasma contains naturally occurring antibodies.
• Antibodies are always present; they will react with unfamiliar antigens, causing clumping (agglutination).

Blood Type

RBC Antigen(s)

Plasma Antibody(ies)

A

A

Anti-B

B

B

Anti-A

AB

A & B

None

O

None

Anti-A & Anti-B

• Critical rule: consider the effect of recipient’s antibodies on donor’s antigens.
• Universal donor: Type O (no antigens on RBCs ➔ recipient antibodies have nothing to attack).
• Universal recipient: Type AB (no antibodies in plasma ➔ will not attack any donor RBC antigens).

Blood Vessels

Major Types & Functional Features

• Arteries – carry blood away from heart (except pulmonary artery). High pressure; thick, elastic muscular walls; small lumen relative to diameter; no valves (pressure prevents backflow).
• Veins – carry blood toward heart (except pulmonary vein). Low pressure; thinner walls with less muscle/elastic tissue; large lumen; valves present to prevent backflow; flow aided by skeletal-muscle contraction.
• Capillaries – microscopic; wall is single layer of endothelial cells; lumen just wide enough for single RBC; partially permeable; form extensive branching networks between arterioles and venules; site of exchange.

One-Way Flow in Veins

• Semilunar valves are folds of the vein’s inner lining.
• Skeletal-muscle contractions compress nearby veins, pushing blood toward the heart; valves prevent retrograde flow.

Exchange Between Capillaries & Tissue Fluid

• Tissue fluid fills minute intercellular spaces.
• Processes:
• Diffusion of \text{O}2 and dissolved nutrients from blood ➔ tissue fluid ➔ cells. • Diffusion of \text{CO}2 and nitrogenous wastes from cells ➔ tissue fluid ➔ blood.
• Some WBCs migrate through capillary walls to patrol tissues.

Advantages of Structural Features (Checkpoint Answers)

• Valves in veins – ensure unidirectional flow under low pressure, preventing pooling and backflow.
• One-cell-thick capillary wall – minimises diffusion distance, maximising rate of exchange.

The Heart – Gross Anatomy

• Location: mediastinum between lungs, behind sternum.
• Four chambers ensure complete separation of oxygenated and deoxygenated blood ➔ greater efficiency.

Atria

• Right atrium (RA) receives deoxygenated blood via superior & inferior vena cavae.
• Left atrium (LA) receives oxygenated blood via pulmonary veins.
• Walls are thin; contract only to top up ventricles.

Ventricles

• Right ventricle (RV) pumps deoxygenated blood through pulmonary artery to lungs (short distance, lower pressure).
• Left ventricle (LV) pumps oxygenated blood through aorta to entire body (long distance, highest pressure); wall significantly thicker than RV.

Valves

• Atrioventricular – tricuspid (RA→RV), bicuspid/mitral (LA→LV); prevent backflow into atria during ventricular systole.
• Semilunar – pulmonary & aortic; prevent backflow into ventricles during diastole.

Septum

• Median muscular wall preventing mixing of the two blood streams.

Double Circulation

• Pulmonary circuit: heart ➔ lungs ➔ heart (lower pressure).
• Systemic circuit: heart ➔ body ➔ heart (higher pressure).
• Benefit: optimises pressure for gas exchange in lungs while ensuring rapid delivery systemically.

Cardiac Cycle (One Heartbeat)

  1. Both atria & ventricles relax (diastole); blood flows into atria from veins.

  2. Atria contract (atrial systole); AV valves open; blood enters ventricles.

  3. Ventricles contract (ventricular systole):
    • AV valves shut ("lub" sound) – prevents backflow.
    • Pressure forces semilunar valves open; blood enters pulmonary artery & aorta.

  4. Ventricles relax (ventricular diastole):
    • Semilunar valves close ("dub" sound) – prevents arterial backflow.
    • As pressure falls below atrial pressure, AV valves reopen ➔ cycle repeats.

Terminology

• Ventricular contraction = ventricular systole.
• Ventricular relaxation = ventricular diastole.

Pressure Relationships

• Arterial blood pressure peaks during ventricular systole (≈ 140\,\text{mmHg} in systemic arteries) and falls during diastole (≈ 60–80\,\text{mmHg}).
• Pressure steadily declines through arterioles, capillaries, venules and veins; venous pressure near RA ≈ 0–10\,\text{mmHg}.

Sequential Events on the Left Side (Pressure Curve Explanation)

  1. Slight ventricular pressure rise as atrial contraction tops up ventricle.

  2. Ventricular systole begins; AV valve closes.

  3. Rapid pressure rise against closed valves.

  4. When ventricular pressure > aortic pressure, aortic valve opens.

  5. As contraction ends, ventricular pressure falls; aortic valve closes.

  6. Ventricular pressure continues to drop.

  7. AV valve reopens when ventricular < atrial pressure.

  8. Passive ventricular filling recommences.

  9. Cycle repeats.

Major Blood Vessels

• Lungs: pulmonary artery (deoxygenated away), pulmonary veins (oxygenated toward heart).
• Liver: hepatic artery (oxygenated in), hepatic vein (deoxygenated out), hepatic portal vein (nutrient-rich blood from gut ➔ liver; not connected to heart).
• Kidneys: renal artery (oxygenated in), renal vein (deoxygenated out).
• Whole body: aorta distributes oxygenated blood; venae cavae return deoxygenated blood.

Hepatic Portal Observation (Checkpoint Answer)

• After a protein-rich meal, amino acids and glucose absorbed from intestines enter hepatic portal vein directly, elevating their concentrations relative to hepatic vein (which carries blood leaving the liver after nutrient metabolism/storage).

Coronary Heart Disease (CHD)

Coronary Arteries

• Supply myocardium with \text{O}_2 and nutrients.
• Blockage ➔ myocardial ischaemia ➔ possible infarction (heart attack).

Pathogenesis

• Diets high in cholesterol & saturated fats deposit fatty plaques (atherosclerosis) within arterial walls.
• Plaque growth narrows lumen ➔ increases blood pressure and risk of thrombosis.
• Smoking introduces nicotine & \text{CO} which damage endothelium and reduce \text{O}_2 delivery.
• Sedentary lifestyle weakens cardiovascular fitness and promotes hypertension.

Risk-Increasing Factors

• Unhealthy diet rich in cholesterol/saturated fats.
• Physical inactivity.
• Smoking.

Preventive Measures

  1. Eat balanced diet low in saturated fat & cholesterol, rich in fruits, vegetables & fibre.

  2. Regular aerobic exercise – strengthens heart and maintains arterial elasticity.

  3. Avoid smoking – eliminates nicotine & \text{CO}-mediated vascular damage.

Summary of Advantages/Functions (Checkpoint Recap)

• Valves in veins ➔ maintain one-way flow toward heart despite low pressure.
• One-cell-thick capillary walls ➔ rapid diffusion of gases, nutrients, wastes.
• Type O blood – universal donor because its RBCs lack antigens A and B, so recipient antibodies cannot agglutinate them.

Quick Fill-In-The-Blank Answers

• Antigens are located on the surface membrane of red blood cells; antibodies reside dissolved in plasma.
• When RA contracts, blood flows into RV.
• Flow occurs when atrial pressure exceeds ventricular pressure.