A Level Biology CIE: Transport in Mammals Revision Notes

Need for a Circulatory System

• Metabolic Requirements: The cells of all living organisms require a constant supply of reactants for metabolism, such as oxygen and glucose.
• Diffusion Limitations:     * Single-celled organisms: Can gain oxygen and glucose directly from surroundings. Molecules diffuse quickly to all parts of the cell due to short diffusion distances.     * Larger organisms: Composed of many layers of cells. The time taken for substances like glucose and oxygen to diffuse to every body cell would be far too long because diffusion distances are too great.
• Mass Transport Systems: To solve the diffusion problem, exchange surfaces are connected to mass transport systems.     * Examples include the digestive system and the lungs being connected to the circulatory system.     * Mass Transport Definition: The bulk movement of gases or liquids in one direction, usually through a system of vessels and tubes.
• Mammalian Circulatory System: A well-studied mass transport system utilizing a one-way flow of blood within vessels to carry nutrients and gases to all body cells.

Open & Closed Circulatory Systems

• Closed Circulatory System: Blood is pumped around the body and is always contained within a network of blood vessels. Found in all vertebrates and many invertebrates.
• Open Circulatory System: Blood is not contained within vessels but is pumped directly into body cavities. Found in arthropods and molluscs.
• Human Double Circulatory System: Humans possess a closed double circulatory system. Blood passes through the heart (the pump) twice in one complete circuit.     * Pulmonary Circulatory System: The right side of the heart pumps deoxygenated blood to the lungs for gas exchange. Blood then returns to the left side of the heart.     * Systemic Circulatory System: Oxygenated blood is pumped from the left side of the heart at high pressure to the rest of the body.

The Main Blood Vessels

• Diffusion Gradients: Oxygen distribution relies on specific concentration gradients:     * Between air in the alveoli and oxygen in the blood (net diffusion into red blood cells).     * Between oxygen in red blood cells and respiring tissues (net diffusion into cell mitochondria).
• Endothelial Layer: Every blood vessel is lined with an endothelial layer of squamous cells.     * Advantages: Reduces friction and reduces damage to the blood vessel.     * Constraint: Squamous epithelial cells do not increase the vessel's elasticity.

Observing & Drawing Blood Vessels

• Structural Composition: Arteries and veins contain the same wall components (elastic tissue, muscular tissue, and collagen fibres) but in different proportions and thicknesses. Capillary walls consist of a single layer of cells.
• Arteries:     * Function: Carry blood at high pressure away from the heart.     * Thick Walls: Necessary to withstand the surge of blood from ventricular contractions.     * Elastic Arteries: Located closer to the heart; they have a higher proportion of elastic fibres to stretch and recoil, preventing bursting and maintaining blood pressure.     * Muscular Arteries: Located further from the heart; they contain more smooth muscle to adjust vessel diameter and alter blood flow to specific tissues. These branch into arterioles.     * Arterioles: Smaller arteries with lower blood pressure than main arteries.     * Lumen: Relatively narrow to ensure high pressure and provide resistance to flow, facilitating efficient gas exchange.
• Capillaries:     * Distribution: Arterioles branch into capillaries, forming capillary beds throughout most tissues. No cell is typically more than a few $\mu m$ away.     * Dimensions: Diameter is between $5-10\,\mu m$. (A typical red blood cell is $7\,\mu m$ in diameter).     * Endothelium: Wall is only one-cell thick for easy diffusion.     * Leaky Walls: Small gaps exist between squamous epithelial cells, allowing small substances to leak into the surrounding fluid.
• Veins:     * Formation: Capillaries join to form venules, which join to form veins.     * Structure: The outer layer is tough (collagen). The middle layer is thin with minimal smooth muscle and elastic fiber because blood pressure is very low.     * Lumen: Characteristically large.     * Flow Assistance: One-way valves keep blood moving toward the heart, and skeletal muscle contraction temporarily raises pressure to aid movement.

Cells of the Blood

• Red Blood Cells (Erythrocytes):     * Approximately $5\times 10^6$ cells per $mm^3$ of blood.     * Contains haemoglobin (quaternary protein with haem iron groups).     * Shape: Biconcave disc (due to lack of nucleus) to increase surface area.
• Monocytes (Leukocyte type):     * Largest of the leukocytes.     * Nucleus: Kidney-shaped or bean-shaped; appears lighter after staining (light blue) with fine, distinct chromatin.
• Neutrophils (Leukocyte type):     * Constitute up to 70% of all leukocytes.     * Nucleus: Multi-lobed.     * Granules: Typically stain pink or purple-blue.
• Lymphocytes (Leukocyte type):     * Constitute 20–25% of leukocytes.     * Nucleus: Very large, typically stains a dark colour.     * Size: Roughly the same size as a red blood cell.

The Role of Water in Circulation

• Composition: Water is the main component of blood (95% of plasma) and tissue fluid.
• Solvent Properties: Ideal for transport. Glucose is transported in solution from the small intestine; urea is transported from the liver to the kidneys.
• Specific Heat Capacity:     * Water has a high specific heat capacity of $4200\,J/kg^{\circ}C$.     * A large amount of energy is required to raise its temperature, allowing it to absorb heat without large fluctuations.     * This is vital for maintaining optimal enzyme activity and transferring heat from active (warmer) regions to maintain constant body temperature.

Blood, Tissue Fluid & Lymph

• Plasma: Straw-coloured liquid (55% of blood). 95% water.
• Tissue Fluid Formation:     * Formed when plasma leaks through gaps in capillary walls to bathe cells outside the circulatory system.     * Content: Virtually identical to plasma but with far fewer proteins (proteins are too large to fit through capillary gaps).     * Arterial End: High hydrostatic pressure pushes molecules out of the capillary. Although protein content creates a water potential gradient drawing water back in, the hydrostatic pressure is greater, resulting in a net movement of fluid out.     * Venous End: Hydrostatic pressure is reduced. The water potential gradient (due to remaining plasma proteins) stays consistent, causing water to flow back into the capillary.     * Oedema: High blood pressure (hypertension) pushes more fluid out at the arterial end than can be reabsorbed, causing fluid accumulation in tissues.
• Lymph:     * Some tissue fluid enters the lymphatic system via lymph capillaries (separate from the circulatory system).     * Lymph Capillaries: Closed ends with large pores to capture large molecules (like escaped plasma proteins or lipids from the intestine).     * Movement: Fluid moves via compression from body movement; one-way valves prevent backflow. Sedentary behavior (e.g., on planes) can cause swelling due to lack of movement.     * Drainage: Lymph eventually re-enters the bloodstream via veins near the heart.     * Protein Regulation: If escaped proteins were not removed, they would lower the tissue fluid's water potential, preventing water reabsorption into the blood.

Transport of Oxygen & Carbon Dioxide

• Oxygen Transport:     * Haemoglobin (Hb) consists of four subunits, each containing a haem group with an iron atom.     * Each haemoglobin molecule can carry four $O_2$ molecules (eight oxygen atoms).     * Equation: $4O_2 + Hb \rightleftharpoons Hb4O_2$     * Cooperative Binding: The binding of the first $O_2$ molecule changes the conformation of the Hb molecule, making it easier for the remaining three molecules to bind.
• The Chloride Shift:     * Occurs to prevent electrical imbalance inside red blood cells.     * Process: $CO_2$ diffuses into RBCs. Carbonic anhydrase catalyses: $CO_2 + H_2O \rightleftharpoons H_2CO_3$ (carbonic acid).     * Carbonic acid dissociates: $H_2CO_3 \rightleftharpoons HCO_3^- + H^+$.     * $HCO_3^-$ (hydrogen carbonate) moves out of the RBC via a transport protein.     * $Cl^-$ (chloride ions) move into the RBC via the same protein to balance the positive charge of the remaining $H^+$ ions.
• Carbon Dioxide Transport:     1. Hydrogen Carbonate Ions ($HCO_3^-$): ~85% of $CO_2$ is transported this way in plasma.     2. Dissolved CO2: ~5% dissolves directly in plasma.     3. Carbaminohaemoglobin: ~10% binds directly to haemoglobin.
• Buffering: Haemoglobin acts as a buffer by binding with $H^+$ ions to form haemoglobinic acid, preventing a drop in pH inside the RBC.

The Oxygen Dissociation Curve

• Definition: Shows the rate at which $O_2$ associates and dissociates with Hb at different partial pressures of oxygen ($pO_2$).
• Affinity: The ease with which Hb binds to $O_2$.     * High affinity: Binds easily, dissociates slowly (occurs at high $pO_2$ like in the lungs).     * Low affinity: Binds slowly, dissociates easily (occurs at low $pO_2$ like in respiring tissues).
• Curve Shape: Sigmoid (S-shaped) due to cooperative binding.     * Left/Bottom: At low $pO_2$, binding is difficult because of the Hb molecule's shape.     * Middle: Once the first molecule binds, shape changes (conformation), and binding speed increases rapidly (steep part of curve).     * Top/Right: As Hb approaches saturation (100%), it takes longer to find the final binding site, and the curve levels off.
• The Bohr Shift:     * High levels of $CO_2$ (in respiring tissues) reduce Hb's affinity for $O_2$.     * The dissociation curve shifts to the right.     * Result: Hb gives up $O_2$ more readily where it is needed most.

Structure of the Heart

• General: Mass of ~300g, located in the chest cavity, protected by the pericardium (a fibrous sac).
• Chambers: Four chambers.     * Upper: Left and right Atria.     * Lower: Left and right Ventricles.
• Septum: Muscular wall separating the sides. Interatrial septum (top) and Interventricular septum (bottom). Prevents mixing of oxygenated and deoxygenated blood.
• Valves:     * Atrioventricular (AV) Valves: Open when pressure behind is greater than in front. Close to prevent backflow.     * Right Side: Tricuspid valve (between right atrium and ventricle).     * Left Side: Bicuspid or Mitral valve (between left atrium and ventricle).     * Semilunar Valves: Pulmonary valve (right ventricle to pulmonary artery) and Aortic valve (left ventricle to aorta).
• Vessels:     * Into Heart: Vena cava (deoxygenated), Pulmonary vein (oxygenated).     * Away from Heart: Pulmonary artery (deoxygenated), Aorta (oxygenated).
• Coronary Arteries: Supply the heart muscle itself with oxygenated blood. Blockage leads to angina or myocardial infarction (heart attack).

Walls of the Heart

• Atria: Thin walls; they only need to generate enough pressure to push blood into the ventricles.
• Ventricles: Thicker, more muscular walls.
• Left vs. Right Ventricle: The left ventricle wall is significantly thicker than the right.     * The right ventricle only pumps blood to the lungs (short distance).     * The left ventricle must generate high pressure to pump blood around the entire body.

The Cardiac Cycle

• Systole: Contraction of the heart.
• Diastole: Relaxation of the heart.
• Duration: One cycle lasts ~0.8 seconds. Ventricular systole occurs ~0.13 seconds after atrial systole.
• Pressure Changes:     * Diastole: Heart is relaxing. AV valves open, semilunar valves closed.     * Systole: Heart contracts. AV valves close (to prevent backflow to atria), semilunar valves open (to push blood to arteries).

Initiation & Control of Heart Action

• Myogenic: The heart beats without external stimulus.
• Sinoatrial Node (SAN): Located in the right atrium wall; acts as the primary pacemaker. It initiates a wave of depolarisation (excitation).
• Annulus Fibrosus: Non-conducting tissue between atria and ventricles. It prevents the depolarisation from spreading directly to the ventricles, ensuring they don't contract simultaneously with the atria.
• Atrioventricular Node (AVN): Carries the depolarisation to the ventricles after a slight delay ($0.1-0.2\,\text{seconds}$). The delay allows atria to empty completely.
• Bundle of His: Conducting tissue in the septum that receives the signal from the AVN.
• Purkyne Tissue (Purkinje): Conducting fibres that branch from the Bundle of His and spread around the ventricle walls. They carry the excitation to the apex (base) of the heart.
• Contraction Direction: Ventricles contract from the apex upwards, pushing blood efficiently into the arteries.

Questions & Discussion

• Q: Which layer of blood vessels does not provide increased elasticity?     * A: The endothelial layer of squamous cells. Its roles are reducing friction and resisting pressure, but it does not contribute to elasticity.
• Q: Why do arteries closer to the heart have more elastic fibres?     * A: To stretch and recoil to accommodate the high-pressure surges of blood, preventing the vessels from bursting.
• Q: Calculate the oxygen capacity in $1\,dm^3$ of blood if there is $150\,g$ of Hb and $1\,g$ of Hb carries $1.3\,cm^3$ of oxygen.     * A: $150\,g \times 1.3\,cm^3/g = 195\,cm^3$ of oxygen.
• Q: Explain the role of the AVN delay.     * A: It ensures that atria have enough time to finish contracting and empty their blood into the ventricles before the ventricles begin their contraction.