8. Transport in Mammals

Mammals have a closed double circulatory system: blood stays inside blood vessels, and one complete journey takes it through the heart twice. The two linked circuits are pulmonary circulation and systemic circulation, not one single loop.

1. Pulmonary circuit: The right side of the heart pumps deoxygenated blood to the lungs. Blood passes through lung capillaries for gas exchange, so this loop links the heart to the lungs.
2. Return and reset: Blood returns from the lungs to the left side of the heart after oxygen loading. The heart therefore receives the blood again before it is sent around the body.
3. Systemic circuit: The left side pumps oxygenated blood at relatively high pressure through the systemic vessels to body tissues, where substances are exchanged.
4. Complete path: heart right side → lungs → heart left side → body tissues → heart right side.

Why double helps: Separating the lung and body circuits lets the heart send blood to the body at high pressure after the lung circuit, while keeping the gas-exchange step as a distinct loop.

“Double” means two circuits in one closed vessel network, not two hearts and not two blood systems that never meet. Arteries are defined by carrying blood away from the heart and veins by returning to it; oxygen content alone does not define those names. Staff-only visual brief: two linked loops with right/left heart sides, pulmonary/systemic labels and arrows; do not generate or bind an image.

Arteries, veins and capillaries are all blood vessels, but their structures match different transport conditions. Vessel names are defined by direction relative to the heart: arteries carry blood away and veins carry blood towards it, not by oxygen content.

• Arteries — away from the heart: blood leaves at relatively high pressure. Thick walls with smooth muscle, elastic tissue and collagen resist the pressure; a relatively narrow lumen helps maintain it. Elastic tissue stretches and recoils with each pulse.
• Veins — towards the heart: pressure is lower and there are no arterial surges. A wider lumen offers less resistance; thinner walls contain less smooth muscle and elastin, and one-way valves prevent backflow.
• Capillaries — exchange with tissues: blood flows relatively slowly through a very small lumen. The wall is only one cell thick and capillaries branch between cells, so diffusion distances are short and exchange can be rapid. Capillaries lack the elastic tissue, smooth muscle and collagen needed in larger vessels.
• Structure → function check: high-pressure delivery favours strong, elastic arterial walls; low-pressure return favours a wide lumen and valves; exchange favours a thin wall, small diameter and extensive branching.

The pulmonary artery carries deoxygenated blood away from the heart and the pulmonary vein carries oxygenated blood towards it, so oxygen content is not the definition. Staff-only visual brief: three aligned vessel profiles with arrows for direction, pressure cues and wall/lumen features; do not generate or bind an image.

The named vessels connect the two linked circuits of a closed double circulation. Capillary beds are the exchange networks between the large vessels and the lungs or body tissues; arteries carry blood away from the heart and veins carry it towards the heart.

1. Pulmonary outflow: right side of heart → pulmonary artery → lung capillary beds. Deoxygenated blood reaches the lungs for gas exchange.
2. Pulmonary return: lung capillary beds → pulmonary vein → left side of heart. Blood returns oxygenated before entering the systemic circuit.
3. Systemic outflow: left side of heart → aorta → systemic arteries and arterioles → body-tissue capillary beds. Oxygenated blood is delivered for exchange with tissues.
4. Systemic return: body-tissue capillary beds → systemic venules and veins → vena cava → right side of heart. Deoxygenated blood returns, completing the route back to the pulmonary circuit.

Whole route cue: heart right side → pulmonary artery → lungs → pulmonary vein → heart left side → aorta → body capillaries → vena cava → heart right side.

Do not define a vessel by oxygen content alone: the pulmonary artery carries deoxygenated blood away from the heart, while the pulmonary vein carries oxygenated blood towards it. The vessel names above describe direction and route; capillaries are the exchange beds, not an extra third circulation.

Recognising a vessel in a micrograph or section is an evidence-matching task. First establish what the image can show; then combine lumen, wall and valve evidence rather than relying on one visual clue or on oxygen content.

1. Set the image boundary: decide whether the vessel is shown in transverse section, longitudinal section or a wider field. Use the scale bar or magnification when available; do not infer a whole-vessel shape from a partial view.
2. Inspect the lumen: compare lumen width with wall thickness. A relatively narrow lumen with a thick wall suggests an artery; a large lumen with a thinner wall suggests a vein.
3. Inspect wall evidence: look for a thick wall containing elastic and muscular tissue in an artery, versus a thinner middle layer and collagen-rich outer support in a low-pressure vein.
4. Test for capillary scale: a very small vessel with a wall only one cell thick, often close to body cells or in a branching network, supports capillary identification and an exchange role.
5. Check valves and shape: visible one-way valves strongly support a vein. A collapsed or irregular lumen can be a useful clue, but it should be checked against the wall evidence and section orientation.
6. Match the complete pattern: artery = thick elastic/muscular wall + relatively narrow lumen; vein = wide lumen + thinner wall/valves; capillary = tiny lumen + one-cell-thick wall. State which observed features support the match.

Evidence boundary: artery and vein names follow direction relative to the heart, not oxygen content, so an image alone cannot justify an oxygen label unless the route is also known.

Do not identify a vessel from colour, a single lumen measurement or apparent wall thickness alone; section angle, scale and image quality can change those appearances. Staff-only visual brief: paired artery/vein transverse sections plus a capillary close-up, with callouts for lumen, wall, valve and scale; do not generate or bind an image.

A plan diagram is a simplified record of the visible organisation of a specimen. It preserves the vessel outline, lumen and relative wall layers without copying every cell or adding structures that the section does not show.

1. Read the evidence first: identify transverse section (TS) or longitudinal section (LS), note the field of view, and record magnification or a scale bar when supplied. Keep orientation and scale consistent.
2. Trace the overall outline: lightly locate the vessel boundary and lumen, then draw a large, clear outline using continuous unbroken lines. Preserve the specimen’s relative proportions rather than inventing a circular or symmetrical shape.
3. Add the visible arrangement: show the wall as the major layers or regions that can be distinguished, including relative thickness and the lumen position. In a longitudinal section, show the lengthwise arrangement rather than forcing a cross-section view.
4. Simplify deliberately: use clean lines and omit individual cells, shading, colour and microscopic texture. A plan diagram communicates pattern and proportions, not every cell boundary.
5. Label only evidence: add ruled, non-crossing labels for visible structures such as lumen, wall layers or a valve if present. Do not label a feature inferred only from oxygen content or from an unseen part of the vessel.
6. Check before finishing: compare lumen-to-wall proportions with the source, make sure lines are continuous and labels are unambiguous, and state any scale or section limitation that affects the interpretation.

Diagram boundary: a plan diagram is not a detailed histological drawing; its value is a faithful, labelled summary of the structures and relative arrangement that are actually visible.

Do not draw every cell, add unobserved layers or use decorative shading. Staff-only visual brief: paired TS/LS plan-diagram examples with continuous outlines, lumen, wall-layer labels and a scale/orientation cue; do not generate or bind an image.

Each blood vessel is adapted to its transport problem. Explain an adaptation as feature → problem → function; do not use oxygen content as the definition of an artery or vein.

• Artery: thick wall containing elastic tissue, smooth muscle and collagen → blood leaves the heart under high pressure and surges with ventricular contraction → the wall resists bursting, elastic tissue stretches and recoils to accommodate the pulse, and the relatively narrow lumen helps maintain pressure.
• Vein: wider lumen, thinner middle wall with less smooth muscle and elastin, plus one-way valves → blood returns at low pressure without arterial surges → the wider lumen reduces resistance and valves prevent backflow, helping keep flow towards the heart.
• Capillary: very small diameter, extensive branching and a wall one cell thick → exchange must occur across a short distance between blood and nearby cells → the thin wall and close contact support rapid diffusion, while slower flow provides more opportunity for exchange.

Boundary: these are structure–function adaptations, not absolute oxygen labels; pulmonary vessels are the familiar counterexample.

Do not list a feature without its problem and function, and do not say that every artery carries oxygenated blood or every vein carries deoxygenated blood. Staff-only visual brief: three aligned feature→problem→function lanes with pressure and exchange cues; do not generate or bind an image.

The formed components of blood have different roles: red blood cells transport respiratory gases, white blood cells contribute to defence, and platelets help blood clot. Identify each group from several structural clues, then connect the clues to its role.

• Red blood cells: no nucleus, biconcave disc shape and haemoglobin → more room for haemoglobin and a large exchange surface → reversible oxygen transport in the blood.
• White blood cells: cells with nuclei and varied specialised forms → they can carry out defence roles rather than acting as uniform gas carriers. In images, use nuclear evidence: monocytes have a kidney/bean-shaped nucleus, neutrophils have a multi-lobed nucleus and lymphocytes have a large, dark-staining nucleus.
• Platelets: small cell fragments rather than complete nucleated cells → they can gather at a damaged vessel and contribute to the clotting response, helping reduce blood loss.

Recognition boundary: colour or size alone is not enough. Combine nucleus/shape evidence with the expected blood role, and do not treat platelets as white blood cells or red blood cells as defence cells.

White blood cells are a varied group, so “white cell” is not one single shape. Staff-only visual brief: three aligned blood-component profiles with red-cell biconcavity/haemoglobin, white-cell nuclear diversity and platelet fragments; do not generate or bind an image.

Water is the main liquid component of plasma and tissue fluid, so its properties make the circulation a useful transport medium. The key applications here are solvent action and heat distribution.

• Solvent → transport of dissolved substances: water dissolves solutes in plasma → they can be carried in solution through blood and tissue fluid → glucose can move from the small intestine to cells for respiration, while urea can move from the liver to the kidneys for excretion.
• High specific heat capacity → temperature distribution: water can absorb substantial heat with relatively little temperature change → plasma and tissue fluid can take up heat from warmer, active regions and redistribute it in the circulation → body temperature is kept more stable, supporting conditions suitable for enzyme activity.

Boundary: these are circulation applications of water’s solvent and thermal properties; they do not require a catalogue of every property of water or invented plasma chemistry.

Do not describe water as merely filling the vessels: connect each property to the transported material or heat. The carrier is plasma/tissue fluid, whose water-rich phase supports these functions. No image generated or bound.

Tissue fluid is formed from plasma at capillary beds, bathes cells for exchange, and is returned either directly to capillaries or, when excess remains, through lymphatic drainage back to the blood.

1. Arterial-end filtration: blood hydrostatic pressure is relatively high at the arterial end of a capillary → water and small dissolved substances are forced through gaps in the capillary wall → tissue fluid forms around nearby cells. Large plasma proteins and blood cells remain in the blood because they cannot pass through the gaps.
2. Cell exchange: tissue fluid bathes the cells → oxygen and solutes can move from the blood towards cells, while carbon dioxide and other waste products move from cells towards the capillary for removal.
3. Venous-end reabsorption: pressure falls along the capillary → the water-potential effect of plasma proteins becomes relatively more important → water moves back into the capillary by osmosis, so much of the tissue fluid is reabsorbed.
4. Lymphatic return: not all filtered fluid returns directly at the venous end → excess tissue fluid enters lymph vessels as lymph → lymph is eventually returned to the bloodstream, helping prevent fluid accumulation in tissues.

Force boundary: hydrostatic pressure pushes fluid out, whereas the protein-related water-potential gradient draws water back in. Changes such as high blood pressure or low blood-protein content can leave more fluid in the tissues.

Tissue fluid is not whole blood: cells and most large plasma proteins remain in the vessels. Lymph is excess tissue fluid within lymph vessels on its return route; this card stops at fluid balance and does not enter later immune functions. Staff-only visual brief: capillary with arterial filtration, venous reabsorption and a lymphatic side-return arrow; do not generate or bind an image.

Red blood cells transport most of the body’s oxygen using haemoglobin. Oxygen bound to haemoglobin forms oxyhaemoglobin; this is distinct from the smaller amount of oxygen carried dissolved in plasma.

• Biconcave shape: the disc-shaped cell has a large surface relative to its volume → oxygen can reach haemoglobin across a short cell distance → loading and unloading are efficient.
• No nucleus: the mature red cell has no nucleus → more internal space is available for haemoglobin → more oxygen can be carried per cell.
• High haemoglobin content: haemoglobin contains four haem groups, each able to bind one oxygen molecule → one haemoglobin molecule can carry four oxygen molecules → red cells provide the main reversible oxygen-carrying capacity of blood.
• Lung–tissue boundary: in the lungs, oxygen binds to haemoglobin and oxyhaemoglobin forms; in respiring tissues, oxygen can dissociate and enter cells. The oxygen-dissociation curve and Bohr shift explain how conditions tune this exchange in later cards.

Do not equate oxygen transport with oxygen dissolved in plasma: most is haemoglobin-bound. Keep carbon-dioxide forms, the oxygen-dissociation curve and the Bohr shift for their dedicated cards. No image generated or bound.

The chloride shift is the exchange of hydrogencarbonate ions and chloride ions across the red-cell membrane during carbon-dioxide transport. It allows hydrogencarbonate to leave the red cell without leaving an electrical imbalance.

1. CO2 enters the red cell: in respiring tissues, carbon dioxide diffuses from cells into the blood and then into red blood cells.
2. Hydrogencarbonate forms: carbonic anhydrase catalyses CO2 + water → carbonic acid; carbonic acid dissociates into hydrogencarbonate ions (HCO3−) and hydrogen ions (H+).
3. HCO3− leaves: negatively charged hydrogencarbonate ions move out of the red cell through a membrane transport protein into the plasma, where they can be carried in solution.
4. Cl− enters: chloride ions move into the red cell through the same exchange system → the negative charge leaving as HCO3− is balanced by negative charge entering as Cl− → electrical neutrality is maintained and conversion can continue.
5. Lung-side reversal: at the lungs, the exchange runs in the opposite direction: HCO3− returns to the red cell and Cl− leaves. The carbon-dioxide chemistry can then be reversed so CO2 is regenerated for removal in exhaled air.

Boundary: this is an ion-exchange mechanism that supports CO2 transport; it is not the Bohr shift, which describes the effect of CO2/H+ conditions on haemoglobin oxygen affinity.

Chloride does not replace carbon dioxide as the transported gas. HCO3− is the carbon-dioxide-derived form carried mainly in plasma; Cl− enters to balance charge. No image generated or bound.

Carbon dioxide from respiring tissues reaches the blood in three forms. Most is converted inside red blood cells and then carried in plasma as hydrogencarbonate ions; smaller amounts remain dissolved in plasma or bind to haemoglobin as carbaminohaemoglobin.

• Dissolved CO2 — a small fraction (about 5% in the SME comparison): CO2 remains in solution in plasma → it can be carried directly in the blood, but this is not the main transport form.
• Carbaminohaemoglobin — a smaller bound fraction (about 10%): CO2 binds to haemoglobin in red blood cells → this carries CO2 in a reversible haemoglobin-bound form.
• Hydrogencarbonate — the major fraction (about 85%): tissue CO2 diffuses into a red blood cell → carbonic anhydrase catalyses reaction with water and carbonic acid dissociates → HCO3− leaves the red cell and enters plasma → plasma carries it in solution towards the lungs.

Division of labour: red blood cells provide the rapid conversion site and haemoglobin binding; plasma provides the main fluid route for hydrogencarbonate. At the lungs, the transport pathway is reversed so carbon dioxide can be regenerated and removed.

Boundary: the detailed HCO3−/Cl− membrane exchange belongs to the chloride-shift card, and CO2/H+ effects on haemoglobin oxygen affinity belong to the Bohr-shift card.

Do not say that all carbon dioxide is dissolved molecular CO2 in plasma or that plasma formed the hydrogencarbonate without red-cell involvement. The percentages are approximate comparison values from the SME note; no image generated or bound.

The oxygen dissociation curve shows haemoglobin saturation against the partial pressure of oxygen (pO2). Its sigmoidal shape reflects changing haemoglobin affinity as oxygen binds and dissociates.

1. Read the axes: x-axis = oxygen partial pressure; y-axis = percentage saturation of haemoglobin with oxygen. The graph is not a time graph and does not show the total oxygen content of all blood.
2. Explain the low-pO2 region: at low pO2, the first oxygen binds less easily, so affinity and saturation are relatively low; dissociation of the final bound oxygen also becomes slower when few binding sites remain occupied.
3. Explain the steep region: after one oxygen binds, haemoglobin changes conformation → the next oxygen molecules bind more easily → a modest pO2 change can produce a relatively large change in saturation. This is cooperative binding.
4. Explain the high-pO2 plateau: as haemoglobin approaches saturation, few binding sites remain → further pO2 increases produce only a small additional saturation change.
5. Apply the shape: high pO2 in the lungs supports loading and keeps haemoglobin near the plateau; the lower pO2 conditions of respiring tissues lie in the unloading region, where oxygen can be released for cellular respiration.
6. Make a cautious read-off: identify the relevant pO2, project to the curve, then read the approximate saturation; report an estimate rather than inventing precision between graph marks.

Boundary: this card explains the curve and cooperative shape. Card 4609 handles the detailed partial-pressure gradient and loading/unloading application; card 4610 handles the Bohr shift.

A right-hand plateau does not mean no oxygen can be released, and a left/steep region does not mean haemoglobin is absent. Saturation is a percentage at a specified pO2. No image generated or bound.

Oxygen partial pressure (pO2) is the pressure contribution of oxygen within a gas mixture. The local pO2 changes haemoglobin’s loading or unloading demand: high pO2 favours binding, while lower pO2 favours dissociation.

• Lungs — high pO2: haemoglobin encounters a high oxygen partial pressure → affinity and percentage saturation are high → oxygen loads onto haemoglobin and blood leaves the lungs carrying a high oxygen load.
• Respiring tissues — lower pO2: cells use oxygen, so tissue pO2 is lower than in the lungs → haemoglobin’s affinity is lower and saturation falls → oxygen dissociates and becomes available for cellular respiration.
• Steep-region demand: when the tissue pO2 lies on the steep part of the curve, a relatively small fall in pO2 can produce a substantial fall in saturation → a useful amount of oxygen is unloaded without requiring complete deoxygenation.
• Active/respiring tissue: greater respiration tends to maintain a lower local oxygen partial pressure → the loading–unloading system is driven towards release where oxygen demand is present, within the conditions represented by the page.

Application method: identify the location, compare its pO2 with the lung/tissue context, locate the corresponding curve region, then state whether saturation rises or falls and why.

Partial pressure is not simply the percentage of oxygen in the whole atmosphere or the total oxygen amount in blood. This card applies pO2 to location; 4608 explains the curve shape/read-off and 4610 explains the Bohr shift when CO2/H+ conditions alter affinity. No image generated or bound.

The Bohr shift is the change in haemoglobin’s oxygen affinity caused by the higher carbon-dioxide conditions of respiring tissues. It shifts the oxygen dissociation curve to the right, so haemoglobin is less saturated at the same oxygen partial pressure.

• Demand signal: active respiring cells produce more carbon dioxide → carbonic acid dissociates and hydrogen ions are formed.
• Affinity change: hydrogen ions bind to haemoglobin and the higher CO2/H+ conditions reduce haemoglobin’s affinity for oxygen → oxygen is released more readily.
• Curve consequence: the dissociation curve shifts right → at the same tissue pO2, haemoglobin has a lower percentage saturation than it would under lower-CO2 conditions.
• Functional result: more oxygen dissociates from haemoglobin in the respiring tissue where demand is high → oxygen becomes available for cellular respiration.
• Lung-side recovery: in the lungs, CO2 is removed and the high-oxygen environment favours restoration of haemoglobin’s higher oxygen affinity, allowing oxygen loading for the next circuit.

Boundaries: the basic curve and cooperative binding belong to 4608; the partial-pressure location application belongs to 4609; the HCO3−/Cl− charge-balancing exchange is the chloride shift in 4606.

A right shift does not mean haemoglobin contains no oxygen or that oxygen cannot load in the lungs. It is a conditional affinity change linked to CO2/H+ conditions, not the chloride-ion exchange. Staff-only visual brief: paired low-CO2/high-affinity and high-CO2/right-shift curves with one shared pO2 read-off; do not generate or bind an image.

The mammalian heart is a hollow muscular pump with two atria above two ventricles. A septum separates the right and left sides, while valves open and close with pressure differences to keep blood moving forwards.

1. Right-side inflow: blood returning from the body enters the right atrium through the vena cava. It passes through the right atrioventricular (tricuspid) valve into the right ventricle.
2. Pulmonary outflow: the right ventricle sends blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs for gas exchange.
3. Left-side inflow: blood returning from the lungs enters the left atrium through the pulmonary vein. It passes through the left atrioventricular (mitral/bicuspid) valve into the left ventricle.
4. Systemic outflow: the left ventricle sends blood through the aortic valve into the aorta, which carries it to the body tissues.
5. Why the route stays one-way: the atrioventricular valves control atrium→ventricle flow and the semilunar valves control ventricle→artery flow; their closure prevents backflow. The septum keeps the two sides separate.

Route cue: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary vein → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.

Artery and vein are named by direction relative to the heart, not by an absolute oxygen rule: the pulmonary artery leaves the heart and the pulmonary vein returns to it. In a diagram, identify the sides by the vessel/chamber route rather than page position. No image generated or bound.

Cardiac muscle thickness is an adaptation to the pressure and distance that each chamber must supply. Thicker muscle can generate a greater pressure when it contracts; wall thickness is not defined by the oxygen content of the blood.

• Atria — thin walls: thin muscular walls → only a short push is needed → atrial contraction generates enough pressure to move blood into the ventricles, but does not need to drive it around a circuit.
• Ventricles — thicker walls: thicker, more muscular walls → a stronger squeeze is possible → ventricular contraction raises pressure and ejects blood through the semilunar valves.
• Right ventricle — thinner than the left: blood travels from the right ventricle to the nearby lungs → a lower pressure is sufficient → its wall can be thinner while still powering the pulmonary circuit.
• Left ventricle — thickest wall: blood must travel from the left ventricle through the systemic circuit to the body → high pressure is required → the much thicker muscle generates the force for systemic delivery.
• Septum: the muscular septum separates the right and left sides → the two circuits remain distinct and blood does not mix between them.

Boundary: this card explains structural pressure demand. The timed systole/diastole and valve sequence belong to the cardiac-cycle card.

The left ventricle is thicker because it pumps farther and needs higher pressure, not because its blood is “more oxygenated”. Do not infer a cardiac-cycle time point from wall thickness alone. No image generated or bound.

One cardiac cycle is one heartbeat: atrial systole, ventricular systole and diastole. Muscle contraction decreases chamber volume and raises pressure; relaxation increases volume and lowers pressure. Valves respond to these pressure differences to keep flow one-way.

1. Atrial systole: atrial muscle contracts → atrial volume falls and atrial pressure rises above ventricular pressure → atrioventricular (AV) valves open → blood is pushed into the relaxed ventricles. Semilunar valves remain closed.
2. Ventricular systole: ventricular muscle contracts → ventricular volume falls and ventricular pressure rises above atrial pressure → AV valves close, preventing backflow into the atria. When ventricular pressure exceeds pressure in the aorta and pulmonary artery, semilunar valves open and blood is ejected.
3. Diastole: atria and ventricles relax → ventricular pressure falls below arterial pressure → semilunar valves close, preventing blood returning from the arteries. The atria fill from the vena cava and pulmonary veins; when atrial pressure rises above ventricular pressure, AV valves open and blood flows passively into the ventricles.
4. Repeat: filling restores chamber volume, the pressure sequence begins again with atrial systole, and blood continues to move without a pause between cycles.

Valve rule: a valve opens when pressure behind it exceeds pressure in front; it closes when the pressure relationship reverses. The closure prevents backflow rather than actively pushing blood.

Do not assign valve states from “systole” alone without checking which chamber pressure is changing. This card covers mechanical pressure and valves; SAN, AVN, bundle of His and Purkyne conduction belong to 4614. No image generated or bound.

The heart is myogenic: its own conduction system starts and distributes the excitation wave. The sequence is SAN → atria → AVN delay → bundle of His → Purkyne tissue → ventricular muscle, so atrial contraction precedes coordinated ventricular contraction.

1. SAN starts the wave: the sinoatrial node in the right atrium generates a wave of excitation, causing the atria to contract.
2. AVN delays the wave: non-conducting tissue between atria and ventricles prevents the wave travelling straight through; the atrioventricular node receives it and delays it, allowing the ventricles time to fill after atrial contraction.
3. His bundle carries it through the septum: after the delay, the wave passes down the bundle of His.
4. Purkyne tissue spreads it from the apex: the bundle divides into Purkyne fibres that spread through the ventricular walls, initiating excitation from the apex upward.
5. Ventricles contract together: this coordinated wave makes the ventricles contract and forces blood into the pulmonary artery and aorta.

The key control relationship is electrical excitation → muscle contraction → coordinated pumping. The AVN delay is essential for atrial-then-ventricular timing. This card covers the conduction pathway, not ECG interpretation, drug effects or other unsupported clinical detail; the mechanical valve-pressure sequence is covered in 4613. No image generated or bound.