Transport in Mammals is Cambridge International Biology 9700 Topic 8. It connects closed double circulation and vessel structure to tissue fluid, blood-cell recognition, oxygen and carbon dioxide transport, haemoglobin behaviour, heart structure and the cardiac cycle. Theory owns structure-function reasoning and trace interpretation; microscope handling, biological drawing execution and physiological investigations remain in the dedicated practical hub.
1. Closed double circulation
Mammals have a closed circulation because blood remains within vessels. It is double because blood passes through the heart twice in one complete circuit. The pulmonary circulation carries blood from the heart to the lungs and back. The systemic circulation carries blood from the heart to body tissues and back.
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein returns oxygenated blood to the left atrium. The aorta carries oxygenated blood from the left ventricle to the body, while the vena cava returns deoxygenated blood to the right atrium. Artery and vein describe direction relative to the heart, not oxygen content.
2. Vessel structure and function
Elastic arteries receive blood at high, fluctuating pressure near the heart. Their thick walls contain abundant elastic tissue, which stretches during ventricular systole and recoils during diastole. Recoil smooths pressure fluctuations and maintains forward flow.
Muscular arteries distribute blood to organs. Their walls contain a relatively large smooth-muscle component, allowing lumen diameter and blood distribution to change. Arterioles provide strong control of local flow because smooth muscle can alter their narrow lumen.
Veins return blood at lower pressure. They have a wider lumen and thinner walls with less muscle and elastic tissue than comparable arteries. Valves prevent backflow where pressure is insufficient to guarantee one-way movement.
Many systemic capillaries are narrow enough for red blood cells to pass close to the wall; lumen size and permeability vary between tissues. Their endothelium is one cell thick, giving a short diffusion distance. Extensive networks provide a large exchange area, while gaps and permeability permit fluid exchange.
3. Recognising vessels and blood cells
In transverse section, an artery usually has a thick wall relative to its lumen and tends to retain a rounded outline. A vein usually has a thinner wall, wider lumen and more irregular collapsed shape. A typical capillary has a thin endothelial layer with a supporting basement membrane and a small lumen; it lacks the thick muscle layers of an artery. Orientation, preparation and scale can alter appearance, so use several features.
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Mature human red blood cells are biconcave discs without nuclei. Neutrophils have a multilobed nucleus and granular cytoplasm. Lymphocytes usually show a large round nucleus with a narrow cytoplasmic rim. Monocytes are larger, often with a kidney-shaped nucleus and more cytoplasm. Recognition belongs in theory, while slide setup and assessed drawing technique remain practical skills.
4. Water in blood and tissue fluid
Water is the main component of plasma and tissue fluid. As a solvent, it carries ions, nutrients, hormones, wastes and dissolved gases. Its high specific heat capacity buffers rapid temperature change while blood redistributes heat.
In the simplified classical tissue-fluid model, hydrostatic pressure at the arterial end favours filtration of water and small dissolved substances across the capillary wall. Cells and most plasma proteins remain in the blood. The resulting tissue fluid surrounds cells, supplies substances for metabolism and receives wastes.
The classical account describes falling capillary hydrostatic pressure and retained plasma proteins favouring venous-end reabsorption. Treat this as a simplified model, not a universal physiological sequence. The revised Starling account includes the endothelial glycocalyx; in many tissues at steady state, slight filtration persists rather than sustained venous reabsorption. Lymphatic drainage returns filtered fluid to the blood. Specialized beds and transient conditions differ, and the required examination treatment still needs official verification. Tissue fluid is derived from plasma but normally contains far fewer large plasma proteins and no red blood cells.
5. Oxygen transport and the dissociation curve
Oxygen diffuses into red blood cells and binds reversibly to haemoglobin, forming oxyhaemoglobin. In the normal adult tetrameric haemoglobin model, each molecule can bind four oxygen molecules. Binding is cooperative: attachment of one oxygen changes haemoglobin's shape and makes further binding easier, while unloading promotes further release.
This behaviour produces a sigmoid oxygen dissociation curve. At the high partial pressure of oxygen in lungs, haemoglobin approaches high saturation and loads oxygen. At lower partial pressures in respiring tissues, the steeper region promotes substantial unloading for a modest fall in partial pressure. The plateau helps preserve loading despite moderate variation in lung oxygen.
6. Carbon dioxide transport and the chloride shift
Some carbon dioxide dissolves in plasma, and some binds to haemoglobin to form carbaminohaemoglobin. Much of it enters red blood cells, where carbonic anhydrase catalyses its reversible reaction with water to form carbonic acid. Carbonic acid dissociates into hydrogen ions and hydrogencarbonate ions.
Hydrogen ions associate with haemoglobin to form haemoglobinic acid, limiting the change in pH. Hydrogencarbonate leaves the red blood cell for plasma through the AE1 anion exchanger as chloride enters in electroneutral exchange. This is protein-mediated exchange, not simple passage through the lipid bilayer. This exchange is the chloride shift and allows continued hydrogencarbonate transport without charge imbalance.
In lung capillaries, the reactions reverse. Hydrogencarbonate re-enters red blood cells as chloride leaves, carbon dioxide is regenerated and diffuses into alveolar air.
7. The Bohr shift
High carbon dioxide concentration increases hydrogen-ion concentration and lowers pH. Haemoglobin's affinity for oxygen decreases, shifting the dissociation curve to the right. At the same partial pressure of oxygen, haemoglobin is less saturated and releases more oxygen.
This Bohr shift is useful in actively respiring tissues, where carbon dioxide production is high and oxygen demand is high. In the lungs, lower carbon dioxide and higher pH favour oxygen loading. Do not describe the shift merely as a curve moving: state the changed affinity and its biological consequence.
8. Heart structure
The right and left atria receive blood and have thinner walls because they move blood a short distance into ventricles. Ventricles eject blood from the heart and have thicker myocardium. The left ventricle is thickest because it generates high pressure for systemic circulation; the right ventricle pumps only through pulmonary circulation.
Atrioventricular valves separate atria from ventricles. Semilunar valves lie at the exits into the pulmonary artery and aorta. Tendinous cords stop atrioventricular valves from inverting during ventricular contraction. The septum separates oxygenated and deoxygenated sides.
Coronary arteries supply cardiac muscle with oxygen and nutrients. The external heart also shows the aorta, pulmonary artery, pulmonary veins and venae cavae connected to their chambers.
9. Pressure changes and valves in the cardiac cycle
During atrial systole, atrial pressure exceeds ventricular pressure, atrioventricular valves remain open and blood enters the ventricles. Ventricular systole begins when ventricular pressure rises. Atrioventricular valves close as ventricular pressure exceeds atrial pressure. Semilunar valves open only when ventricular pressure exceeds pressure in the aorta or pulmonary artery.
During ventricular diastole, ventricular pressure falls. Semilunar valves close when arterial pressure exceeds ventricular pressure. Atrioventricular valves reopen once ventricular pressure falls below atrial pressure, allowing filling. Valves respond passively to pressure differences; they do not actively decide when to open.
10. Electrical coordination
The sinoatrial node in the right atrial wall initiates excitation and acts as the heart's pacemaker. Excitation spreads across atrial muscle, causing atrial systole. Non-conducting tissue between atria and ventricles prevents direct spread, so excitation reaches the atrioventricular node.
The atrioventricular node imposes a delay, allowing atrial contraction and ventricular filling to occur before ventricular systole. Excitation then travels through conducting tissue to the apex and through Purkyne tissue in ventricular walls. Ventricular contraction proceeds from the apex upward, efficiently ejecting blood into the arteries. This note focuses on the intrinsic conduction pathway; any claim that nervous or hormonal control is excluded from assessment requires checking the current official syllabus.
Worked application: linking tissue activity to blood transport
During intense muscle activity, local carbon dioxide production rises. More carbon dioxide enters red blood cells, where carbonic anhydrase supports hydrogencarbonate formation. Hydrogen ions bind haemoglobin, and hydrogencarbonate leaves in exchange for chloride. The lower pH reduces haemoglobin's oxygen affinity, shifting its dissociation curve to the right and increasing oxygen unloading at the tissue partial pressure. Greater local flow also raises delivery. On return to lung capillaries, oxygen loading and carbon dioxide removal favour the reverse reactions. This explanation connects cellular respiration, chloride shift, haemoglobinic acid and the Bohr effect rather than treating them as unrelated facts.
Common misconceptions and corrections
Calling pulmonary arteries oxygenated. They carry deoxygenated blood away from the heart.
Defining arteries by oxygen content. They carry blood away from the heart.
Saying elastic arteries mainly control organ distribution. Muscular arteries and arterioles provide stronger control.
Giving veins thicker walls than arteries. Veins operate at lower pressure.
Putting valves throughout capillaries. Capillary exchange depends on a thin endothelial wall.
Identifying a vessel from shape alone. Use wall, lumen, layers and scale.
Giving mature red blood cells nuclei. Mammalian red blood cells lack them.
Confusing lymphocytes with neutrophils. Lymphocytes have a large round nucleus; neutrophils are multilobed.
Saying tissue fluid contains normal numbers of red blood cells. Cells remain in vessels.
Treating sustained venous-end reabsorption as universal. The classical diagram is simplified; glycocalyx-dependent exchange and lymphatic return must be distinguished from that model.
Saying haemoglobin binds oxygen irreversibly. Loading and unloading are reversible.
Drawing the adult dissociation curve as a straight line. Cooperative binding produces a sigmoid curve.
Calling every transported carbon dioxide hydrogencarbonate. Some is dissolved or carbamino-bound.
Giving atria thicker walls than ventricles. Ventricles generate ejection pressure.
Saying valves contract to open. Pressure differences move them.
Starting ventricular excitation at the base. Purkyne conduction supports contraction from the apex upward.
Assessment guidance
Organise circulation answers by route, direction and oxygenation rather than by memorised colour. For vessel or cell images, cite multiple visible features and respect section orientation. Tissue-fluid explanations should identify the model used and compare hydrostatic and osmotic effects without treating venous-end reabsorption as universal. Red cells normally remain intravascular; large-protein movement is restricted rather than absolutely zero. On dissociation graphs, read both axes, compare saturation at the same partial pressure and translate a shift into affinity and unloading. Carbon dioxide answers should follow reactions and ion movements in order. Cardiac-cycle explanations must use chamber and arterial pressures to derive valve state, then place SAN, AVN delay and Purkyne conduction in sequence.
Retrieval practice
Trace one blood cell through pulmonary and systemic circuits. Build a structure-function table for elastic arteries, muscular arteries, veins and capillaries, then identify four blood-cell images. Explain tissue-fluid formation and return. Reconstruct carbon dioxide transport with carbonic anhydrase, haemoglobinic acid, carbaminohaemoglobin and chloride shift. Interpret normal and right-shifted oxygen curves. Finish by narrating one cardiac cycle from pressure changes and overlaying the SAN-to-Purkyne conduction pathway.
