Animal and plant transport: active recall

Blood passes through the heart twice during one complete circuit: through pulmonary and systemic circuits. This allows lower pressure in lung vessels and higher pressure supplying body tissues. Oxygenated and deoxygenated streams are normally kept separate in the four-chambered heart.

A pressure difference across the valve drives its movement. Valves permit flow in the forward direction and oppose backflow. They do not actively contract to push blood. Relate the pressure difference to contraction, relaxation and the chamber or vessel on each side.

Arteries have walls suited to pressure and its fluctuations; veins return blood at lower pressure and many have valves. Capillaries have thin walls for exchange and form extensive networks. Vessel type is defined by direction relative to the heart, not simply by whether its blood is oxygenated.

Cardiac output equals heart rate multiplied by stroke volume. For example, beats per minute multiplied by millilitres per beat gives millilitres per minute. A change in heart rate alone does not determine output if stroke volume also changes, so use both supplied values.

Binding of one oxygen molecule changes haemoglobin’s conformation and can increase affinity at other binding sites. This contributes to a sigmoid dissociation curve. The curve relates saturation to oxygen partial pressure; saturation is not the same quantity as the concentration of haemoglobin.

Increased carbon dioxide and associated changes in acidity can lower haemoglobin’s oxygen affinity, shifting its dissociation relationship. At a given oxygen partial pressure, less oxygen is retained. This helps delivery in active tissues, but does not mean carbon dioxide is converted into oxygen.

Pressure can drive fluid from capillaries into surrounding spaces while cells and most large plasma proteins remain within the vessels. Solutes exchange between tissue fluid and cells. Fluid returns to circulation through capillary exchange and lymphatic drainage; lymph is not produced by red cells leaving intact vessels.

Mature vessel elements form hollow pathways with lignified walls that provide support and resist collapse under tension. Connections allow movement between pathways. The conducting elements are dead at maturity, so their own respiration is not what pumps the transpiration stream upwards.

Evaporation lowers water potential in leaf tissues and draws water from xylem. Tension can be transmitted along cohesive water columns, supported by adhesion and the vessel structure. This cohesion–tension mechanism links the leaf to water uptake; it is not a series of active xylem pumps.

A potometer measures water uptake by a shoot, often inferred from movement of an air bubble over time. Uptake is related to transpiration, but some water can be retained or used and leaks can affect the reading. It therefore does not directly measure every molecule of water evaporating from leaves.

Sieve-tube elements form conducting pathways and have reduced internal contents, while companion cells support metabolic functions and loading or unloading processes. Phloem carries assimilates such as sucrose. It differs from dead xylem conduits and should not be described as simply the downward half of water transport.

Loading solute at a source lowers water potential, encouraging water entry and higher hydrostatic pressure. Unloading at a sink helps maintain a pressure difference that drives bulk flow. Sources and sinks depend on the organism’s state, and different sieve tubes can carry material in different directions.