Transport and gas exchange

Transport and gas exchange

Multicellular organisms need transport systems because diffusion over long distances is too slow to meet all cellular demands. Exchange surfaces obtain useful substances or remove wastes, while transport connects these surfaces to internal tissues. The effectiveness of diffusion depends on surface area, concentration gradient and distance. Thin, extensive exchange surfaces can work together with flowing transport fluids to maintain useful gradients.

In humans, the heart pumps blood through a double circulation. One circuit takes blood to the lungs and back; the other supplies the rest of the body. Arteries carry blood away from the heart, veins carry blood towards it and capillaries connect small vessels near tissues. These names describe direction, not oxygen content: the pulmonary artery carries relatively deoxygenated blood towards the lungs.

Red blood cells contain haemoglobin for oxygen transport and have adaptations that support their function. White blood cells contribute to defence, platelets help blood clotting and plasma carries dissolved substances and suspended cells. Oxygen moves from lung air into blood down a partial-pressure gradient, while carbon dioxide moves the other way. Haemoglobin does not make a blood vessel an artery or a vein.

In the lungs, numerous alveoli provide a large area, thin moist exchange surfaces and close capillary contact. Ventilation refreshes alveolar air, while blood flow transports gases away and towards the surface. Breathing is the mechanical movement of air; gas exchange is diffusion across a surface; respiration is a cellular energy-releasing process. These terms describe linked but distinct activities and should not be used interchangeably.

Plants also have transport tissues. Xylem carries water and mineral ions mainly upwards from the roots and gives structural support. Phloem translocates organic products such as sucrose between sources and sinks. Transport direction in phloem depends on the locations of sources and sinks and is not always simply downwards. Water loss from aerial plant surfaces is called transpiration and occurs largely through stomata.

Environmental conditions affect transpiration through their effects on evaporation and water-vapour gradients. Higher wind speed often removes humid air near a leaf, increasing water loss if other factors permit it. Stomatal closure reduces water loss but can also limit carbon dioxide entry. In experimental explanations, identify the changing factor, relate it to the gradient or stomatal response, and avoid treating correlations as universal rules without stated conditions.

The mammalian circulation as two linked circuits

The right side of the heart receives blood returning from the body and pumps it to the lungs through the pulmonary artery. Oxygenated blood returns through pulmonary veins to the left atrium, passes to the left ventricle and is pumped through the aorta to the body. This is a double circulation because blood passes through the heart twice during a complete journey through lung and body circuits. The left ventricular wall is thicker because it must generate the pressure needed for systemic circulation. The two sides remain separated, so blood destined for the lungs is not normally mixed directly with blood being sent to body tissues.

Arteries carry blood away from the heart, while veins carry it towards the heart. Oxygen content is not the defining distinction: the pulmonary artery carries relatively deoxygenated blood and pulmonary veins carry oxygenated blood. Arterial walls contain substantial muscle and elastic tissue that withstand and maintain pressure. Many veins have valves that help prevent backward flow under lower pressure. Capillaries have extremely thin walls and narrow lumens, bringing blood close to cells for exchange. A capillary network has a large combined cross-sectional area, which helps slow flow and allows time for transfer.

Blood plasma carries dissolved substances, heat and cells. Red blood cells carry much of the oxygen using haemoglobin. White blood cells contribute to immune defence, and platelets participate in clotting. At tissues, oxygen dissociates from haemoglobin and moves towards cells that are using it in respiration. Carbon dioxide moves in the opposite direction and is carried largely after conversion to hydrogencarbonate in blood. These descriptions connect transport to concentration differences and chemical binding rather than treating blood as a liquid that simply “contains air”.

Ventilation is different from cellular respiration

During quiet inhalation, the diaphragm contracts and flattens while external intercostal muscles lift the ribs. Thoracic volume increases and pressure falls slightly below atmospheric pressure, drawing air in. Quiet exhalation follows relaxation and elastic recoil, reducing volume. Oxygen reaches alveoli, crosses their thin moist surfaces and enters surrounding capillaries. Ventilation refreshes alveolar air; blood flow removes absorbed oxygen. Together they maintain gradients for diffusion. Lung tissue is not a muscle that actively sucks oxygen molecules into red cells.

Fish gills have filaments and lamellae that provide extensive exchange area. Counter-current flow of water and blood helps maintain a gradient along much of the exchange surface. In insects, a tracheal system delivers air through branching tubes close to tissues; insect blood generally does not carry oxygen in the same way as mammalian blood. Plants exchange gases through stomata and other surfaces, but rates and directions depend on photosynthesis and respiration. At night, photosynthesis stops without light while respiration continues. Avoid asserting that a plant only takes in carbon dioxide or only gives out oxygen.

Xylem, phloem and water balance

Xylem transports water and mineral ions mainly from roots towards shoots. Its conducting elements are largely dead, hollow and strengthened by lignin. Water loss from leaves by transpiration creates tension that contributes to a continuous water column being pulled upward. Cohesion between water molecules and adhesion to walls help maintain the column. Root pressure may contribute under some conditions but does not adequately explain water transport to the tops of tall trees. The account must connect evaporation at leaves with movement through xylem rather than claiming that roots alone pump every drop upward.

Phloem transports organic solutes from sources to sinks. A source may be a photosynthesising leaf or a storage organ releasing reserves. A sink may be a growing root, developing fruit or storage organ accumulating material. The same organ can change role over time. Transport in different sieve tubes can occur in different directions according to source-sink relationships. A simple “phloem always carries food downward” rule therefore fails. Phloem contains living conducting cells and associated companion cells, unlike the main conducting xylem elements.

Interpreting measurements and variation

A potometer usually estimates water uptake by a cut shoot, which is used as a proxy for transpiration under suitable conditions. Uptake is not identical to water loss because some absorbed water contributes to metabolism or cell expansion. Leaks and changes in temperature can affect readings. Air movement may remove humid air around a leaf, while high humidity reduces the water-vapour gradient and may reduce transpiration. When testing one factor, keep others as constant as possible and use repeated readings. A longer bubble movement in the same capillary and time indicates a greater volume of water uptake, provided the tube diameter is constant.

Self-check and misconceptions

Trace an oxygen molecule from inhaled air to a muscle mitochondrion: airway, alveolar surface, blood, systemic circulation, tissue fluid and cell. This joins ventilation, diffusion, circulation and respiration without confusing them. Then trace water from soil to atmosphere through root tissues, xylem, leaf cells, air spaces and stomata. In both cases, structure and gradients explain movement. Remember that valves guide bulk blood flow, not individual oxygen diffusion, and that transpiration is the loss of water vapour from aerial plant parts rather than liquid leaking from a cut stem.

Comparing vessels and internal plant anatomy

The hepatic portal vein carries nutrient-rich blood from the digestive organs towards the liver, while the renal artery supplies a kidney and the renal vein drains it. Lymph returns excess tissue fluid towards the circulation and contributes to immune and lipid transport. In an open circulatory system, circulating fluid is not confined to a closed capillary network throughout its route. In a closed system, blood remains within vessels, allowing different control of pressure and distribution.

A root's outer tissues and root hairs support uptake; the cortex provides a route towards central vascular tissues, and the endodermis helps regulate entry to the vascular region. In many dicot stems, vascular bundles occur in a ring, while typical monocot stem bundles are scattered. In a leaf, veins connect xylem supply with phloem transport, while mesophyll and air spaces support photosynthesis and gas diffusion. Guard-cell turgor changes alter stomatal aperture: greater turgor commonly opens stomata in the standard model, and reduced turgor closes them. Lenticels provide gas-exchange routes in some woody organs. Read the actual section supplied, since plant anatomy varies.