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The thorax is a mobile osteocartilaginous enclosure with two pleural compartments and a central mediastinum; its visceral boundaries change normally with breathing and posture. The sketch is an anterior projection, with the subject's right on the viewer's left. It distinguishes the lower margin of a lung from the lower reflection of its pleura: those are different boundaries.

The posterior wall is formed by the twelve thoracic vertebrae and adjacent parts of the ribs; laterally the ribs and intercostal muscles form a flexible wall. Anteriorly lie the sternum, costal cartilages and anterior intercostal structures. The superior aperture communicates with the neck, while the respiratory diaphragm closes the inferior aperture. The lower thoracic wall therefore overlaps upper abdominal organs: not every organ under a lower rib is inside the pleural compartment.

Each lung is covered by visceral pleura. Parietal pleura lines the chest wall, upper surface of the respiratory diaphragm and side of the mediastinum, becoming continuous with visceral pleura at the root. The potential pleural space contains a lubricating fluid film; it is not normally an air-filled gap. The right lung has three lobes, the left two, with the cardiac notch and lingula accommodating the leftward projection of the heart. Each apex projects above the first rib into the root of the neck. The roots carry the bronchi, pulmonary vessels, nerves and lymphatic connections.^[ref]

The heart and pericardium occupy chiefly the middle part of the inferior mediastinum, behind the sternum and between the lungs, with most of the cardiac bulk to the left of the median plane. The great vessels approach superiorly. The trachea divides into main bronchi behind the upper central region; the oesophagus descends behind the airway and heart. The descending thoracic aorta lies posteriorly, chiefly left of the vertebral bodies; the azygos venous route lies chiefly on the right. The thoracic duct and sympathetic chains belong to the posterior relations. A frontal projection superposes these structures and must not be mistaken for their order from front to back.

For a conventional quiet-breathing surface description, the lower lung border reaches approximately the sixth rib in the midclavicular line, eighth in the midaxillary line and tenth posteriorly near the scapular line. The pleural reflection descends approximately two ribs farther at corresponding landmarks: eighth, tenth and twelfth. These are orientation points, not immutable lines in every healthy individual. Inferiorly, the extra pleural extent forms the costodiaphragmatic recess. The right diaphragmatic dome usually stands higher, related to the underlying liver; the left is related to the stomach and other abdominal viscera.

In inspiration the respiratory diaphragm descends, the rib cage expands and lower lung margins move into the recesses; in expiration they retreat. The heart also shifts with the diaphragm and changes its surface projection as neighbouring lung expands. Deep inspiration, quiet expiration and forced expiration therefore cannot share a single normal map. The heart's own beat adds smaller cyclic movements.

Posture shifts the diaphragm and mediastinal contents: a supine position generally raises the domes compared with standing, while lateral positioning displaces mobile contents towards the dependent side. Body build, age, abdominal fullness and pregnancy change the available thoracic shape and diaphragmatic level. A distended stomach can raise the left dome and alter the cardiac projection without implying primary cardiac disease. The clinically useful comparison records posture and respiratory phase before judging an apparent difference in organ position.

Arterial recoil, the muscle pump, respiratory movements, venous tone and the mechanics of cardiac filling assist blood circulation. The heart remains its principal continuing source of mechanical energy. These agencies differ in whether they store cardiac work, contribute new muscular work, or direct flow without supplying power.

During ventricular systole, the aorta and other large elastic arteries expand and store elastic potential energy. During diastole, their recoil continues to drive blood towards the peripheral vessels, while closure of the aortic valve limits return into the heart. This elastic reservoir smooths an intermittent cardiac input into a more continuous downstream flow. It does not manufacture extra energy: it releases part of the work performed during the preceding ejection.

Contraction of surrounding skeletal muscles compresses veins. Venous valves favour displacement towards the heart and limit backward flow when the compression is released. Alternating contraction and relaxation therefore produces a muscle pump, especially useful for return from dependent limbs. The added work comes from the contracting muscles; valves guide the displacement but supply no power themselves. Persistent compression without refilling is not equivalent to a series of pumping strokes.

Spontaneous inspiration supplies another assistance. Descent of the respiratory diaphragm raises abdominal pressure while expansion of the thorax lowers intrathoracic pressure. The resulting changes favour a pressure gradient towards the thoracic veins and right side of the heart. The effect depends on posture, the respiratory phase and whether vessels remain open. Positive-pressure inflation does not reproduce the same pressure changes as spontaneous inspiration.^[ref]

Venous smooth muscle can reduce the storage capacity of peripheral veins, transferring blood towards the central circulation. This changes the volume available for cardiac filling; it does not require a travelling wave of venous peristalsis. Relaxation and recoil of the heart, together with movement of the atrioventricular plane, also influence filling pressures. The older expression cardiac suction is therefore best interpreted through specific changes of chamber volume and pressure, rather than a separate force pulling uniformly through every vessel.

Gravity assists a downward column and opposes an upward one. For a complete circuit returning to its initial height it supplies no net gravitational work. Its important effects include hydrostatic pressure differences and dependent pooling. Likewise, the residual pressure gradient across the capillaries is chiefly the downstream consequence of the cardiac pump. Ordinary capillary action at a liquid meniscus is not a universal extra pump inside a closed, fluid-filled vascular network. Local vasodilation can improve perfusion by reducing vascular resistance, but it does not create the necessary pressure source.

With $Q$ the mean volumetric flow rate, $R$ an effective vascular resistance, and $\Delta P$ the driving pressure difference, the useful bookkeeping relation isIt separates an increase of driving pressure from a reduction of resistance. An instructive apparatus compares the outflow of a pulsatile pump through rigid tubing with its outflow through an elastic reservoir feeding the same resistance. Continued outflow between strokes demonstrates stored elastic potential energy. A separately compressed valved flexible tube demonstrates the additional muscle pump principle. Neither observation requires a mysterious attraction of blood by the tissues.

Quiet breathing chiefly uses active inspiration and passive expiration; deeper or more forceful breathing recruits additional muscles and changes their coordination. The vertical, anteroposterior and transverse dimensions of the thorax can all change, and different patterns need not represent different mechanisms of gas exchange.

The respiratory diaphragm is the principal inspiratory muscle. Its contraction lowers and flattens its domes, increases the thoracic vertical dimension and displaces abdominal contents so the relaxed abdominal wall moves outwards. Its lower-rib attachments also contribute to expansion of the costal margin. The phrenic nerves, chiefly C3–C5, provide its motor supply. The external intercostal muscles elevate and stabilize the ribs, while the interchondral or parasternal portions of the internal intercostal muscles also assist inspiration. It would be wrong to give every internal intercostal fibre an exclusively expiratory function.

Elevation of upper ribs and the sternum increases the anteroposterior dimension, the pump-handle component. Outward movement of the lateral portions of lower ribs increases transverse diameter, the bucket-handle component. These are components of oblique rib rotation rather than two mechanically isolated systems. Abdominal or diaphragmatic breathing makes descent of the respiratory diaphragm particularly conspicuous; costal breathing makes chest-wall expansion more conspicuous. Both contribute in ordinary respiration, with their proportions changing with posture, mechanical loading and individual anatomy.

As the thorax expands, pleural pressure becomes more negative and transpulmonary pressure increases. The lungs expand and alveolar pressure briefly falls below atmospheric pressure, admitting air. Relaxation of inspiratory muscles lets elastic recoil reverse the flow in quiet expiration. The abdominal wall and respiratory diaphragm return towards their resting arrangement without requiring active abdominal compression for each quiet breath.^[ref]

With deeper inspiration, the scalene muscles help stabilize or elevate the upper ribs, and the sternocleidomastoid muscle can elevate the sternum when the head is fixed. With the upper limbs or shoulder girdle fixed, muscles such as pectoralis minor and serratus anterior can assist expansion in greater effort. Recruitment is graded; an anatomical list of possible accessory muscles does not mean all are active in every ordinary breath.

Active expiration uses the abdominal muscles, including rectus, oblique and transversus components, to compress the abdominal contents and drive the respiratory diaphragm upwards. The interosseous portions of the internal intercostal muscles help depress the ribs. This is important in vigorous exercise and expulsive actions. At rest, elasticity supplies much of the expiratory work; during high demand, active muscle contraction adds to it.

The observable pattern also changes with the physiological task. Exercise increases breath depth and usually frequency. A sigh is an enlarged inspiration followed by expiration; yawning includes a prolonged deep inspiratory movement. Coughing coordinates an inspiration, closure of the glottis, pressure-generating expiratory effort and sudden glottic opening. Speech imposes a controlled prolonged expiration with laryngeal regulation. Swallowing briefly interrupts ordinary airflow. Thus respiratory observation should include phase, depth, frequency and coordination, not merely count which ribs move. Fixed claims that men must breathe abdominally and women must breathe costally overstate a variable tendency. Pregnancy or a change of posture can alter mechanical contributions without replacing the basic pressure mechanism.

The nervous system regulates visceral movement, glandular secretion, blood flow and coordinated bodily responses, while also influencing nutrition indirectly through those functions. Organic or vegetative life here means activities maintaining the body, such as blood circulation, digestion, pulmonary ventilation, secretion and excretion. It does not designate an independent form of living substance.

One influence is directly motor. Autonomic nerves alter the rate and force of the heart, contract or relax smooth muscle in the intestine, urinary bladder, airways and blood vessels, and coordinate movement with the opening and closure of sphincters. Their effect need not be excitatory: an active vagus nerve can slow the heart, while other pathways accelerate it. In the modern account, sympathetic and parasympathetic effects depend on the target and its receptors; they are not respectively universal accelerator and brake systems. Some organs receive both divisions, while others do not have a simple balanced double supply.^[ref]

A second influence is secretomotor. Activity in appropriate nerves can increase salivary secretion, alter gastric acid secretion and promote sweating. This is distinguishable from moving a duct or changing its calibre. A convincing experiment collects a secretion from a cannulated duct while a nerve is stimulated, rather than inferring secretion from a change of colour or organ size. For example, stimulation of the chorda tympani can evoke fluid from the submandibular gland. The response depends on functioning secretory biological cells; the nerve communicates a command rather than supplying the material constituents of saliva.

A third influence is vasomotor. Changing the calibre of arterioles changes local vascular resistance and perfusion. The classic comparison of an ear before and after interruption of its cervical sympathetic supply illustrates loss of vasoconstrictor tone: the vessels dilate and the ear becomes warmer because more warm blood reaches it. Stimulation of the peripheral cut pathway can produce the converse vascular effect. Warmth therefore need not mean that the local nerves themselves generate more heat. Vascular control also changes the delivery of oxygen and nutrients to a gland or tissue, so it can modify function indirectly.

These outputs are organized by sensory feedback and reflexes. Distension of the intestine can evoke coordinated movement; changes in blood pressure or blood-gas composition influence heart rate and breathing. Local circuits in the enteric nervous system permit some digestive activity without an intact connection to the brain, while longer reflexes involve the spinal cord or brainstem. Central states also matter: the sight or expectation of food can initiate preparatory digestive responses, and fear can alter heart rate, sweating and visceral movement.^[ref]

Somatic motor activity also supports vegetative functions. Breathing uses the respiratory diaphragm and other skeletal muscles; swallowing and continence use coordinated skeletal muscle as well as smooth muscle. Thus organic function cannot be identified solely with the anatomical distribution of the sympathetic nervous system.

The historical idea of a direct trophic influence requires discrimination. A denervation region may change through loss of movement, altered perfusion, loss of protective sensation or changed secretion. Those observations alone do not establish a special nutritive fluid flowing down the nerve. Modern biology recognizes molecular signalling between nerves and their targets, as well as neuroendocrine signalling pathways: the hypothalamus can influence hormone release, and sympathetic input can evoke adrenal medulla secretion. These are mechanisms added to, rather than evidence for an undefined historical nutritive force. A useful experimental design measures the motor, vascular and secretory consequences separately before assigning a residual effect to direct trophic action.

The liver has a substantial normal range of size, determined by age, body dimensions, nutritional state and its variable contents; there is no universal absolute minimum and maximum valid for every healthy person. One must also distinguish true organ volume from the position of its lower border, which moves with the respiratory diaphragm during breathing.

An ordinary adult liver is on the order of 1.5 kg and roughly a few percent of body mass, but neither figure is a diagnostic boundary. As an explicitly modern example, an autopsy study of adult men reported a mean hepatic mass of 1561 g and observed values from 838 to 2584 g in its selected population. Those are sample observations, not a declaration that every liver inside those limits is healthy or that every one outside is diseased. A newer large-sample study similarly found an association of liver mass with body mass; much of an apparent sex difference disappears when body size is taken into account.^[ref][ref]

Age changes the proportion as well as the absolute size. The fetal and infant liver is relatively large; the adult organ is smaller in proportion to the whole body. Its fetal role in haematopoiesis and subsequent changes of growth and function make an adult mass standard inappropriate for an infant. Constitutional body size and normal anatomical shape also vary: a long lower projection is not necessarily the same thing as an enlarged total organ.

Reversible differences within one person arise because the liver is a store and a vascular organ:

A meal therefore changes hepatic activity and splanchnic blood flow without requiring a large immediate increase in total liver volume. In a modern paired study of 20 healthy volunteers, single-photon emission computed tomography before and after a standardized meal showed a nonsignificant 1.5% decrease in estimated liver volume. The authors judged the change to be within methodological error. This is a useful check against assuming that greater digestive activity proves gross swelling.^[ref]

The experimental approach is to compare like with like: specify age and body mass, use a fixed interval after food, standardize hydration, posture and respiratory phase, and distinguish a volume measurement from a palpable edge or a single linear dimension. Magnetic resonance imaging can measure volume while magnetic resonance spectroscopy measures glycogen or lipid signals; the two observations answer different questions. The size of an excised organ additionally depends on how much blood remains in it. The physiological point appropriate to the 1866 question is reversible variation with growth, alimentation and vascular filling. Modern imaging results refine that point; they are not numerical standards that the historical exam could have expected.

Chyle is intestinal lymph enriched with absorbed dietary lipids; its usual route ends in the venous circulation at the left jugulosubclavian junction. The milky appearance after a fat-containing meal is chiefly due to suspended lipid-rich particles, now called chylomicrons.

Formation begins with lipid digestion in the small intestine. Pancreatic lipase helps hydrolyse dietary triglycerides; bile salts help the digestion products reach the intestinal absorptive surface in mixed micelles. Enterocytes take up the products, reconstruct much of the long-chain triglyceride, and package it with other lipids and apolipoproteins into chylomicrons. These leave the basolateral side of the enterocytes and enter the central lacteals of the intestinal villi. Thus the particles carried in chyle are not simply intact globules of a meal passing through an open hole from the intestinal lumen.^[ref]

From the lacteals, chyle enters lymphatic channels in the intestinal wall and then collecting vessels in the mesentery. These vessels pass through mesenteric lymph nodes, with afferent vessels entering and efferent vessels leaving the nodes. Successive collectors join the intestinal lymphatic trunk or trunks. They usually communicate with the cisterna chyli, an expanded origin of the thoracic duct in the upper lumbar region, near the bodies of L1–L2 and to the right of the aorta. The cisterna chyli also receives lymph from lumbar trunks; the stream beyond it therefore contains material from more than the intestine. A conspicuous sac is not present in every individual.

The thoracic duct passes through the aortic hiatus of the respiratory diaphragm and ascends in the posterior mediastinum, typically between the aorta and azygos vein, behind the oesophagus. It crosses towards the left in the middle or upper thorax, continues to the root of the neck and arches to its terminal venous opening. The usual endpoint is the left venous angle, where the left internal jugular vein joins the left subclavian vein; terminal branches and the precise opening show anatomical variation. It is not normally the right venous angle or a direct entrance into the portal vein.^[ref]

The flow is promoted by intrinsic contraction of collecting lymphatic vessels, movements of surrounding muscles and respiratory changes of pressure. Lymphatic valves limit backward movement. Lymph nodes are interposed biological structures, not pumps that propel every particle independently.^[ref]

This route applies particularly to the long-chain dietary lipids carried by chylomicrons. Most absorbed glucose and amino acids, and much shorter-chain fatty acid material, follow portal vascular routes instead. Confusing these pathways would obscure the reason for asking separately about chyle. In a classical anatomical demonstration, the mesenteric channels of an appropriately fed animal appear white, and an injected or dissected preparation can trace their continuity towards the thoracic duct. The timing after a fatty meal matters: an empty-looking channel in a fasted preparation does not prove absence of a lymphatic vessel. The molecular account of chylomicrons is a modern explanation of the older gross observation.

Ordinary inspiration is active enlargement of the thorax; ordinary expiration is chiefly passive recoil after the inspiratory muscles relax. The movements increase and decrease the vertical, anteroposterior and transverse dimensions together. Their relative prominence varies with posture and the individual; there is no exclusive division into abdominal and costal kinds of breathing.

The principal inspiratory muscle is the respiratory diaphragm. Its peripheral fibres shorten towards the central tendon, lowering and flattening its domes. This increases the vertical dimension of the thorax and displaces the abdominal contents downwards and forwards, making the relaxed abdominal wall project. Its costal attachments also act on the lower ribs: with abdominal support opposing unlimited descent, shortening can lift and expand the lower costal margin. Thus the respiratory diaphragm does more than pull down a piston. Its motor supply is through the phrenic nerves, principally from cervical segments C3–C5.

The external intercostal muscles elevate the ribs and help keep the intercostal spaces from being drawn inwards as pleural pressure falls. The interchondral, or parasternal, portions of the internal intercostal muscles also contribute to inspiration. It would therefore be inaccurate to label every part of the internal intercostal muscles expiratory. Coordinated activity braces the rib cage so that the shortening of one layer moves the ribs rather than simply deforming the spaces between them.

Rotation about the posterior articulations raises the anterior ends of the upper ribs and the sternum, increasing the anteroposterior diameter: the usual pump-handle description. The more laterally directed portions of the lower ribs rise and move outwards, increasing the transverse diameter: the bucket-handle description. These are descriptions of components of an oblique movement, not two independent sets of hinges. The scalene muscles can stabilize and elevate the uppermost ribs, with recruitment varying in quiet breathing. The sternocleidomastoid muscle, when the head is fixed, is an accessory elevator of the sternum in greater inspiratory effort; it is not required for an ordinary quiet breath.

The lungs follow the expanding chest wall because their surfaces remain coupled to it through the thin fluid-filled pleural cavity. More negative pleural pressure increases the distending transpulmonary pressure; enlargement of the alveoli briefly makes alveolar pressure lower than atmospheric pressure, so air enters. At the end of inspiration, airflow stops when these last two pressures equalize. During quiet expiration, relaxation of the respiratory diaphragm and inspiratory intercostal muscles allows the stretched lungs and chest-wall system to recoil towards their resting balance. The domes rise, the ribs and sternum descend, and alveolar pressure becomes slightly positive relative to atmospheric pressure while air leaves. Active abdominal muscles and the interosseous portions of the internal intercostal muscles become important in forced expiration, rather than supplying the main force in quiet expiration.^[ref]

A simple mechanical demonstration uses two elastic bags in an airtight transparent vessel, connected through its stopper to the air, with a movable rubber sheet forming the base. Pulling the sheet down expands the vessel and inflates the bags; letting it return allows deflation. A side-tube manometer can show the associated pressure change. This illustrates expansion by lowering the floor, but its rigid vessel cannot demonstrate the movements of the ribs, the action of the intercostal muscles, or the real geometry of the pleural cavity. Observing lower-chest widening and abdominal displacement during quiet breathing supplies those missing observations without confusing the model with the anatomy.

The two pneumogastric nerves are the right and left vagus nerves, the tenth cranial nerves: mixed sensory, branchial motor and parasympathetic pathways from the medulla oblongata to the neck, thorax and abdomen. A useful description separates their apparent origin at the surface of the brain from the origins and terminations of their component fibres.

Rootlets emerge in the groove behind the inferior olive, between the glossopharyngeal nerve above and the accessory rootlets below. Branchial motor fibres arise chiefly in the nucleus ambiguus; preganglionic parasympathetic fibres arise in the dorsal motor nucleus of the vagus and, particularly for cardiac control, the nucleus ambiguus. Visceral sensory fibres have their cell bodies chiefly in the inferior vagal ganglion and terminate centrally in the nucleus of the solitary tract; somatic sensory fibres associated with the superior vagal ganglion project to the spinal trigeminal nucleus. Consequently a sensory fibre is not accurately described as originating in a motor nucleus simply because its peripheral nerve emerges nearby.^[ref]

Each vagus nerve leaves the skull through the jugular foramen, with the glossopharyngeal nerve and accessory nerve. It bears the small superior vagal ganglion at the foramen and the larger inferior vagal ganglion below it. Connections include communications with adjacent cranial nerves and the cervical sympathetic nervous system. Traditional descriptions assign fibres joining the vagus nerve from the cranial part of the accessory nerve to the latter peripheral nerve; their modern classification is more nuanced and does not change the peripheral branches being described. In the neck the vagus nerve descends in the carotid sheath, behind and between the internal jugular vein and the internal, then common, carotid artery.

Its meningeal and auricular branches supply small sensory territories of the cranial covering and external ear. Pharyngeal branches contribute to the pharyngeal plexus, supplying much of the soft palate and pharynx; the stylopharyngeus muscle belongs to the glossopharyngeal nerve and the tensor veli palatini muscle to the mandibular division of the trigeminal nerve. The internal branch of the superior laryngeal nerve supplies sensation above the vocal folds; its external branch supplies the cricothyroid muscle. The recurrent laryngeal nerve supplies the other intrinsic laryngeal muscles and sensation below the vocal folds. The right recurrent laryngeal nerve loops beneath the right subclavian artery; the left loops under the aortic arch beside the ligamentum arteriosum. Both return towards the larynx near the groove between trachea and oesophagus.^[ref]

In the thorax, both vagus nerves pass behind the roots of the lungs, contrasting with the anterior course of the phrenic nerves. Cardiac branches join the cardiac plexus; pulmonary branches join the pulmonary plexus. The trunks break up into the oesophageal plexus, then collect principally as an anterior trunk derived from the left vagus nerve and a posterior trunk derived from the right. Through the oesophageal hiatus they reach the stomach; hepatic and coeliac branches distribute to abdominal viscera. Their parasympathetic territory includes foregut and midgut derivatives, extending into the proximal colon, rather than every abdominal and pelvic organ.^[ref] This extensive distribution explains why the level of experimental section is indispensable information.

The main consequences of section can be organized by the interrupted functions:

A discriminating historical experiment would record respiratory movements and heart rate before and after section, distinguish unilateral from bilateral section, and compare an intact airway with an airway bypass. A tracheal cannula can separate obstruction at the glottis from changes in the respiratory rhythm, while the central-versus-peripheral stimulation comparison separates incoming reflex traffic from outgoing motor effects. Later separate-lung recordings confirmed that a change following unilateral section could affect both lungs, supporting a central reflex interpretation rather than simple local paralysis.^[ref] The gastric distinction is also borne out by measurements of persistent acidity after surgical vagotomy, despite major reductions in stimulated gastric acid secretion.^[ref]

The heart supplies the principal continuing mechanical input to blood circulation; arterial recoil, respiratory movements and the action of skeletal muscles redistribute and supplement that input. They have different sources of energy and should not all be called independent propulsive powers. The pressure gradient remaining after passage through the capillaries is still a consequence of the upstream pump.

Arterial elasticity. During ventricular systole, ejected blood expands the aorta and large arteries, storing elastic potential energy in their walls. During diastole, recoil continues to drive blood into the peripheral vessels while closure of the aortic valve limits return towards the heart. This reduces the intermittency of flow downstream. The recoil is a temporary store of cardiac work, rather than a fresh source of energy. A vessel that is too rigid provides less of this smoothing; a compliant vessel with an open backward outlet would not provide the same useful result.

The skeletal-muscle pump. Contracting skeletal muscles compress adjacent veins. Venous valves make the displacement preferentially towards the heart; when compression ceases, refilling permits another stroke. Walking therefore aids return from dependent limbs. The source of the added work is muscle contraction, and the valves guide it without producing energy. Merely possessing valves cannot make a stationary column circulate.

The respiratory pump. During inspiration, falling intrathoracic pressure and displacement of the abdominal contents favour a pressure gradient from abdominal and peripheral veins towards the thoracic veins and right side of the heart. The respiratory diaphragm thus participates indirectly in venous return as well as pulmonary ventilation. The exact flow varies with posture, vessel collapse and the phase of the respiratory cycle; positive-pressure inflation does not reproduce the same mechanical circumstances as spontaneous inspiration.^[ref]

Venous tone and cardiac filling. Sympathetic contraction of venous smooth muscle can reduce venous storage capacity and transfer blood towards the central circulation. This alters the volume available for filling rather than requiring a wave of venous peristalsis. Cardiac relaxation, recoil and movement of the atrioventricular plane also affect filling pressure. Some traditionally described cardiac suction is therefore a feature of the cardiac cycle and mechanical coupling, not a separate organ pulling blood through every vessel.^[ref]

Gravity and supposed capillary attraction. Gravity assists a downward column but opposes an upward one. In a complete circuit returning to its original height it supplies no net gravitational work; its important practical effects include hydrostatic pressure differences and dependent venous pooling. Ordinary capillary action in an open narrow tube, driven by a liquid meniscus, is not a general pump in a fluid-filled capillary network. Nor does a vague attraction of blood by tissues replace a measurable pressure gradient. Local vasodilation can increase perfusion by lowering vascular resistance, but that change does not manufacture the necessary pressure source.

For a simple mean-flow description, with $Q$ the volume flow rate, $\Delta P$ the driving pressure gradient and $R$ the effective vascular resistance,This relation distinguishes a source of driving pressure from a change of resistance. In a lumped compliant arterial reservoir with vascular compliance $C$ and no inflow between beats, volume balance giveswhere $P_v$ is the downstream pressure. The exponential decay follows from stored volume leaving through the resistance. It explains continued flow during diastole without claiming that the idealization describes every reflected arterial wave.

A physical demonstration can compare a pulsatile pump feeding a narrow outlet through rigid tubing with the same pump feeding it through an elastic reservoir. Collecting the outflow over the pumping and resting phases reveals the reservoir's smoothing action; a side-arm manometer records its falling pressure. Separately compressing a valved flexible tube shows the role of a peripheral muscle pump. Keeping pump stroke and outlet resistance comparable prevents the apparatus from conflating a smoother flow with creation of extra energy.

Boyle's law states that a fixed amount of gas at constant temperature obeyswhere $p$ is absolute pressure, not gauge pressure. It is an ideal-gas approximation, not an assertion that every rapid compression is isothermal.

The respiratory diaphragm contracts and lowers its dome, increasing thoracic volume. Mechanical coupling through the pleural space lets the lungs expand; alveolar pressure falls below atmospheric pressure, drawing air in. Relaxation of the respiratory diaphragm and elastic recoil drive quiet expiration. These respiratory mechanics describe ventilation, distinct from chemical cellular respiration.^[ref]

For the stipulated sealed, isothermal gas, initial and final volumes are $1600$ and $2000\mathrm{c}{\mathrm{m}}^3$. Expressing absolute pressure as a mercury-column height,The mercury column is supported by the atmospheric-to-internal pressure difference:The specified volume change belongs to the historical idealized model. Real tube-air volume, temperature changes and limits on respiratory-muscle pressure generation would affect a physical apparatus.

Motor supply to the respiratory diaphragm, arising chiefly from cervical spinal segments C3–C5.

Respiratory mechanics relates the movements of the thorax and respiratory diaphragm to pressure changes and airflow.