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Exhaled carbonic acid here means carbon dioxide; its amount per unit time must be distinguished from its percentage in expired air. A person can breathe a larger volume of more dilute expired gas and still exhale more total carbon dioxide.

At metabolic steady state, pulmonary carbon dioxide output approximately reflects its production by cellular respiration. Increased muscular activity is therefore a major physiological cause of increased output: walking, climbing and strenuous exercise increase substrate oxidation and usually increase both ventilation and total carbon dioxide elimination. Rest lowers the requirement; during ordinary sleep, reduced activity and metabolic demand generally lower output relative to the waking active state. A comparison needs the same time interval and should not confuse a short transient with a sustained resting measurement.

Food affects output in two ways. Digestion, absorption and processing increase metabolic expenditure after a meal, with a time course depending on the meal. The material oxidized also matters. The respiratory quotient is the ratio of metabolic carbon dioxide production to oxygen use. For complete oxidation of glucose,A typical fatty acid gives less carbon dioxide per unit oxygen consumed; for palmitic acid,These are substrate comparisons, not a rule that any high-fat meal immediately reduces a person's total daily output. Fasting changes fuel selection as well as metabolic rate, so both processes must be considered.^[ref]

Cold exposure increases output when maintaining body temperature requires extra thermogenesis, especially shivering. Moving from cold to a thermally comfortable environment can reduce that extra production. Heating does not simply increase or decrease output in proportion to air temperature: passive warming, heat stress, activity and the thermoregulatory response differ. Growth, body size, age, pregnancy and lactation can change whole-body metabolic requirements. Comparisons between people therefore need to specify whether output is absolute or normalized to body mass or another appropriate size measure. Biological sex alone is not an explanation independent of these factors. Daily variation also mixes circadian physiology with sleep, meals and activity.

Ventilatory changes require a separate account. Voluntary hyperventilation can initially wash carbon dioxide out of body stores faster than it is being produced; breath holding temporarily prevents its pulmonary escape while production continues. Subsequent recovery changes the output again. The ratio measured at the mouth, the respiratory exchange ratio, need not equal the tissue respiratory quotient during those transitions or during intense exercise with acid buffering. Reduced expired output for a few seconds is therefore not evidence that tissue metabolism has stopped.^[ref]

A measurement should collect total expired gas over a known time and determine its dry carbon dioxide fraction, allowing for inspired carbon dioxide. With inspired and expired dry gas volumes expressed under the same reference conditions,Here ${\dot{V}}_E$ and ${\dot{V}}_I$ are expired and inspired volume rates and the $F$ terms their respective fractions. Recording food intake, physical activity, ambient temperature and time since the preceding intervention makes the comparison interpretable. Carbon dioxide output is a measured flux, not merely an observation that an animal makes a gas collection vessel cloudy.

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.