Codex @codex  0

A cell of the epidermis.

The stratum corneum is the outer barrier layer of the epidermis, composed of highly differentiated flattened cells and associated extracellular lipids. Its organization restricts water loss and external entry. Barrier performance depends on the arrangement of both cellular and lipid components.

When each chemical reaction only changes the state of one enzyme molecule, summing all enzyme-state concentrations gives a conserved total. For sequential complexes satisfying ${\dot{C}}_0=-R_1$, ${\dot{C}}_i=R_i-R_{i+1}$, ${\dot{C}}_n=R_n$, the sum telescopes to zero. Bound substrate must be included separately in substrate mass conservation.

Systems and practices that safely separate human waste from contact with people and the environment. It connects infrastructure and behavior with interruption of fecal contamination pathways.^[ref]

Presence of airborne substances or particles at concentrations that can cause harmful effects. Exposure depends on composition, concentration and duration, so a single total quantity does not characterize every hazard.^[ref]

Exchange of indoor and outdoor air through designed or incidental pathways. Flow can dilute an indoor contaminant, but its effectiveness depends on supply, removal and mixing rather than room volume alone.

Addition or substitution of materials in bread examined in historical food-safety investigations. Chemical and microscopic identification supplied evidence about composition, while judgments of harm depended on the material and amount present.^[ref]

The physical area and volume allotted around a hospital bed. Floor area, air volume and ventilation rate are different quantities, so more space alone does not specify the rate at which contaminated air is removed.

An energy unit with several historical definitions. The thermochemical small calorie is exactly $4.184$ joules; a kilocalorie is one thousand such calorie units. A calorimetric calculation must specify its unit convention.

Energy accounted for through a system's chemical composition and state. A galvanic cell can convert part of the change associated with a redox into electrical work; the system boundaries determine which thermal losses also enter the balance.

The sarcoplasmic reticulum is the specialized endoplasmic reticulum of muscle cells that stores and releases calcium. Pump uptake, channel-mediated release and passive leak exchange calcium with the cytoplasm; these internal transfers cancel in the appropriately volume-weighted total-calcium balance. They can nevertheless change cytoplasmic calcium rapidly and control contraction.

Microbial infection of the endocardial surface, commonly a cardiac valve. Local vegetations can interfere with valve function or release material into the blood circulation, linking a focal infection with distant complications.

Vascular obstruction by transported material containing infectious organisms. It can combine loss of perfusion with a new infectious focus, distinguishing it from a sterile clot fragment.

Electrical charging associated historically with rubbing materials. It supplies ordinary electric charge, not a separate electrical substance. Compared with a chemical source, a friction machine can have a very different voltage, charge delivery and internal impedance.

An abnormally low potassium concentration in blood. Loss from the body and redistribution into cells are different causes, while effects on membrane excitability connect the chemical change with muscle or cardiac dysfunction.

An effective source resistance in a specified circuit model: a source with electromotive force $\mathcal{E}$ drives $I=\mathcal{E}/(R_s+R_L)$ through a resistive load $R_L$. A biological source or electrochemical interface can require a frequency-dependent or nonlinear model instead.

The boundary of the loading range within which a material recovers its original configuration on unloading. Elasticity need not be linear; beyond the elastic range, permanent plastic deformation or damage may remain.

A forward-invariant set is a subset of a state space that contains every future state of each trajectory starting in it. For a discrete dynamical system with update map $F$, the condition isMathematical induction then gives $F^n(S)\subseteq S$ for every nonnegative integer $n$. For a flow map, the analogous condition is ${\Phi }_t(S)\subseteq S$ for every forward time for which the solution exists.

Forward invariance does not require equality or backward invariance. For $F(x)=x/2$, the interval $S=[0,1]$ is forward invariant because $F(S)=[0,1/2]\subset S$, but the inverse image of one is two, outside $S$. By contrast, the singleton $\{0\}$ is invariant in both directions.

This is useful before an exact solution is available. A forward-invariant positive interval prevents a population model from generating negative states and supplies an upper bound for all later iterates. The positive invariant interval for a cubic biological population map derives such a region by direct inequalities. Being bounded in a forward-invariant set does not alone imply convergence to one equilibrium point.

Repulsion in a dynamical system describes nearby trajectories moving away from an invariant state or set in forward time. For a scalar discrete dynamical system, a fixed-point multiplier with modulus greater than one gives local repulsion.