Table of Contents

The historical eponymous gas laws as identified below have subsequently been combined into the unifying Ideal Gas Law, which is discussed last. A brief summary of the important features is available elsewhere, for the purposes of revision. This chapter is more of a reflective digression on the history of gas research, lingering delicately on the important roles played in it by lamb bladders and mobs of angry French villagers.

The principles governing the behaviour of gases in solution are fundamental to the understanding of gas exchange and gas transport in the blood. The major topics of this chapter are Dalton's and Henry's Laws, and the influence of temperature on the solubility of gases in body fluids.

Cell membranes are not particularly water-impermeable, and so cells are prone to shrinking and swelling as they are battered by osmotic forces. Furthermore, they are affected by the Gibbs-Donnan effect, which results in increased intracellular osmolality. Regulatory mechanisms exist, which allow them to compensate for anisotonic extracellular fluids. These mainly consist of changes to the ion permeability of the cell membrane, active transport of ions, and modifying the concentration of organic intracellular solutes ("idiogenic osmoles")

Mitochondria are the most complex organelle, a "domesticated" bacterial species used by eukaryotes for energy production. Structurally, they consist of two membrane layers and a proteinaceous inner matrix. Their functions consist of ATP synthesis, haem metabolism, urea metabolism, production of reactive oxygen species, participation in apoptosis, and numerous others.

The intracellular organelles are minor players on the stage of Intensive Care Medicine, and it is no wonder that the college has never raised these in a question (outside of the example of mitochondria). For some nighmarish future scenario where the ICU trainee might be asked an exam question on this topic, this chapter offers short notes on what the organelles do, and what they are supposed to look like. Particular emphasis is placed upon the possibility that labelled diagrams might one day be asked for. The possibility of this is quite remote, and one can safely skim over this section, expecting never to be questioned on the structure or function of the Golgi apparatus.

The Gibbs-Donnan effect is the phenomenon of predictable and unequal distribution of permeant charged ions on either side of a semipermeable membrane, in the presence of impermeant charged ions. On each side of the membrane, each solution will be electrically neutral; and the product of diffusible ions on one side of the membrane will be equal to the product of diffusible ions on the other side of the membrane. At a Gibbs-Donnan equilibrium, there will be an osmotic diffusion gradient attracting water into the compartment which contains impermeant ions.

The Gibbs-Donnan effect is the phenomenon of predictable and unequal distribution of permeant charged ions on either side of a semipermeable membrane, in the presence of impermeant charged ions. On each side of the membrane, each solution will be electrically neutral; and the product of diffusible ions on one side of the membrane will be equal to the product of diffusible ions on the other side of the membrane. At a Gibbs-Donnan equilibrium, there will be an osmotic diffusion gradient attracting water into the compartment which contains impermeant ions.

Before allowing the gentle reader to proceed, I should mention that specifically for the CICM trainees there is in fact an Official Blood Gas Diagnostic Sequence, promoted by Oh's Manual. This sequence is the nearest thing to a gold standard for blood gas interpretation and should probably be used to answer all those ABG-related CICM Fellowship SAQs.

The resting membrane potential is the voltage (charge) difference between the intracellular and extracellular fluid, when the cell is at rest (i.e not depolarised by an action potential). It is generated by the opposing effects of chemical concentration gradients drivingionic diffusion across a selectively permeable membrane, and electostatic forces which oppose these gradients.  Specific ion contributions to this equlibrium can be calculated from the Nernst equation, and the net effect of all ions is represented by the Goldman-Hodgkin-Katz equation.  At equilibrium, the cell membrane has a slightly negatively charged inner surface (-90 mV). 

Transport across the cell membrane can occur by diffusion or by active transport. Diffusion can take the form of simple passive diffusion or "facilitated" diffusion which occurs via pores or protein channels. Active transport can be primary (where ATP is used as the energy source) or secondary (where concentration gradients are used as the source of energy). Endocytosis and exocytosis are mechanisms of delivering larger molecules into and out of the cell, and involve the transport of such molecules inside membrane-bound vesciles.

The cell membrane is a physical and chemical barrier which separates the inside of the cell from the outside environment. It is a soft, flexible liquid bilayer of lipid with embedded proteins (a "fluid mosaic"), 5 nm thick, which contains proteins, lipids, polysaccharides, water and adsorbed ions.

The non-cardiopulmonary effects of positive pressure ventilation include raised intracranial pressure, water retention, sodium retention, decreased renal perfusion, decreased hepatic perfusion, decreased intestinal motility and poor gastric emptying, increased risk of stress ulceration, neutrophil retention in the pulmonary capillaries and impaired lymphatic drainage from the lungs.

This chapter focuses on the physiological effects of CO₂, taking care to view it as a drug, including pharmacokinetics and pharmacodynamics. In summary, CO₂ is a potent inotrope and vasopressor which exerts its effects mainly by being an indirect sympathomimetic. It is also an anaesthetic, a bronchodilator, a cerebral vasodilator, and it causes QT prolongation.

Volume of distribution is a pharmacokinetic concept which is used to describe the distribution of drugs in the body as relative to the measured concentration. In brief, it is the apparent volume into which the drug appears to be distributed when only the sample concentration is considered. It is a purely theoretical volume, which can substantially exceed the total body volume, or potentially even be infinite in size. Among many other uses, the volume of distribution (V_D) plugs into loading dose calculations. It can also help you decide instantly whether your drug is going to be easily cleared by dialysis.