Factors which affect airway resistance relate to the components which make up Reynold's number, i.e. the airway diameter, airway length, gas density, gas flow rate, and everything which affects these variables. Increasing lung volume also tends to decrease airway resistance.
Amiodarone is a Class III antiarrhythmic with Class I, Class II and Class IV effects. Given acutely as an infusion, it mainly acts as a beta-blocker and calcium channel blocker, increasing the refractory period of the AV node and therefore counteracting supraventricular tachycardias. Prolonged therapy reveals more of a Class I and Class III effect, which decreases the velocity of action potential propagation and decreases the risk of reentry from ectopic minipacemakers. Amiodarone has uniquely tenacious pharmacokinetics, is extensively tissue bound, and has a half life measured in tens of days. ,/p>
West's Zones of the lung are physiologically distinct but anatomically blurry regions which represent different relationships between alveolar pressure, arterial pressure and venous pressure. Zone 1 is characterised by minimal perfusion (it is essentially dead space), Zone 2 is perfused intermittently, and Zone 3 has good perfusion and receives most of the blood flow. An additional fourth zone represents the region of lung which is collapsed and in which interstitial fluid pressure is the dominant Starling resistor.
Tests of liver function include tests of liver enzymes which leak following hepatic injury, and tests of the synthetic and metabolic functions of the liver. Of the latter, albumin, glucose, bilirubin, ammonia and coaguilation studies are the most commonly used. In all cases, failure of liver function is not the only possible explanation for an abnormal biochemistry result.
Cardiac output is defined as the volume of blood ejected by the heart per unit time. It is usually presented as [stroke volume × heart rate], in L/min. Its main determinants are heart rate and stroke volume. Stroke volume, in turn, is determined by preload afterload and contractility, each of which have their own determinants, and each of which influence one another.
Local anaesthetics are voltage-gated sodium channel blockers that must penetrate the cell to act on the inside part of the channel (locking it in its inactive state). This means their lipid solubility is the most important determinant of potency. Their most interesting pharmacodynamic properties are use-dependent block (which increases with neuronal activity) and differential block, which is the phenomenon of their selectivity for pain and temperature fibres.
Compartment models simulate drug absorption distribution and elimination. They are a convenient oversimplification used to predict the concentration of a drug at any given time in any given body fluid or tissue. A single compartment model is the least accurate, as it assumes a homogeneous distribution of the drug in the body. A two-compartment model is a satisfactory oversimplification to produce reasonably accurate results, as it takes into account the redistribution of drugs into the tissues and back out of the tissues. A three-compartment model is the most useful for anaesthetic substances as it discriminates between fast-redistributing tissues (muscle) and slow tissues (fat). The net effect of having multiple compartments is that there is an initial fast distribution phase, followed by a slower elimination phase during which the concentration is maintained by redistribution from drug stores in the tissues.
The hypothalamus and pituitary are small CNS endocrine organs that are involved in the regulation of numerous processes, among them the autonomic nervous system, temperature, body water volume, reproductive function, emotion, metabolic rate, satiety, and stress responses.
A portal system is two capillary beds not separated by the heart, i.e. some sort of conduit "portal" vessel connects the two sets of capillaries. The three main portal systems are the hepatic, pituitary and renal.
Glucose release and storage are regulated by the balance between the actions of insulin and glucagon. Insulin promotes glucose uptake and storage, whereas glucagon promotes glucose release from stores and glucose synthesis from amino acids and lactate. The pancreatic β-cells and α-cells are the main sensory organ for the detection of glucose, with the hypothalamus playing a minor role in the control of satiety and autonomic responses to hyper and hypoglycaemia.
Adrenaline is an endogenous catecholamine, a sympathomimetic drug with little receptor selectivity. In its role as a haemodynamic support infusion, adrenaline acts as an inodilator, increasing cardiac output and increasing blood pressure while decreasing the afterload (mainly by its effect on the arterioles of the skeletal muscle). It also has several unpleasant side effects (hyperglycaemia, hypokalemia, lactate elevation) which decrease its popularity somewhat.
Glucagon is a counterregulatory peptide hormone that is released from pancreatic α-cells in response to hypoglycaemia, as well as a range of other modulatory stimuli. Its main effects are experienced by the liver, which produces glucose in response - mainly as the result of accelerated glycogenolysis and gluconeogenesis.
The liver stores metabolic fuel as glycogen (which can be rapidly mobilised) and fat (which can be slowly mobilised). There may be 75-100g of glycogen (400 kcal) and up to 75g (675 kcal) of fat in a normal liver, with more fat being deposited in times of dietary carbohydrate excess. The liver also stores micronutrients such as fat-soluble vitamins (A, D, E and K), the water-soluble vitamin B12, iron (as ferritin and haemosiderin), and trace elements (zinc, copper, selenium, etc). The liver also acts as a venous blood reservoir used to buffer fluctuations in blood volume.
Somatostatin is a counter-secretory hormone which acts as the antagonist for the secretory activity of most endocrine and exocrine glands. It is secreted from numerous tissue sources, including the intestine (65%) brain (25%) and pancreas (5%). The main mechanism of effect is mediated by the inhibition of adenylyl cyclase, which decreases cAMP concentrations and inhibits exocytosis.