Final answer:
Positive and negative factors can affect heart contractility, and aspects such as the Frank-Starling mechanism and baroreceptor responses influence stroke volume and cardiac output. Drug doses in children are calculated based on weight or surface area, taking into account pharmacokinetic factors such as age, gender, genetics, and diet.
Step-by-step explanation:
Heart contractility can be influenced by various factors which can be categorized as either positive or negative. Positive factors that increase heart contractility include stimulants such as adrenaline that enhance the calcium availability in the heart muscle, thereby increasing the strength of contraction. On the other hand, negative factors such as acidosis or hypoxia can decrease contractility by altering the acid-base balance or oxygen supply to the myocardium.
Stroke volume and cardiac output are also affected by multiple factors. The Frank-Starling mechanism plays a pivotal role by dictating that the more the heart fills with blood during diastole, the more forcefully it contracts. Additionally, changes in blood volume and pressure are sensed by baroreceptors, which adjust the heart rate and contractility in response to maintain adequate organ perfusion and blood pressure. For instance, a decrease in blood volume will trigger baroreceptors to increase heart rate and contractility to compensate.
Drug doses for children are often calculated based on body weight or surface area because they have smaller body sizes and a reduced weight compared to adults, which affects pharmacokinetics. As such, drug pharmacokinetics must be carefully evaluated, considering age, gender, genetic characteristics, and food intake, as these factors influence drug absorption, distribution, metabolism, and excretion.