Vaccination stimulates ________ immunity.

Choice A rationale

Positive feedback mechanisms amplify the initial stimulus, leading to an increased response. While important in certain physiological processes like childbirth (oxytocin release) or blood clotting, it is less common for target organs to regulate pituitary function through this method, as it can lead to instability.

Choice B rationale

Negative feedback is the predominant mechanism by which target organs regulate pituitary function. High levels of target gland hormones inhibit the release of stimulating hormones from the pituitary and hypothalamus, thus maintaining hormonal homeostasis and preventing overproduction or underproduction.

Choice C rationale

Neural stimulation involves direct nerve impulses influencing hormone release, as seen in the adrenal medulla's response to sympathetic nervous system activation. While the hypothalamus has neural connections to the pituitary, direct neural stimulation from target organs is not the primary regulatory mechanism for pituitary control.

Choice D rationale

Direct hormonal release from target organs without involving feedback loops would result in unregulated hormone secretion, disrupting endocrine balance. Target organs typically release hormones that, in turn, exert feedback effects on the pituitary or hypothalamus, ensuring precise control and preventing dysregulation.

Choice A rationale

Increased viscosity directly impedes blood flow. Viscosity refers to the "thickness" or internal friction of a fluid. Higher viscosity, often due to increased hematocrit or plasma protein concentration, increases resistance to flow, requiring greater pressure to maintain the same flow rate, thus reducing velocity according to Poiseuille's Law.

Choice B rationale

Increased blood pressure provides a greater driving force for blood flow. According to Ohm's Law for fluid flow, flow rate is directly proportional to the pressure gradient. Therefore, a higher pressure propels blood through vessels more quickly, leading to an increased velocity of blood flow, assuming resistance remains constant.

Choice C rationale

Increased vessel radius significantly decreases resistance to blood flow. Resistance is inversely proportional to the fourth power of the radius (Poiseuille's Law). A larger lumen allows blood to flow with less friction against the vessel walls, thereby increasing the velocity of blood flow for a given pressure gradient.

Choice D rationale

Increased afterload refers to the resistance the ventricles must overcome to eject blood. While increased afterload initially reduces stroke volume, the body often compensates with increased contractility or heart rate. However, a sustained increase in systemic vascular resistance, a major component of afterload, can reduce the overall systemic blood flow velocity by increasing the impedance to ejection.

Choice E rationale

Decreased vasomotion implies reduced changes in vessel diameter, potentially leading to less efficient blood distribution and potentially increased overall peripheral resistance if arterioles remain constricted. Reduced vasomotion might hinder the body's ability to adjust blood flow to meet metabolic demands, potentially decreasing localized or overall blood flow velocity.