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Angiotensin II: Angiotensin II is commonly released during prolonged exercise because dehydration and sweating reduce plasma volume, activating the renin–angiotensin system. Its release helps constrict blood vessels and maintain blood pressure despite fluid losses. This hormone therefore supports circulatory stability during long-duration exertion such as a marathon.

Epinephrine: Epinephrine release increases significantly during endurance events due to activation of the sympathetic nervous system. It supports cardiac output, mobilizes glucose and fatty acids, and maintains perfusion to active muscles. Elevated epinephrine is therefore a natural and necessary response to prolonged physical stress.

Antidiuretic hormone (ADH): ADH secretion rises during prolonged exercise because reduced blood volume and increased plasma osmolality stimulate its release. This hormone promotes water reabsorption in the kidneys, helping conserve fluid and maintain blood pressure. Its release is essential to counteract dehydration during a marathon.

Aldosterone release increases as part of the renin–angiotensin–aldosterone system during extended physical activity. It enhances sodium and water reabsorption, supporting blood pressure and fluid balance as the runner loses electrolytes through sweat. This hormone plays a key role in maintaining volume homeostasis during endurance exercise.

Tunica intima: The tunica intima is the innermost layer composed mainly of endothelial cells that provide a smooth surface to reduce friction and regulate vessel tone chemically. While it plays roles in coagulation and permeability, it does not generate the force needed to maintain blood pressure. Its structure is not designed for active contraction or pressure regulation.

Tunica externa: The tunica externa is the outer connective tissue layer that protects the vessel and anchors it to surrounding structures. Although it provides structural support, it does not contain the smooth muscle necessary to adjust vessel diameter. Without this contractile ability, it cannot directly influence blood pressure.

Subendothelial: The subendothelial layer is a thin connective tissue region beneath the endothelium. It provides minor structural support but lacks smooth muscle or elastic fibers needed for active regulation. Because it cannot constrict or dilate, it plays little to no role in controlling blood pressure.

Increased urine output: Diuretics promote renal excretion of sodium and water, leading to a noticeable rise in urine volume. This effect helps lower circulating fluid levels and reduces preload on the heart. Increased urine output is therefore an expected and intended response to these medications.

Lower plasma volume: By enhancing fluid loss through the kidneys, diuretics decrease overall plasma volume. Reduced intravascular volume helps lessen the workload on the cardiovascular system and supports blood pressure control. This decrease is a predictable physiological effect of diuretic therapy.

Decreased blood pressure: Lower plasma volume reduces venous return and cardiac output, leading to a reduction in arterial pressure. Many diuretics are prescribed specifically to manage hypertension by decreasing this circulating volume. A drop in blood pressure is therefore consistent with the drug’s mechanism of action.

Muscular arteries and arterioles: Both muscular arteries and arterioles contain smooth muscle and some elastic tissue within their walls. These elastic components help them withstand pressure changes and regulate blood flow. Because they possess measurable elasticity, they cannot be categorized as vessels lacking elastic tissue.

Venules and veins: Veins and venules do contain small amounts of elastic fibers, even though they are far less elastic than arteries. These fibers help accommodate changes in blood volume and allow the vessel walls to stretch as they act as capacitance vessels. Since elastic tissue is present, they do not fit the criterion of lacking elasticity.

Arterioles and capillaries: Arterioles possess smooth muscle and contain elastic tissue that allows them to regulate resistance and control blood flow into capillary beds. Capillaries alone lack elastic tissue, but arterioles do not.

Inversely; directly: Blood flow rises when the pressure gradient increases, so an inverse relationship does not fit normal cardiovascular physiology. The pressure gradient is the main driving force that propels blood forward, meaning higher gradients produce greater flow. Although resistance does influence blood flow, pairing it with an incorrect pressure-flow relationship makes this option inaccurate.

Directly; directly: While blood flow does increase with larger pressure differences, it does not increase with increased resistance. Higher resistance, such as from vasoconstriction, reduces flow by making it harder for blood to move through the vascular system. The second relationship therefore prevents this option from aligning with physiological reality.

Inversely; inversely: Suggesting that flow decreases when pressure gradients rise conflicts with the fundamental mechanism that drives circulation. Although flow does decline with increased resistance, an inverse pressure relationship fully contradicts how blood is propelled through vessels, making this option invalid.

Lymph nodes: Lymph nodes serve as sites where B lymphocytes encounter antigens and become activated, but they do not confer immunocompetence. They primarily function in filtering lymph and supporting the immune response rather than the initial development of B cells.

Spleen: The spleen acts as a secondary lymphoid organ where B cells can encounter blood-borne antigens and participate in immune responses. However, it does not provide the environment for B cells to acquire immunocompetence initially.

Thymus: The thymus is essential for T lymphocyte maturation and development of immunocompetence, but it is not involved in B cell maturation. B cells do not pass through the thymus during their development.

Phagocytosis: Phagocytes can engulf and destroy pathogens and debris, but they primarily target extracellular pathogens and particles. They do not specifically recognize and kill cancer cells or virus-infected body cells before adaptive immunity is activated.

B-lymphocytes: B cells mediate adaptive immunity by producing antibodies after activation. They require antigen recognition and helper T cell signals, so they cannot kill infected or cancerous cells immediately in the absence of adaptive immune activation.

Pinocytosis: Pinocytosis is a cellular process of “cell drinking” where extracellular fluid and dissolved substances are taken up. It does not target abnormal or infected cells for destruction and plays no role in killing cancer or virus-infected cells.

Cytotoxic T cells can kill virus-infected or cancer cells, but only after activation through antigen presentation by MHC molecules. They require adaptive immune activation, unlike natural killer cells that act immediately.

Lymph node: Lymph nodes are secondary lymphoid organs where lymphocytes encounter antigens and mount immune responses. They do not provide the environment for initial lymphocyte development or immunocompetence.

Tonsils: Tonsils are secondary lymphoid tissues that trap pathogens entering through the oral and nasal cavities. They function in immune surveillance but do not serve as sites for lymphocyte maturation.

Appendix: The appendix contains lymphoid tissue that contributes to immune defense, but it is a secondary lymphoid structure involved in antigen exposure, not lymphocyte development.

Spleen: The spleen filters blood and supports immune responses to blood-borne antigens. It is a secondary lymphoid organ and does not generate immunocompetent lymphocytes initially.

All contain valves to prevent the backflow of blood: Valves are present in veins, not arteries. Arteries rely on high pressure generated by the heart to maintain forward blood flow, so valves are generally unnecessary.

All carry oxygenated blood to the heart: Arteries do not carry blood to the heart; veins do. Additionally, some arteries (like the pulmonary arteries) carry deoxygenated blood away from the heart, so oxygen content is not a defining feature.

Only large arteries are lined with endothelium: All arteries, regardless of size, have an endothelial lining (tunica intima). This layer is essential for maintaining a smooth blood flow and regulating vascular functions.

Veins: Veins primarily function as capacitance vessels, returning blood to the heart and serving as blood reservoirs. They are not specialized for nutrient or gas exchange because their walls are thicker and less permeable.

Arteries: Arteries carry blood away from the heart and help maintain blood pressure, but their thick muscular walls prevent direct exchange of nutrients and gases with surrounding tissues. Their primary role is transport and pressure regulation.

Arterioles: Arterioles regulate blood flow into capillary beds through vasoconstriction and vasodilation. While they influence perfusion, they are not the primary site for actual exchange of gases and nutrients.