The immune system spans nearly every organ and tissue in the human body.

A. Red bone marrow:Red bone marrow is the site of red blood cell production, not destruction. Mature RBCs are released into circulation from the marrow.

B. Spleen and liver:Most red blood cells are removed from circulation by the spleen and, to a lesser extent, the liver. Macrophages in these organs phagocytize aged or damaged RBCs, recycling components like iron and hemoglobin for reuse.

C. Lymph nodes and thymus:Lymph nodes and the thymus are primarily involved in immune functions, such as lymphocyte maturation and immune surveillance, not in RBC destruction.

D. Stomach and liver:The stomach does not play a significant role in RBC destruction. Only the liver contributes partially, but RBC breakdown predominantly occurs in the spleen.

E. Stomach and small intestine:The gastrointestinal tract is not involved in normal RBC destruction; it plays a role in digestion and nutrient absorption, not hemolysis.

A. A chemical messenger that is triggered by the binding of a ligand on the cell surface and causes a change in DNA transcription:This describes the mechanism of some hormones or intracellular signaling molecules, particularly steroid hormones, but it is not a complete definition of a hormone itself.

B. Junctions between cells interconnecting their cytoplasm:This describes gap junctions, which allow direct cytoplasmic communication between adjacent cells. It is a structural feature, not a hormone.

C. A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away:This accurately defines a hormone. Hormones are secreted by endocrine glands into the blood and travel to distant target cells, eliciting specific physiological responses.

D. Chemical messengers that travel across a synapse to stimulate another cell:This describes neurotransmitters, which act locally at synapses rather than being transported through the bloodstream.

E. Chemical messengers that diffuse from their point of origin locally to affect other cells physiology:This describes paracrine signaling, which is local and short-range. Hormones, in contrast, typically act on distant cells via the circulatory system.