Monocytes differentiate into large phagocytic cells.

A. Regulation of blood pressure:Carbon dioxide levels influence vascular tone indirectly, but blood pressure is primarily regulated by cardiac output, blood volume, and peripheral resistance. CO₂ production does not have a direct regulatory role in maintaining systemic arterial pressure.

B. Regulation of pH:Carbon dioxide combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. This relationship directly links CO₂ production to acid–base balance, making CO₂ a key determinant of blood pH through respiratory compensation.

C. Regulation of body temperature:Temperature regulation involves metabolic heat production, sweating, and vasomotor changes. While CO₂ is a byproduct of metabolism, its production does not directly regulate body temperature.

D. The synthesis of vasodilators:Vasodilators such as nitric oxide and prostaglandins are synthesized by endothelial and other cells. Their production is not directly dependent on the respiratory generation of CO₂, although CO₂ may influence vascular tone secondarily.

E. Aids in defecation:Defecation is controlled by the enteric nervous system, abdominal pressure, and sphincter relaxation. CO₂ production has no functional role in initiating or supporting this process.

A. Weeks:While some activated lymphocytes may only survive for a few weeks after an immune response, memory lymphocytes are designed to persist much longer to provide lasting immunity.

B. Days:Effector lymphocytes, such as short-lived plasma cells, may function for days during an active immune response. However, this does not reflect the longevity of memory lymphocytes.

C. Decades:Memory lymphocytes, particularly memory T and B cells, can survive for decades. This long lifespan allows the immune system to “remember” pathogens encountered earlier in life and respond more rapidly upon re-exposure.

D. Months:Some lymphocytes can survive for months, but this is still short compared to memory cells that provide long-term immunity over decades.

E. Years:While memory lymphocytes can survive for many years, the most accurate term emphasizing their extreme longevity is decades, capturing their ability to persist through much of a person’s lifetime.