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. 5%:This percentage is far too low to represent the volume of fluid returned by the lymphatic system. Only a small portion of capillary filtrate would be recovered at this rate, which would lead to significant interstitial fluid accumulation.

B. 85%:Approximately 85% of filtered fluid is reabsorbed directly at the venous end of capillaries, while the remaining 15% enters lymphatic vessels. The lymphatic system consistently retrieves this portion to prevent edema and maintain fluid balance.

C. 25%:A recovery of 25% would greatly overestimate the lymphatic contribution compared with the actual capillary reabsorption dynamics. This value does not match known physiological distribution of fluid return pathways.

D. 50%:If lymphatics recovered half of the capillary filtrate, the role of venous reabsorption would be significantly reduced, which does not reflect actual fluid handling. This percentage does not align with established cardiovascular–lymphatic physiology.

The immune system spans nearly every organ and tissue in the body because immune cells circulate continuously through the bloodstream and lymphatic system. Lymphoid tissues such as the bone marrow, thymus, lymph nodes, spleen, tonsils, Peyer’s patches, and mucous membranes all play key roles in immune defense. Organs like the skin, lungs, and gastrointestinal tract contain specialized immune cells that detect and respond to pathogens.

Pus forms as part of the body’s inflammatory response to infection or tissue injury. Neutrophils arrive first and engulf pathogens, then die off, accumulating at the site. Macrophages follow and continue clearing debris. The mixture of dead neutrophils, macrophages, bacteria, and broken-down tissue creates the thick, often cloudy fluid known as pus.

A. Weeks:Memory T cells persist far longer than a few weeks. While effector T cells die off quickly after an immune response, memory T cells are designed for long-term immunity rather than short-term survival.

B. Days:A lifespan of only days reflects the behavior of effector T cells during an acute response. Memory T cells, however, undergo homeostatic proliferation and survive well beyond this timeframe.

C. Months:Although memory T cells can certainly survive for months, this does not represent their full potential lifespan. They are known to persist long after the initial antigen exposure.

D. Years:Many memory T cells do survive for years, providing extended protection. However, in humans, the lifespan of memory T cells can surpass even this range, especially after certain viral infections.

E. Decades:Memory T cells can remain functional for decades, offering long-term immunity. Memory T cells from childhood infections can persist into old age, highlighting their exceptional longevity

A. Granzymes:Granzymes are proteolytic enzymes released by NK cells into target cells after perforin creates entry pores. They trigger apoptosis by activating caspases and degrading intracellular components, allowing NK cells to eliminate virus-infected or cancerous cells efficiently.

B. Selectins:Selectins are adhesion molecules that help leukocytes migrate during inflammation. They are not enzymes and are not secreted by NK cells, as their role involves cell-to-cell binding rather than inducing apoptosis.

C. Cytokines:Cytokines are signaling molecules used for communication within the immune system. Although NK cells secrete cytokines like IFN-γ, cytokines are not proteolytic enzymes and do not directly induce programmed cell death.

D. Interferons:Interferons are antiviral signaling proteins that enhance immune defense, but they are not proteolytic and do not mediate direct killing. NK cells may secrete interferons, yet these are regulatory rather than destructive molecules.

E. Perforins:Perforins create pores in the target cell membrane, enabling granzymes to enter. While essential to NK-cell function, perforins themselves are not proteolytic enzymes, but rather pore-forming proteins.

A. Reticular cells:Reticular cells provide structural support within lymphoid tissues by producing the stroma, but they do not engage in direct destruction of abnormal cells. Their role centers on organizing the tissue microenvironment rather than conducting immune surveillance.

B. Macrophages:Macrophages phagocytose pathogens and debris and help present antigens, but their actions depend on encountering targets rather than performing continuous nonspecific monitoring. Their surveillance is limited compared to NK cells’ rapid response capabilities.

C. Natural killer (NK) cells:NK cells constantly patrol the body and nonspecifically kill virus-infected and cancerous cells through perforin and granzyme release. They do not require prior antigen exposure, making them central to immune surveillance and early defense against abnormal cells.

D. T lymphocytes (T cells):T cells require antigen presentation and activation before responding, so they do not provide the immediate, nonspecific surveillance carried out by NK cells. Their function is part of adaptive immunity rather than constant innate monitoring.

E. Dendritic cells:Dendritic cells specialize in capturing antigens and migrating to lymph nodes to activate T cells. They contribute to immune activation but are not responsible for nonspecific detection and destruction of diseased cells.

Red bone marrow is the primary site where all blood cells, including immune cells, originate from hematopoietic stem cells. B lymphocytes mature directly in the bone marrow, while T lymphocytes migrate to the thymus to complete maturation. Other immune cells, such as natural killer cells, monocytes, and granulocytes, also develop from progenitor cells in the red bone marrow, making it the central origin for the lymphatic and immune system.

A. Interferons:Interferons are signaling proteins produced by virus-infected cells that warn neighboring cells of infection. They stimulate the production of antiviral proteins in surrounding cells, limiting viral replication and spread, and enhancing the innate immune response.

B. Granzymes:Granzymes are proteolytic enzymes secreted by NK cells and cytotoxic T cells to induce apoptosis in infected or cancerous cells. They act directly on target cells rather than alerting neighboring cells.

C. Complement system globulins:Complement proteins help lyse pathogens and promote opsonization, but they do not act as signaling molecules to warn uninfected cells of viral threats. Their action is primarily destructive, not protective for nearby healthy cells.

D. Perforins:Perforins form pores in target cell membranes to allow granzymes entry. They are involved in direct cytotoxicity and do not provide antiviral signaling to surrounding cells.

E. Pyrogens:Pyrogens induce fever by acting on the hypothalamus and are part of systemic inflammatory responses, but they do not protect neighboring cells from viral infection directly.

This phenomenon is called molecular mimicry, where antibodies generated against foreign antigens (like viruses or bacteria) mistakenly recognize similar structures on the body’s own cells. This cross-reactivity can trigger an autoimmune response, leading the immune system to attack healthy tissues. Conditions such as rheumatic fever or type 1 diabetes can result from this type of immune misidentification.