Cross Cutting Skills>Concept Application Practice Test

Microvascular control of tissue perfusion relies on smooth muscle rings around terminal arterioles, known as precapillary sphincters. In resting skeletal muscle, many sphincters are partially constricted; during exercise, vasodilators released by hypoxic fibers relax these rings, permitting more red blood cells (RBCs) to enter downstream capillary networks. Within individual capillaries, blood exhibits largely laminar flow, and whole-blood viscosity over physiologic hematocrit ranges changes little on the timescale of seconds. Because capillaries are arranged in parallel beds, local changes in radius at the arteriole level can strongly bias how much flow enters a given bed without greatly altering systemic pressure. Physiologists often model an arteriole segment as a rigid cylindrical tube of length that is short compared to the upstream arterial tree, with flow driven by a roughly constant pressure drop over that segment. Under these conditions, even modest relaxation that increases radius by a few percent can produce multipliers of added volumetric flow, rapidly restoring oxygen delivery. The passage focuses on the mechanistic relationship between vessel geometry and volumetric flow in laminar regimes.

Metabolically active tissues acidify their microenvironment by producing carbon dioxide and lactic acid. In capillaries surrounding such tissues, hemoglobin within red blood cells encounters elevated CO2 and H+ levels, as well as slightly higher temperature. Hemoglobin binds oxygen reversibly; in simplified form, Hb + O2 ⇌ HbO2. Protons and CO2 bind at distinct sites on hemoglobin and stabilize conformations with lower oxygen affinity, facilitating oxygen release to the tissue. This behavior is often depicted as a rightward shift of the oxygen-hemoglobin dissociation curve under acidic, warm conditions, enhancing unloading without requiring drastic changes in arterial oxygen partial pressure. From a chemical perspective, HbO2 formation and dissociation behave like an equilibrium subject to perturbations by ligands that change the relative stability of bound and unbound states. The passage emphasizes that adding species that preferentially stabilize deoxygenated hemoglobin drives the system toward more oxygen release, which complements diffusion driven by partial pressure gradients.