BPK 105 Study Guide - Final Guide: Vasomotor Center, Medulla Oblongata, Autonomic Nervous System
27 Apr 2018
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Modules 7 & 8 - Review Questions - Part 8
14. Describe the changes in blood flow that occur during the onset of exercise. Include
detailed descriptions of the local and nervous control mechanisms. [6 marks]
Blood flow provided to the tissues by the circulatory system is highly controlled and matched
closely to the metabolic needs of tissues. Mechanisms that control blood flow through tissues
are classified as (1) local control or (2) nervous and hormonal control.
Local Control of Blood Flow
- Local control of blood flow is achieved by the periodic relaxation and contraction of the
precapillary sphincters.
- When the sphincters relax, blood flow through the capillaries increases.
- When the sphincters contract, blood flow through the capillaries decreases.
- The precapillary sphincters are controlled by the metabolic needs of the tissues.
- For example, blood flow increases when by-products of metabolism buildup in tissue
spaces.
- During exercise, the metabolic needs of skeletal muscle increase dramatically, and the
by-products of metabolism are produced more rapidly. The precapillary sphincters relax,
increasing blood flow through the capillaries.
Other factors that control blood flow through the capillaries are the tissue concentrations of O2
and nutrients, such as glucose, amino acids, and fatty acids (figure 13.25 and table 13.1). Blood
flow increases when O2 levels decrease or, to a lesser degree, when glucose, amino acids,
fatty acids, and other nutrients decrease. An increase in CO2 or a decrease in pH also causes
the precapillary sphincters to relax, thereby increasing blood flow.
In addition to the control of blood flow through existing capillaries, if the metabolic activity of a
tissue increases often, additional capillaries gradually grow into the area. The additional
capillaries allow local blood flow to increase to a level that matches the metabolic needs of the
tissue. For example, the density of capillaries in the well-trained skeletal muscles of athletes is
greater than that in skeletal muscles on a typical nonathlete (table 13.1).
Nervous and Hormonal Control of Blood Flow
Nervous control of blood flow is carried out primarily through the sympathetic division of the
autonomic nervous system. Sympathetic nerve fibers innervate most blood vessels of the body,
except the capillaries and precapillary sphincters, which have no nerve supply (figure 13.26).
An area of the lower pons and upper medulla oblongata, called the vasomotor center,
continually transmits a low frequency of action potentials to the sympathetic nerve fibers.
- As a consequence, the peripheral blood vessels are continually in a partially constricted
state, a condition called vasomotor tone.
Document Summary
Modules 7 & 8 - review questions - part 8: describe the changes in blood flow that occur during the onset of exercise. Include detailed descriptions of the local and nervous control mechanisms. Blood flow provided to the tissues by the circulatory system is highly controlled and matched closely to the metabolic needs of tissues. Mechanisms that control blood flow through tissues are classified as (1) local control or (2) nervous and hormonal control. Local control of blood flow is achieved by the periodic relaxation and contraction of the precapillary sphincters. When the sphincters relax, blood flow through the capillaries increases. When the sphincters contract, blood flow through the capillaries decreases. The precapillary sphincters are controlled by the metabolic needs of the tissues. For example, blood flow increases when by-products of metabolism buildup in tissue spaces. During exercise, the metabolic needs of skeletal muscle increase dramatically, and the by-products of metabolism are produced more rapidly.
