In most forms of artificial respiration, such as mouth-to-mouth resuscitation and mechanical respiration, the lungs are inflated by applying pressure above atmospheric pressure into the endotracheal tube. This inflation of the lungs causes an increase in intrathoracic pressure. During positive-pressure lung inflation, venous flow through the vena cava decreases sharply. On the other hand, when negative endotracheal pressure is used to facilitate lung deflation, venous flow through the vena cava accelerates more than when the lungs are allowed to deflate passively.

When a person makes sustained expiratory efforts, it increases intrathoracic pressure, which hinders venous return. Straining against a closed glottis, known as Valsalva's maneuver, commonly occurs during activities like coughing, defecation, and heavy lifting. Trumpet players have recorded intrathoracic pressures exceeding 100 mm Hg, while pressures higher than 400 mm Hg have been observed during bouts of coughing. These increased pressures are directly transmitted to the lumens of the intrathoracic arteries. After coughing stops, arterial blood pressure may drop rapidly due to the previous impediment to venous return.

The circulatory system's main function is to supply the necessary nutrients for tissue metabolism and growth and eliminate metabolic waste products. Previously, we discussed how the different components of the cardiovascular system contribute to maintaining adequate tissue perfusion under various physiological conditions. In this section, we will explore the interrelationships among these components. The autonomic nervous system, baroreceptors, and chemoreceptors play crucial roles in regulating the cardiovascular system. Additionally, the kidneys play an essential role in controlling fluid balance and maintaining a constant blood volume.

To assess the extent and sensitivity of the regulatory mechanisms in a well-regulated system, one can disturb the system and observe how it restores the previous steady state. We will discuss two such disturbances: physical exercise and hemorrhage. These examples will illustrate the operation of various regulatory factors.

During exercise, cardiovascular adjustments are influenced by a combination of neural and local (chemical) factors. Neural factors include central command, reflexes originating in the contracting muscle, and the baroreceptor reflex. Central command refers to the activation of the sympathetic nervous system by the cerebrum, which leads to cardiac acceleration, increased myocardial contractile force, and peripheral vasoconstriction. Reflexes are activated within the muscles through mechanoreceptors (sensitive to stretch and tension) and chemoreceptors (responsive to metabolic products) in response to muscle contraction. Impulses from these receptors travel through small myelinated (group III) and unmyelinated (group IV) afferent nerve fibers to the central nervous system. The efferent limb of this reflex consists of sympathetic nerve fibers that innervate the heart and peripheral blood vessels. The baroreceptor reflex and local factors that affect skeletal muscle blood flow, such as metabolic vasodilators, also contribute to cardiovascular regulation during exercise. Vascular chemoreceptors are particularly important in this regulation. It is worth noting that arterial blood levels of PaCO2, PaO2, and pH remain normal during exercise.

In humans or trained animals, anticipation of physical activity suppresses vagal nerve impulses to the heart and increases sympathetic discharge. This leads to an increase in heart rate and myocardial contractility, resulting in an elevated cardiac output.
Inmostformsofartificialrespiration(mouth-to-mouthresuscitation,mechanicalrespiration),endotrachealpressureaboveatmosphericpressureisusedtoinflatethelungs,andexpirationoccursbypassiverecoilofthethoraciccage(see ).Thuslunginflationisaccompaniedbyanappreciableriseinintrathoracicpressure.Venacavalflowdecreasessharplyduringthephaseofpositive-pressurelunginflationwhentheendotrachealpressureprogressivelyrises.Whennegativeendotrachealpressureisusedtofacilitatedeflation,venacavalflowacceleratesmorethanwhenthelungsareallowedtodeflatepassively.
Sustained expiratory efforts increase intrathoracic pressure and thus impede venous return. Straining against a closed glottis (Valsalva’s maneuver) regularly occurs during coughing, defecation, and heavy lifting. Intrathoracic pressures in excess of 100 mm Hg have been recorded in trumpet players, and pressures higher than 400 mm Hg have been observed during paroxysms of coughing. Such increases in pressure are transmitted directly to the lumens of the intrathoracic arteries. After coughing stops, arterial blood pressure may fall precipitously because of the preceding impediment to venous return.
Interplay of Central and Peripheral Factors in Control of the Circulation
The primary function of the circulatory system is to deliver the nutrients needed for tissue metabolism and growth and to remove the products of metabolism. The contributions of the components of the cardiovascular system to maintain adequate tissue perfusion under different physiological conditions were discussed previously. In this section, the interrelationships among the various components of the circulatory system are explored. The autonomic nervous system and the baroreceptors and chemoreceptors play key roles in regulating the cardiovascular system. Control of fluid balance by the kidneys, with maintenance of a constant blood volume, is also very important.
In any well-regulated system, one way to evaluate the extent and sensitivity of its regulatory mechanisms is to disturb the system and to observe how it restores the preexisting steady state. Two such disturbances, physical exercise and hemorrhage, are discussed in the following sections to illustrate operation of the various regulatory factors.
The cardiovascular adjustments that occur during exercise consist of a combination of neural and local (chemical) factors. Neural factors include (1) central command, (2) reflexes that originate in the contracting muscle, and (3) the baroreceptor reflex. Central command is the cerebrocortical activation of the sympathetic nervous system that produces cardiac acceleration, increased myocardial contractile force, and peripheral vasoconstriction. Reflexes are activated intramuscularly by stimulation of mechanoreceptors (by stretch, tension) and chemoreceptors (by metabolic products) in response to muscle contraction. Impulses from these receptors travel centrally via small myelinated (group III) and unmyelinated (group IV) afferent nerve fibers. Group IV unmyelinated fibers may represent the muscle chemoreceptors, inasmuch as no morphological chemoreceptor has been identified. The central connections of this reflex are unknown, but the efferent limb consists of sympathetic nerve fibers to the heart and peripheral blood vessels. The baroreceptor reflex is described in , and local factors that influence skeletal muscle blood flow (metabolic vasodilators) are described in . Vascular chemoreceptors are important in regulation of the cardiovascular system during exercise. Evidence for this assertion comes from the observations that the PaCO2, the PaO2, and the pH of arterial blood remain normal during exercise.
Mild to Moderate Exercise
In humans or trained animals, anticipation of physical activity inhibits vagal nerve impulses to the heart and increases sympathetic discharge. The result is an increase in heart rate and myocardial contractility. The tachycardia and enhanced contractility increase cardiac output.