Regulation of blood flow takes precedence, but it is subordinate to metabolic regulation during muscle contractions, such as exercise (Kara et al., 2003).

Metabolic factors, particularly CO2, K+, and adenosine, predominantly regulate cerebral blood flow. Enhanced blood flow in the activated region of the contralateral cerebral cortex accompanies increased regional cerebral activity elicited by stimuli like touch, pain, hand motion, talking, reading, reasoning, and problem-solving. The neurovascular unit, consisting of the microcirculation, pericytes, extracellular matrix, astrocytes, and neurons, is believed to connect brain activity with heightened blood flow and oxygenation, functioning as a component of the blood-brain barrier (Kara et al., 2003).

The microcirculation in intestinal villi serves as a countercurrent exchange system for oxygen. This system places the villi at risk in conditions of low blood flow. Splanchnic resistance and capacitance vessels demonstrate great responsiveness to alterations in sympathetic neural activity (Kara et al., 2003).

Approximately 25% of cardiac output is directed towards the liver, with about three-quarters originating from the portal vein and one-quarter from the hepatic artery. In the event of reduced flow in either the portal or hepatic system, flow usually increases in the other system but not proportionately. The liver is adept at maintaining a constant oxygen consumption due to its highly efficient mechanism for extracting oxygen from the blood. The liver typically harbors around 15% of the total blood volume, making it a crucial blood reservoir for the body (Kara et al., 2003).

During fetal development, a substantial portion of blood from the right atrium passes through the foramen ovale to the left atrium, and a significant proportion of pulmonary arterial blood flows through the ductus arteriosus to the aorta. At birth, the muscle layers of the umbilical vessels, ductus venosus, and ductus arteriosus contract, resulting in their closure. The decrease in pulmonary vascular resistance triggered by lung inflation is the primary factor that reverses the pressure gradient between the atria, leading to the closure of the foramen ovale (Kara et al., 2003).

Acetylcholine-induced paradoxical vasoconstriction in atherosclerotic coronary arteries has been reported by Ludmer et al. (1986). The requisite role of Kv1.5 channels in coronary metabolic dilation has been demonstrated by Ohanyan et al. (2015). Potente et al. (2011) discuss the basic and therapeutic aspects of angiogenesis. The impact of fluid shear stress on the vascular endothelium is examined by Resnick et al. (2003).

Upon completion of this chapter, the student should be able to answer the following questions:
1. How do the parasympathetic and sympathetic nervous systems regulate the functions of the heart and vasculature?
2. What factors influence the differential sympathetic regulation of resistance and capacitance vessels?
3. How does the baroreceptor-mediated reflex mimic the operation of the skeletal muscle proprioceptor reflex?
4. What are the two primary mechanisms intrinsic to heart muscle that govern myocardial performance?
5. Which major hormones regulate myocardial performance?
6. How are changes in arterial blood concentrations of O2, CO2, and H+ implicated in myocardial performance?
7. What is the myogenic mechanism of vascular smooth muscle, and how does it contribute to the regulation of tissue blood flow?
Kara T, Narkiewicz K, Somers VK. Chemoreflexes—physiology and clinical implications. Acta Physiol Scand. 2003;177:377.
regulation of blood flow is paramount, but it yields to metabolic regulation during muscle contractions (as during exercise).
15.
Cerebral blood flow is regulated predominantly by metabolic factors, especially CO2, K+ , and adenosine. The increased regional cerebral activity produced by stimuli such as touch, pain, hand motion, talking, reading, reasoning, and problem solving are associated with enhanced blood flow in the activated area of the contralateral cerebral cortex. The neurovascular unit (microcirculation, pericytes, the extracellular matrix, astrocytes and neurons), a component of the blood-brain barrier, is thought to link brain activity with increased blood flow and oxygenation.
16.
The microcirculation in intestinal villi constitutes a countercurrent exchange system for O2. Because of the presence of this countercurrent exchange system, the villi are in jeopardy in states of low blood flow. The splanchnic resistance and capacitance vessels are very responsive to changes in sympathetic neural activity.
17.
The liver receives approximately 25% of cardiac output; approximately three fourths of this output is from the portal vein and approximately a fourth from the hepatic artery. When flow is diminished in either the portal or hepatic system, flow in the other system usually increases, but not proportionately. The liver tends to maintain constant O2 consumption, in part because its mechanism for extracting O2 from blood is so efficient. The liver normally contains approximately 15% of the total blood volume. It serves as an important blood reservoir for the body.
18.
In the fetus, a large percentage of right atrial blood passes through the foramen ovale to the left atrium, and a large percentage of pulmonary arterial blood passes through the ductus arteriosus to the aorta. At birth, the umbilical vessels, ductus venosus, and ductus arteriosus close by contraction of their muscle layers. The reduction in pulmonary vascular resistance caused by lung inflation is the main factor that reverses the pressure gradient between the atria and thereby causes the foramen ovale to close.
Ludmer PL, Selwyn AP, Shook TL, et al. Paradoxical vasoconstriction induced by acetylcholine in atherosclerotic coronary arteries. N Engl J Med. 1986;315:1046.
Ohanyan V, Yin L, Bardakjian R, et al. Requisite role of Kv1.5 channels in coronary metabolic dilation. Circ Res. 2015;117:612.
Potente M, Gerhardt H, Carmeliet P. Basic and therapeutic aspects of angiogenesis. Cell. 2011;146:873.
Resnick N, Yahav H, Shay-Salit A, et al. Fluid shear stress and the vascular endothelium: for better and for worse. Prog Biophys Mol Biol. 2003;81:177.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
How do the parasympathetic and sympathetic nervous systems regulate the functions of heart and vasculature?
2.
What factors affect the differential sympathetic regulation of resistance and capacitance vessels?
3.
How does the baroreceptor-mediated reflex mimic the operation of skeletal muscle proprioceptor reflex?
4.
What are the two major mechanisms, intrinsic to heart muscle, that regulate myocardial performance?
5.
What are the major hormones that regulate myocardial performance?
6.
How is myocardial performance affected by changes in the arterial blood concentrations of O2, CO2, and H+?
7.
What is the myogenic mechanism of vascular smooth muscle, and how does it participate in regulation of tissue blood flow?