Regional Circulation and its regulation Dr. Shafali singh

Learning objectives Describe the phasic flow of blood to the ventricular myocardium through an entire cardiac cycle. Explain how arterio-venous O2 difference and oxygen extraction in the heart is unique when compared with other body organs. Contrast the local and neural control of cerebral blood flow. Discuss the relative importance of O2, CO2, and pH in regulating cerebral blood flow

Learning objectives Contrast the local and neural control of the splanchnic circulation. . Contrast local and neural control of cutaneous blood flow Contrast the blood flow pattern in the fetus with that of a normal neonate, including the source of oxygenated blood

left coronary artery blood flow decreases dramati-cally during the isovolumetric phase of systole, prior to opening of the aortic valve. Left coronary artery blood flow remains lower during systole than during diastole because of compression of the coronary blood vessels in the contracting myocardium. The left ventricle receives most of its arterial blood inflow during diastole. Right coronary artery blood flow tends to be sustained during both systole and diastole because lower intraventricular pressures are developed by the contracting right ventricle, resulting in less compression of coronary blood vessels

Coronary Circulation Coronary flow patterns Characteristics of left coronary flow (flow to the left ventricular myocardium): Left ventricular contraction causes severe mechanical compression of subendocardial vessels. Therefore: Very little if any blood flow occurs during systole. Most of the blood flow is during diastole. Some subepicardial flow occurs during systole.

Significant flow can occur during systole. Characteristics of right coronary blood flow (flow to the right ventricular myocardium): Right ventricular contraction causes modest mechanical compression of intramyocardial vessels. Therefore: Significant flow can occur during systole. The greatest flow under normal conditions is still during diastole.

Oxygenation In the coronary circulation, the tissues extract almost all the oxygen they can from the blood, even under “basal” conditions. Therefore: The venous PO2 is extremely low. It is the lowest venous PO2 in a resting individual. Because the extraction of oxygen is almost maximal under resting conditions, increased oxygen delivery to the tissue can be accomplished only by an increased blood flow. Coronary blood flow is most closely related to cardiac tissue oxygen consumption

Pumping action Coronary blood flow (mL/min) is determined by the pumping action, or stroke work times heart rate, of the heart. Increased pumping action means increased metabolism, which means increased production of vasodilatory metabolites, which means increased coronary flow. Increased pump function occurs with: Exercise: increased volume work (more volume pumped at the same pressure) Increased arterial pressure (hypertension): increased pressure work (a similar volume pumped against a greater pressure) Pressure work has a higher oxygen cost than volume work; therefore, increased systolic ventricular pressure development will require a greater increase of coronary blood flow than a similar increase in stroke volume only.

Cerebral Circulation Flow is proportional to arterial PCO2. Hypoventilation increases arterial PCO2, thus it increases cerebral blood flow. Hyperventilation decreases arterial PCO2, thus it decreases cerebral blood flow. As long as arterial PO2 is normal or above normal, cerebral blood flow will be regulated via arterial PCO2. Therefore: If a normal person switches from breathing room air to 100% oxygen, there will be no significant change in cerebral blood flow. Under normal conditions, arterial PCO2 is the main factor regulating cerebral blood flow.

However, a (large) decrease in arterial PO2 will increase cerebral blood flow. Under these conditions, it is the low arterial PO2 that is determining flow. Intracranial pressure is an important pathophysiologic factor that can affect cerebral blood flow.

Regional increases in cerebral flow that accompany increased activity are typically matched by opposing changes in a different brain area

Flow interruption A 20%–30% decrease in cerebral flow causes lightheadedness. A 40%–50% decrease causes fainting (syncope). Complete interruption of flow for 4–5 minutes can cause organ failure and death. Cerebral vessels that have narrowed with age or disease may cause a transient ischemic attack (TIA), a localized reduction in flow and loss of cerebral function lasting minutes or hours. Interruptions in cerebral flow (cerebrovascular accidents, or strokes) occur when a cerebral vessel is occluded. Such events cause infarction and more permanent neurologic defects. The brain has a very low tolerance for ischemia

Cutaneous Circulation Almost entirely controlled via sympathetic adrenergic nerves Large venous plexus innervated by sympathetics A-V shunts innervated by sympathetics

Sympathetic stimulation to the skin will cause: Constriction of arterioles and a decrease in blood flow Constriction of the venous plexus and a decrease in blood volume in the skin Increase in velocity of blood (decreased cross-sectional area) Sympathetic activity to the skin varies mainly with the body’s need for heat exchange with the environment. Increased skin temperature directly causes vasodilation, which increases heat loss

A person has cold, painful fingertips because of excessively constricted blood vessels in the skin. Which of the following alterations in autonomic function is most likely to be involved? (A) Low concentration of circulating epinephrine (B) High sensitivity of arterioles to norepinephrine (C) High sensitivity of arterioles to nitric oxide (D) Low parasympathetic nerve activity (E) Arterioles insensitive to Epinephrine

Renal and Splanchnic Circulation A small change in blood pressure will invoke an autoregulatory response, to maintain renal blood flow. Thus, under normal conditions, the renal and splanchnic circulations demonstrate autoregulation. Situations in which there is a large increase in sympathetic activity (e.g., hypotension) usually cause vasoconstriction and a decrease in blood flow. Renal circulation is greatly overperfused in terms of nutrient requirements, thus the venous PO2 is high.

Pulmonary Circuit Characteristics Low-pressure circuit, arterial = 15 mm Hg, venous = 5 mm Hg High flow, receives entire CO Low-resistance circuit Hypoxic vasoconstriction (low alveolar PO2 causes local vasoconstriction) Very compliant circuit; both arteries and veins are compliant vessels Blood volume proportional to blood flow – Because of the passive nature of the pulmonary circuit, pulmonary pressures are proportional to the output of the right ventricle. – Because of the very compliant nature of the pulmonary circuit, large changes in the output of the right ventricle are associated with only small changes in pulmonary pressures

Pulmonary response to hemorrhage A large decrease in CO means decreased volume pumped into the circuit. This will produce a decrease in pulmonary pressures. Because of the passive, compliant nature of the circuit, the response to a decrease in pressure is vessel constriction. This results in a large increase in resistance. Consequently, during hemorrhage, there is often only a slight decrease in pulmonary pressures. Vessel constriction also means less blood is stored in this circuit.

Blood flow (CO): large increase Pulmonary Circuit Blood flow (CO): large increase Pulmonary arterial pressure: slight increase Pulmonary vascular resistance: large decrease Pulmonary blood volume: increase Number of perfused capillaries: increase Capillary surface area: increase, i.e., increased rate of gas exchange The following assumes the person is in a steady state, performing moderate exercise at sea level

The bolded numbers refer to the percent hemoglobin (%HbO2) saturation. Shunting occurs because fetal pulmonary vascular resistance is very high, so 90% of the right ventricular output flows into the ductus arteriosus and only 10% to the lungs. The percent HbO2 saturation of aortic blood is 60%.

Which of the following sequences is a possible anatomic path for a red blood cell passing through a fetus and back to the placenta? (Some intervening structures are not included.) (A) Umbilical vein, right ventricle, ductus arteriosus, pulmonary artery (B) Ductus venosus, foramen ovale, right ventricle, ascending aorta (C) Spiral artery, umbilical vein, left ventricle, umbilical artery (D) Right ventricle, ductus arteriosus, descending aorta, umbilical artery (E) Left ventricle, ductus arteriosus, pulmonary artery, left atrium

Fetal Circulation Of the fetal CO, 55% goes to the placenta. The umbilical vein and ductus venosus have highest %HbO2 saturation(80%). When mixed with inferior vena caval blood (26% HbO2), the %HbO2 saturation of blood entering the right atrium is 67%. This blood is directed through the foramen ovale to the left atrium, left ventricle, and ascending aorta to perfuse the head and the forelimbs. Superior vena caval blood (40% HbO2) is directed through the tricuspid valve into the right ventricle and pulmonary artery and shunted by the ductus arteriosus to the descending aorta Shunting occurs because fetal pulmonary vascular resistance is very high, so 90% of the right ventricular output flows into the ductus arteriosus and only 10% to the lungs. The percent HbO2 saturation of aortic blood is 60%.

Fetal Circulation- At birth The loss of the placental circulation increases systemic resistance. The subsequent rise in aortic blood pressure (as well as the fall in pulmonary arterial pressure caused by the expansion of the lungs) causes a reversal of flow in the ductus arteriosus, which leads to a large enough increase in left atrial pressure to close the foramen ovale.

After birth, Left atrial pressure is raised above that in the inferior vena cava and right atrium by (1) the decrease in pulmonary resistance, with the consequent large flow of blood through the lungs to the left atrium; (2) the reduction of flow to the right atrium caused by occlusion of the umbilical vein; and (3) the increased resistance to left ventricular output produced by occlusion of the umbilical arteries. Reversal of the pressure gradient across the atria abruptly closes the valve over the foramen ovale, and the septal leaflets fuse over a period of several days

Q At birth, changes that occur in the fetal circulation include a. Increased systemic arterial pressure b. Increased pulmonary vascular resistance c. Increased pulmonary arterial pressure d. Decreased left atrial pressure e. Decreased pulmonary blood flow

For an arterial blood content of 20 mL oxygen per 100 mL blood and venous blood content of 15 mL oxygen per 100 mL of blood, how much oxygen is transferred from blood to tissue if the blood flow is 200 mL/min? (A) 5 mL/min (B) 10 mL/min (C) 15 mL/min (D) 20 mL/min (E) 25 mL/min

Arterial-venous difference

Arterial-venous difference: Is positive if substance extracted by the organ, e.g., O2, substrates like glucose Is negative if substance produced by the organ, e.g., CO2, glucose in liver, lactate in skeletal muscle and ischemic heart muscle

Q Resting muscle venous PO2 ~ 45 mm Hg Exercising muscle venous PO2 ~ 20 mm Hg 1. What is the A-V PO2 difference in this resting muscle? 2. What is the A-V PO2 difference in this exercising muscle? 3. What happens to the A-V difference with exercise?

4. During exercise, increased oxygen delivery to the muscle is accomplished by: A. increased blood flow B. increased extraction C. Both 5. With exercise, which increases more in skeletal muscle, flow or metabolism? Because extraction does increase in exercising muscle, there is a greater rise in metabolism than blood flow.

6. How does flow versus metabolism change in the heart with exercise? 7. How does the A-V PO2 differ in the renal circuit compare with the coronary circuit? 8. Assuming no effect on metabolism, what consequences does a vasodilatory drug have on the A-V PO2 difference in resting skeletal muscle? 9. What are the direct effects of an α agonist on the A-V PO2 difference in resting skeletal muscle? Flow must increase in proportion to metabolism in order to meet tissue demands.