Special Circulations

Mark T Ziolo, PhD, FAHAAssociate Professor, Physiology & Cell Biology019 Hamilton Hall614-688-7905ziolo.1@osu.edu

Learning Objectives

• Describe the regulation of coronary, cerebral, and skeletal muscle blood flow

• Differentiate flow regulation in cutaneous, splanchnic, renal and pulmonary circuits

Detailed Objectives

Understand how coronary blood flow is regulated Know why coronary blood flow must be increased and the primary

factor responsible for coronary blood flow Understand extravascular compression in the heart Understand how cerebral blood flow is regulated Know why cerebral blood flow is always maintained Know what is the purpose of Cushing’s Phenomenon Know how skeletal muscle blood flow is regulated Understand why skeletal muscle blood flow switches from neuronal

to local (metabolic) regulation Know the role of the “muscle-pump mechanism” Know the purpose of blood flow to the following organs: cutaneous,

splanchnic, renal, and pulmonary Understand the major mechanisms of blood flow regulation in the

following organs: cutaneous, splanchnic, renal, and pulmonary

• Mohrman DE, Heller LJ. Cardiovascular Physiology 8th Edition. Lange Medical Books/McGraw-Hill Publishers, 2014.

• Berne RM, Levy MN. Cardiovascular Physiology Sixth Edition. Mosby-Year Book, Inc., 2010.

• MediaPhys 3.0. An Introduction to Human Physiology. The McGraw-Hill Publishers, 2010.

References

Coronary Blood Flow

Myocardium extracts ~75% of the oxygen Increase in myocardial O2 consumption must be

accompanied by an increase in blood flow 1° factor responsible for perfusion is the aortic pressure Local control (metabolic)

Flow directly related to O2 consumption

Extravascular Compression

MyocardialO2 demand:Myocardial

O2 supply

MyocardialMetabolic rate

Vasodilatormetabolites

Coronaryblood flow

Arterial O2

content

Coronary Blood Flow, cont

Local Control

Vasodilator metabolites Adenosine

Activates adenosine receptor O2, CO2, H+, K+

Neuronal Control Sympathetic activation

Vasodilation (increase myocardial contractility)

Lack of Blood Flow

Myocardial Ischemia Arrhythmias

Endocardial layer of left ventricle

Cerebral Blood Flow

In ALL situations, blood flow to the brain is preserved (55 ml/min/100g)

Whole brain has a nearly constant metabolic rate

Blood flow to discrete regions is not constant Regulated almost entirely by local mechanisms

O2, H+ (PCO2), K+, adenosine

Excellent autoregulation Some sympathetic vasoconstriction

Hand Reasoning

Problem Solving

Cerebral Blood Flow, cont

Cerebral Blood Flow, cont

No reserves- very intolerant to ischemia 5 sec: fainting Minutes: death

Cushing’s Phenomenon

Tumor

Intracranial pressure

CBF

Metabolicvasodilation

Ischemic stimulation of vasomotorregions in medulla

systemic blood pressure

Maintain CBF

Skeletal Muscle

Rate of blood flow directly related to contractile state of muscle

At rest, large percentage of capillary bed is not perfused Regulation of flow

Neuronal and local influences Physical factor- squeezing effect of contracting muscle

Skeletal Muscle, cont

Neuronal Influence High basal tone Sympathetic fibers elicits vasoconstriction

Predominates in resting muscle

Skeletal Muscle, cont

Local Influence Very strong in working muscle

Muscle O2 consumption, adenosine, H+, K+, lactic acid

Neuronal and local influences oppose each other, in working muscle the local (metabolic) influence predominates

Skeletal Muscle, cont

“muscle-pump mechanism” Contracting muscles push blood in veins towards thorax

Cutaneous

Very low O2 and nutrient requirements Maintain constant body temperature Arterioles and arteriovenous anastomoses AV anastomoses shunt blood from arterioles to

venules Governed by nervous system in response to

temperature receptors NE and E elicit vasoconstriction Chiefly influenced by environmental temperature

Splanchnic

GI tract, spleen, pancreas, and liver ~25% of resting cardiac output Neuronal and local influences

Sympathetic causes vasoconstriction Shifts blood to central venous pool (liver important blood reserve)

Gastrointestinal hormones- functional hyperemia Autoregulation not well developed

Renal Blood Flow

0.5% TBW but 20% of cardiac output Strong autoregulation

Regulate GFR Myogenic mechanism (stretch) Tubuloglomerular feedback

Tubular flow sensed by macula densa sends signal via juxtaglomerular apparatus to afferent arterioles

JGA also releases renin (angiotensin II) Neuronal Influence

Sympathetic decreases RBF, but GFR only slightly

Pulmonary Blood Flow

Vascular system is low-resistance and highly distensible Capillaries aligned in thin sheets between adjacent

alveoli Gravitational effects (regional distribution) Hypoxia most important influence on tone Low alveolar PO2 leads to shunting of blood from poorly

ventilated regions to better ventilated regions

Summary

Coronary blood flow is regulated by metabolic influences and the primary factor responsible is arterial pressure

With increased oxygen demand, coronary blood flow must be increased because of the bulk flow principle

Extravascular compression occurs in the heart due to high systolic forces Cerebral blood flow in mostly under metabolic influence Cerebral blood flow is always maintained since it is the least tolerant organ

to ischemia and there are no reserves Cushing’s Phenomenon is elevation of intracranial pressure results in an

increase in systemic blood pressure to maintain cerebral blood flow Skeletal muscle blood flow is regulated by the neuronal influence at rest, and

by the metabolic influence in working muscle The muscle pump mechanism pushes blood back towards the heart

Summary, cont

Purpose of cutaneous blood flow is temperature regulation and is under neuronal control

Purpose of splanchnic blood flow is nutrient reabsorption and is under neuronal control and functional hyperemia

Purpose of renal blood flow is filtration and has strong autoregulation and is under neuronal control

Purpose of pulmonary blood flow is gas exchange and has hypoxic vasoconstriction

Special Circulations Quiz

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