Special Circulations Mark T Ziolo, PhD, FAHA Associate Professor, Physiology & Cell Biology 019 Hamilton Hall

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, Berne RM, Levy MN. Cardiovascular Physiology Sixth Edition. Mosby-Year Book, Inc., MediaPhys 3.0. An Introduction to Human Physiology. The McGraw-Hill Publishers, References

Coronary Blood Flow  Myocardium extracts ~75% of the oxygen  Increase in myocardial O 2 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 O 2 consumption

Extravascular Compression

Myocardial O 2 demand : Myocardial O 2 supply Myocardial Metabolic rate Vasodilator metabolites Coronary blood flow Arterial O 2 content Coronary Blood Flow, cont

Local Control  Vasodilator metabolites  Adenosine Activates adenosine receptor  O 2, CO 2, 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 O 2, H + (PCO 2 ), K +, adenosine  Excellent autoregulation  Some sympathetic vasoconstriction

Hand Reasoning Problem Solving Cerebral Blood Flow, cont

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

Cushing’s Phenomenon Tumor Intracranial pressure CBF Metabolic vasodilation Ischemic stimulation of vasomotor regions 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 O 2 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 O 2 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 PO 2 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

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