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11/7/12 Cardiac Output Volume of blood pumped by each ventricle per minute Cardiac Output = CO = SV x HR Equal on both sides of the heart Average CO = 5 litres/min at rest (70ml/beat x 70beat/min) ie at rest you circulate twice your entire blood volume every minute Can increase 5-fold during exercise Regulation of Cardiac Output Regulate heart rate and stroke volume These can change from moment to moment Extrinsic and Intrinsic regulation Extrinsic - outside hormones (adrenaline) nerves (autonomic nervous system) Intrinsic - local Autonomic nervous system Efferent nervous system Autonomic (involuntary) Somatic (voluntary) Parasympathetic Sympathetic Motor Parasympathetic and sympathetic have opposing effects 1 11/7/12 Factors Affecting Cardiac Output: Heart Rate Neuronal: Pacemaker (SA node) initiates contraction – innervated by autonomic nerves Parasympathetic slows heart rate (dominant at rest) Sympathetic increases heart rate Balance between sympathetic and parasympathetic is key Hormonal: Adrenaline (epinephrine) - same effect as sympathetic nervous system Glucagon - increases heart rate Factors Affecting Cardiac Output: Stroke Volume Primary factors affecting stroke volume 1) Ventricular contractility 2) End-diastolic volume (preload) 3) Afterload Afterload = pressure in aorta during ejection This is the force that the heart must pump against 2 11/7/12 1) ventricular contractility If ventricles contract with more force they eject more blood – SV increases Sympathetic nerves increase SV Parasympathetic have no effect Adrenaline increases SV 2) EDV End-diastolic volume: intrinsic control mechanism Length-Tension Curve (Starling Curve) for cardiac muscle EDV is the preload – the work the heart must do Increased EDV stretches the myocardium (cardiac muscle) The resulting contraction is greater Increased EDV= increased SV: this is called the Starling Effect This mechanism ensures that venous return matches cardiac output – the heart pumps out the blood that is returned to it Increase EDV stretches muscle fibres: closer to optimum length Greater strength of contraction: increased SV 3 11/7/12 Autonomic regulation of Cardiac Output Remember: CO = HR x SV Autonomic nerves affect both variables to influence CO Increases in Cardiac Output: exercise CO can increase 5 fold during exercise Exercise affects HR and SV Influence of autonomic nerves Increased venous return eg via skeletal muscle pump One-way valves in peripheral veins Skeletal muscle contracts and squeezes veins: increased pressure Blood moves toward heart Blood cannot move backwards (valves) Skeletal muscle relaxes Blood flows into veins 4 11/7/12 Distribution of Cardiac Output at rest and during exercise Independent regulation of blood flow during exercise Cardiac output increases during exercise Distribution of blood does not increase proportionally Dilation to skeletal muscle and heart increases blood flow Constriction to GI tract and kidneys decreases blood flow Overview of the Vasculature Heart →Arteries → Arterioles → Capillaries → Venules → Veins – Arteries – relatively large, branching vessels that conduct blood away from the heart. Major artery is aorta – Microcirculation • • • Arterioles – small branching vessels with high resistance Capillaries – site of exchange between blood and tissues Venules – small converging vessels - drain blood to veins – Veins – relatively large converging vessels that conduct blood to the heart. Major vein is vena cava (superior and inferior) – Closed system – Arteries branch; veins converge – Differences between blood vessel types: • Structure: diameter; composition of walls • Function 5 11/7/12 Blood Vessel anatomy Arteries Carry blood away from heart Thick, elastic walls Large diameter, therefore low resistance to blood flow Pressure remains relatively constant through arteries. Pressure reservoirs because of thick elastic walls that stretch during systole - elastic recoil helps maintain blood pressure during diastole 6 11/7/12 Physiology research focus: Arteries and disease Atherosclerosis - ‘hardening of the arteries’: wall thickens and hardens One of the leading causes of death worldwide. A plaque composed of cholesterol, calcium and other substances builds up in an artery Plaques reduce blood flow They can rupture and cause clots - heart attacks or strokes can result Often occurs with age Smoking, diabetes and obesity are other risk factors Angioplasty or stent implantation can be used as treatments Arterioles Arterioles are resistance vessels Small diameter Walls contain smooth muscle: regulation of radius, and thus, resistance Functions: Regulate blood pressure • Control distribution of blood to body organs Effect of diameter. • Wider diameter – vasodilation (vasodilatation) • Narrower diameter - vasoconstriction • Greater diameter, greater blood flow Influenced by nerves, hormones and local effects in tissues eg. during exercise 7 11/7/12 Arterioles: vasoactivity – Radius dependent on contraction state of smooth muscle in arteriole wall – Vasoconstriction: increased contraction (decreased radius) – Vasodilation: decreased contraction (increased radius) – Functions of Varying Arteriole Radius – Controlling blood flow to individual capillary beds – Regulating mean arterial pressure Factors that influence vasodilation and vasoconstriction Autonomic nerves (sympathetic constricts) Hormones (eg adrenaline constricts) Metabolism (eg. decreased O2 causes dilation) These factors therefore influence blood flow 8 11/7/12 Capillaries Site of exchange between blood and tissue 5-10 µm diameter - small diffusion distance Walls - 1 endothelial cell layer plus basement membrane (small diffusion barrier) 10-40 billion per body Total SA = 600 m2 Most cells within 2 or 3 cell diameters of a capillary 1 mm long Materials exchanged: O2 CO2 Glucose Fatty acids Hormones etc. Capillaries can be continuous or fenestrated Fenestrated capllaries located in kidneys, liver, intestines, bone marrow 9 11/7/12 Blood-brain barrier Tight junctions between endothelial cells lining cerebral blood vessels Prevents easy passage of large macromolecules and pathogens between the circulation and the brain The arborizing network of cerebral arteries is demonstrated here in this cerebral angiogram seen laterally after injection of contrast into the right internal carotid artery 10 11/7/12 Physiology research focus: cerebral blood flow and disease Pathophysiology of the cerebral vasculature is associated with many brain diseases/injuries including stroke, traumatic brain injury and Alzheimer’s Disease. However we still do not understand the underlying mechanisms – this is a very active area of research in Physiology. Venules Smaller than arterioles Connect capillaries to veins Thin walls Little smooth muscle in walls Some exchange of material between blood and interstitial fluid Veins Large diameter, but thin walls, which contain muscle and elastic tissue Valves allow unidirectional blood flow Volume reservoirs: at rest, systemic veins contain 60% of total blood volume Return of blood to heart from veins is called venous return 11