Cardiac Muscle Found only in heart Striated Each cell usually has one nucleus Has intercalated disks and gap junctions Autorhythmic cells Action potentials of longer duration and longer refractory period Ca2+ regulates contraction

Cardiac Muscle Elongated, branching cells containing 1-2 centrally located nuclei Contains actin and myosin myofilaments Intercalated disks: Specialized cell-cell contacts Desmosomes hold cells together and gap junctions allow action potentials Electrically, cardiac muscle behaves as single unit

Cardiac myocyte action potential:

Refractory Period Absolute: Cardiac muscle cell completely insensitive to further stimulation Relative: Cell exhibits reduced sensitivity to additional stimulation Long refractory period prevents tetanic contractions

AP-contraction relationship: AP in skeletal muscle is very short-lived AP is basically over before an increase in muscle tension can be measured. AP in cardiac muscle is very long-lived AP has an extra component, which extends the duration. The contraction is almost over before the action potential has finished.

Functions of the Heart Generating blood pressure Routing blood Heart separates pulmonary and systemic circulations Ensuring one-way blood flow Heart valves ensure one-way flow Regulating blood supply Changes in contraction rate and force match blood delivery to changing metabolic needs

Orientation of cardiac muscle fibres: Unlike skeletal muscles, cardiac muscles have to contract in more than one direction. Cardiac muscle cells are striated, meaning they will only contract along their long axis. In order to get contraction in two axis, the fibres wrap around.

Circulation circuits:

Heart Wall Three layers of tissue Epicardium: This serous membrane of smooth outer surface of heart Myocardium: Middle layer composed of cardiac muscle cell and responsibility for heart contracting Endocardium: Smooth inner surface of heart chambers

Valve function:

Coronary circulation:

Cardiac conducting system:

Pacemaker potential:

EKG:

Heart sounds:

Heart Sounds First heart sound or “lubb” Second heart sound or “dupp” Atrioventricular valves and surrounding fluid vibrations as valves close at beginning of ventricular systole Second heart sound or “dupp” Results from closure of aortic and pulmonary semilunar valves at beginning of ventricular diastole, lasts longer Third heart sound (occasional) Caused by turbulent blood flow into ventricles and detected near end of first one-third of diastole

Cardiac Arrhythmias Tachycardia: Heart rate in excess of 100bpm Bradycardia: Heart rate less than 60 bpm Sinus arrhythmia: Heart rate varies 5% during respiratory cycle and up to 30% during deep respiration Premature atrial contractions: Occasional shortened intervals between one contraction and succeeding, frequently occurs in healthy people

Cardiac Cycle Heart is two pumps that work together, right and left half Repetitive contraction (systole) and relaxation (diastole) of heart chambers Blood moves through circulatory system from areas of higher to lower pressure. Contraction of heart produces the pressure

Pressure relationships:

Mean Arterial Pressure (MAP) Average blood pressure in aorta MAP=CO x PR CO is amount of blood pumped by heart per minute CO=SV x HR SV: Stroke volume of blood pumped during each heart beat HR: Heart rate or number of times heart beats per minute Cardiac reserve: Difference between CO at rest and maximum CO PR is total resistance against which blood must be pumped

Factors Affecting MAP

Regulation of the Heart Intrinsic regulation: Results from normal functional characteristics, not on neural or hormonal regulation Starling’s law of the heart Extrinsic regulation: Involves neural and hormonal control Parasympathetic stimulation Supplied by vagus nerve, decreases heart rate, acetylcholine secreted Sympathetic stimulation Supplied by cardiac nerves, increases heart rate and force of contraction, epinephrine and norepinephrine released

Heart Homeostasis Effect of blood pressure Baroreceptors monitor blood pressure Effect of pH, carbon dioxide, oxygen Chemoreceptors monitor Effect of extracellular ion concentration Increase or decrease in extracellular K+ decreases heart rate Effect of body temperature Heart rate increases when body temperature increases, heart rate decreases when body temperature decreases

Baroreceptor and Chemoreceptor Reflexes

Cardian innervation:

Pacemaker regulation: Once the pacemaker cells reach threshold, the magnitude and duration of the AP is always the same. In order to change the frequency, the time between APs must vary. The interval can only be changed in two ways. The rate of depolarization can be changed The amount of depolarization required to reach threshold can be changed.

Vascular physiology:

Peripheral Circulatory System Systemic vessels Transport blood through most all body parts from left ventricle and back to right atrium Pulmonary vessels Transport blood from right ventricle through lungs and back to left atrium Blood vessels and heart regulated to ensure blood pressure is high enough for blood flow to meet metabolic needs of tissues

Blood Vessel Structure Arteries Elastic, muscular, arterioles Capillaries Blood flows from arterioles to capillaries Most of exchange between blood and interstitial spaces occurs across the walls Blood flows from capillaries to venous system Veins Venules, small veins, medium or large veins

Structure of Arteries and Veins Three layers except for capillaries and venules Tunica intima (interna) Endothelium Tunica media Vasoconstriction Vasodilation Tunica adventitia (externa) Merges with connective tissue surrounding blood vessels Note mistake on figure

Structure of Arteries Elastic or conducting arteries Largest diameters, pressure high and fluctuates Muscular or medium arteries Smooth muscle allows vessels to regulate blood supply by constricting or dilating Arterioles Transport blood from small arteries to capillaries

Structure of Veins Venules and small veins Medium and large veins Tubes of endothelium on delicate basement membrane Medium and large veins Valves Allow blood to flow toward heart but not in opposite direction Atriovenous anastomoses Allow blood to flow from arterioles to small veins without passing through capillaries

Blood Vessel Comparison:

Capillaries: Capillary wall consists mostly of endothelial cells Types classified by diameter/permeability Continuous Do not have fenestrae Fenestrated Have pores Sinusoidal Large diameter with large fenestrae

Capillary Network: Blood flows from arterioles through metarterioles, then through capillary network Venules drain network Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow

Muscular contractions aid venous return:

Pulmonary Circulation Moves blood to and from the lungs Pulmonary trunk Arises from right ventricle Pulmonary arteries Branches of pulmonary trunk which project to lungs Pulmonary veins Exit each lung and enter left atrium

Systemic Circulation: Arteries Aorta From which all arteries are derived either directly or indirectly Parts Ascending, descending, thoracic, abdominal Coronary arteries Supply the heart

Systemic Circulation: Veins Return blood from body to right atrium Major veins Coronary sinus (heart) Superior vena cava (head, neck, thorax, upper limbs) Inferior vena cava (abdomen, pelvis, lower limbs) Types of veins Superficial, deep, sinuses

Dynamics of Blood Circulation Interrelationships between Pressure Flow Resistance Control mechanisms that regulate blood pressure Blood flow through vessels

Blood Pressure Measure of force exerted by blood against the wall Blood moves through vessels because of blood pressure Measured by listening for Korotkoff sounds produced by turbulent flow in arteries as pressure released from blood pressure cuff

Pressure and Resistance Blood pressure averages 100 mm Hg in aorta and drops to 0 mm Hg in the right atrium Greatest drop in pressure occurs in arterioles which regulate blood flow through tissues No large fluctuations in capillaries and veins

Blood Pressure Measurement

Pulse Pressure Difference between systolic and diastolic pressures Increases when stroke volume increases or vascular compliance decreases Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity

Blood Flow, Poiseuille’s Law and Viscosity Amount of blood moving through a vessel in a given time period Directly proportional to pressure differences, inversely proportional to resistance Poiseuille’s Law Flow decreases when resistance increases Flow resistance decreases when vessel diameter increases Viscosity Measure of resistance of liquid to flow As viscosity increases, pressure required to flow increases

Critical Closing Pressure, Laplace’s Law and Compliance Vascular compliance Tendency for blood vessel volume to increase as blood pressure increases More easily the vessel wall stretches, the greater its compliance Venous system has a large compliance and acts as a blood reservoir Critical closing pressure Pressure at which a blood vessel collapses and blood flow stops Laplace’s Law Force acting on blood vessel wall is proportional to diameter of the vessel times blood pressure

Physiology of Systemic Circulation Determined by Anatomy of circulatory system Dynamics of blood flow Regulatory mechanisms that control heart and blood vessels Blood volume Most in the veins Smaller volumes in arteries and capillaries

Cross-Sectional Area As diameter of vessels decreases, the total cross-sectional area increases and velocity of blood flow decreases Much like a stream that flows rapidly through a narrow gorge but flows slowly through a broad plane

Laminar and Turbulent Flow Laminar flow Streamlined Outermost layer moving slowest and center moving fastest Turbulent flow Interrupted Rate of flow exceeds critical velocity Fluid passes a constriction, sharp turn, rough surface

Aging of the Arteries Arteriosclerosis Atherosclerosis General term for degeneration changes in arteries making them less elastic Atherosclerosis Deposition of plaque on walls

Capillary Exchange and Interstitial Fluid Volume Regulation Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries A net movement of fluid occurs from blood into tissues. Fluid gained by tissues is removed by lymphatic system.

Fluid Exchange Across Capillary Walls

Vein Characteristics and Effect of Gravity on Blood Pressure Venous return to heart increases due to increase in blood volume, venous tone, and arteriole dilation Effect of Gravity In a standing position, hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart

Control of Blood Flow by Tissues Local control In most tissues, blood flow is proportional to metabolic needs of tissues Nervous System Responsible for routing blood flow and maintaining blood pressure Hormonal Control Sympathetic action potentials stimulate epinephrine and norepinephrine

Local Control of Blood Flow by Tissues Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism Vasodilator substances produced as metabolism increases Vasomotion is periodic contraction and relaxation of precapillary sphincters

Nervous Regulation of Blood Vessels

Short-Term Regulation of Blood Pressure Baroreceptor reflexes Change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure Chemoreceptor reflexes Sensory receptors sensitive to oxygen, carbon dioxide, and pH levels of blood Central nervous system ischemic response Results from high carbon dioxide or low pH levels in medulla and increases peripheral resistance

Baroreceptor Reflex Control

Local mechanisms affect MAP:

Effects of pH and Gases

Long-Term Regulation of Blood Pressure Renin-angiotensin-aldosterone mechanism Vasopressin (ADH) mechanism Atrial natriuretic mechanism Fluid shift mechanism Stress-relaxation response

General control of MAP:

Renin-Angiotensin-Aldosterone Mechanism

Vasopressin (ADH) Mechanism

Long Term Mechanisms Which Lower Blood Volume Fluid shift Movement of fluid from interstitial spaces into capillaries in response to decrease in blood pressure to maintain blood volume Stress-relaxation Adjustment of blood vessel smooth muscle to respond to change in blood volume Atrial natriuretic factor Hormone released from cardiac muscle cells when atrial blood pressure increases, simulating an increase in urinary production, causing a decrease in blood volume and blood pressure

Chemoreceptor Reflex Control

Shock Inadequate blood flow throughout body Three stages Compensated: Blood pressure decreases only a moderate amount and mechanisms able to reestablish normal blood pressure and flow Progressive: Compensatory mechanisms inadequate and positive feedback cycle develops; cycle proceeds to next stage or medical treatment reestablishes adequate blood flow to tissues Irreversible: Leads to death, regardless of medical treatment

Fetal circulation: