Lecture 15 Hemodynamics PDF

General Organization of Cardiovascular System Right Heart Pulmonary Circuit Systemic Circuit Left Heart • 2 pumps (right and left ventricles) • 2 circuits (pulmonary and systemic) connected in a series. • When circuits are connected in a series (in a steady state), flow must be equal in the two...

General Organization of Cardiovascular System Right Heart Pulmonary Circuit Systemic Circuit Left Heart • 2 pumps (right and left ventricles) • 2 circuits (pulmonary and systemic) connected in a series. • When circuits are connected in a series (in a steady state), flow must be equal in the two circuits. • Cardiac output of the left heart = cardiac output of the right heart • Venous return to the left heart = venous return to the right heart • Cardiac output from the heart equals venous return to the heart Objective 1 3 General Organization of Cardiovascular System Head & neck ARTERIAL CIRCULATION 1. High resistance Upper limbs Lungs Bronchial 2. High pressure (120/80 mmHg) Left atrium Right atrium 3. Low compliance Right vent. Left vent. Coronary Thorax VENOUS CIRCULATION 1. Low resistance 2. Low pressure (25/10 mmHg) 3. High compliance Hepatic Portal Liver Spleen Hepatic Peritubular Glomerular Kidneys Large & small intest. • Blood can take many different pathways from left heart to right heart: • Single capillary, coronaries • Two capillaries in series, glomerular and peritubular • Two capillaries beds in parallel, splenic and mesenteric merge to enter the portal circulation • In contrast, blood going from rt to lt heart can only take a single pathway, across single capillary bed in the lungs • Parallel arrangement prevents changes in blood flow to one vascular bed from significantly affecting blood flow to another organ. • Therefore, blood flow to a specific organ can be adjusted to meet its metabolic demands independent of other organs. Pelvic Lower limbs Objective 2 Figure 17-3 from Boron and Boulpaep’s Medical Physiology 2nd ed 4 Organization of the Cardiovascular System Direction of blood flow • From the lungs to the left atrium via the pulmonary vein • From the left atrium to the left ventricle through the mitral valve • From the left ventricle to the aorta through the aortic valve • From the aorta to the systemic arteries and the systemic tissues (i.e. cerebral, coronary, renal , splanchnic, skeletal, and skin) [parallel arrangement] • From the tissues to the systemic veins and vena cava • From the vena cava (mixed venous blood) to the right atrium • From the right atrium to the right ventricle through the tricuspid valve • From the right ventricle to the pulmonary artery through the pulmonic valve • From the pulmonary artery to the lungs for oxygenation Objective 3 5 Distribution of Blood in the Circulatory System • • • • Objective 4 Body contains about 5 L of blood High pressure (15%) vs low pressure (60%) the venous system acts as a volume reservoir Changes in the diameter of veins has a major impact on the amount of blood they contain 6 Hemodynamics Objective 5 What is Hemodynamics? Principles that govern blood flow in the cardiovascular system. The concepts of flow, pressure, resistance, and capacitance are applied to blood flow to and from the heart and within the blood vessels. 7 Objective 5 Hemodynamics: Velocity What is Velocity, v? Linear distance travelled per unit time: v = Δx / Δt A red blood cell is moving through an artery at the same velocity that the blood is moving. If that red blood cell moves 1 cm along the vessel in 1 sec, what is its velocity? Spoiler alert: Velocity = Δx / Δt à 1 cm / 1 sec à 1 cm / sec learn the units for each parameter… makes the relationships much easier to follow. But blood is a fluid and fluid has a volume…right? 8 Hemodynamics: Flow What is Flow, Q? The linear movement of a volume of a fluid Think: how blood much flows through a vessel per unit time? Units: ml / sec Objective 5 Remember the Spoiler : learn the units for each parameter… makes the relationships much easier to follow. Remember the red blood cell moving at 1 cm/sec (velocity). How can we know how much blood is flowing to carry that red blood cell along? Use the area of the blood vessel: Area = π r2 with units of cm2 Multiply the velocity (cm/sec) of the blood X the area (cm2) of the blood vessel à cm3/sec = ml / sec This brings up a CENTRAL principle in hemodynamics: The radius (or cross-sectional area) of a blood vessel is a MAJOR determinant for how much blood can flow in that vessel. First look: consider blood flow in the aorta (large radius) versus a typical artery (smaller radius). 9 Hemodynamics: Pressure Objective 5 units What is Pressure, P? Units = mm Hg The action of a force against an opposing force... Force per area. Think about air escaping out of a balloon: High pressure inside the balloon… low pressure outside the balloon: ΔP Air moves out of the balloon and the pressure drops from inside to outside The ventricle creates pressure in the circulation by pushing blood into the arteries. Blood that returns to the heart has significantly lower pressure… ΔP Therefore, blood will continuously move through the circulation. First Look: the radius of the vessel matters (see note) 10 Hemodynamics: Resistance Objective 5 What is Resistance? The opposition to the movement of an object / substance. Think: what restricts blood flow through a vessel? Greater restriction = greater resistance …… Less restriction = less resistance. 8ηl See note on units For blood, three main variables (Poiseuille’s equation): R = πr( • Viscosity (η)à how “stiff” a fluid is… think mineral oil (low) versus syrup (high) • Length (l) of the vessel à the longer the vessel, the more resistance • Radius of vessel à KEY variable… R(esistance) inversely related to 1/r(adius)4 11 Hemodynamics: Capacitance Objective 5 What is Capacitance, C (ml/mm Hg)? The ability to hold or store Veins have large cross-sectional area and are compliant They can hold a large volume of blood Velocity is low but flow is high, even at very low pressures Think of a deep, wide river: lots of water moves slowly while looking calm C = V⁄P and C = ΔV⁄ΔP Capacitance (ml/mmHg) = volume (ml) / pressure (mmHg) At a constant capacitance, C When the pressure increases, veins will expand and hold more blood When the pressure decreases, veins will hold less blood 12 Hemodynamics: Pressure, Resistance, Flow 8ηl ⁄ Blood Flow: Q = ∆ P R where R = πr, Objective 6 Blood flow is greater when the pressure difference is greater. Blood flow is greater when vessel resistance (R ) is smaller. Resistance is lower when the radius (r ) is larger Want to change blood flow? Change pressure created by ventricle (more or less forceful contractions, SV) Change the cross-sectional area (or radius, r) of the vessel. ß BIGGEST EFFECT… 1/r4 First look: vasoconstriction and vasodilation major way to alter blood flow 13 Hemodynamics: Whole Body Objective 6 MAP/RAP Cardiac Output = TPR Cardiac Output is Blood Flow driven by the Heart Cardiac Output = the volume of blood pumped by the left ventricle per unit time MAP = Mean Arterial Pressure, the average blood pressure in all arteries RAP = Right Atrial Pressure, the lowest pressure of the systemic circulation TPR = Total Peripheral Resistance, the resistance of all blood vessels opposing flow MAP-RAP represents ΔP for the systemic circulation à Q = ∆ P⁄R 14 Hemodynamics: Everything Needed Velocity of blood flow: • V= velocity (cm/sec) • Q= blood flow (mL/sec) • A= cross-sectional area (cm2) Blood flow: • Q= flow or cardiac output (mL/min) • ΔP= pressure gradient (mm Hg) • R= resistance or total peripheral resistance (mm Hg/mL/min) • MAP = mean arterial pressure • RAP= right atrial pressure • TPR= total peripheral resistance Resistance: • R= resistance • η= viscosity of blood • l= length of blood vessel • r4= radius of vessel to the fourth power Capacitance (compliance): • C= capacitance or compliance (mL/ mmHg) • V= volume (mL) • P= pressure (mm Hg) 15 Velocity of Blood Flow Objective 8 The velocity of blood flow is the rate of displacement of blood per unit time. • The relationship between velocity, flow, and cross-sectional area is as follows: v = Q⁄A • V : velocity of blood flow is linear velocity expressed in units of distance per unit time (cm/sec) • Q : is volume flow per unit time (mL/sec) • A : The cross-sectional area of the blood vessel (e.g., aorta versus arteriole) Figure 4-3 and 4-4 from Costanzo’s Physiology 5th Ed. 20 Blood Flow Depends on the Pressure Gradient Objective 8 • Blood Flow through a blood vessel (or series of blood vessels) is determined by 2 factors: • Pressure Difference (ΔP) between in the 2 ends of the vessel • Resistance (R) • The equation for blood flow is expressed as follows: Q = ΔP/R Q = Flow (mL/min) ΔP = Pressure Difference (mmHg) R = Resistance (mm Hg/mL/min) • The magnitude of blood flow (Q) is directly proportional to the pressure gradient. • The direction of blood flow is determined by direction of pressure gradient and is always from high to low pressure. • Blood flow is inversely proportional to resistance (R): ↑ R (e.g. vasoconstriction) ↓ Q -or- ↓ R (e.g. vasodilation)↑ Q 22 Resistance to Blood Flow r r r  Poiseuille’s Law R= Flow ∝ r 4 8ηl ∆P πr ( = Q R= Resistance η = Viscosity of blood L= length of blood vessel r4 =radius of blood vessel Objective 8 Important Concepts: • Resistance to flow is directly proportional to viscosity (η) of blood • Resistance to flow is directly proportional to length (l) of blood vessel • Resistance to flow is inversely proportional to the fourth power of the radius (r4) of the blood vessel • Flow increases with the 4th power of the vessel radius (at constant pressure) 23 Arterial Pressure in the Systemic Circulation Systolic blood pressure 120 Dicrotic notch Pulse pressure Pressure 100 (mmHg) Mean arterial 80 blood pressure Aortic valve opens • The pulsatile arterial blood flow is caused by phasic cardiac ejection • Each pulsation in the arteries coincides with one cardiac cycle Diastolic blood pressure Normal Values • Systolic blood pressure (SBP): peak pressure during contraction of the heart (systole) 120 mmHg • • Diastolic blood pressure (DBP): minimum arterial pressure during relaxation of the heart (diastole) 80 mmHg • Pulse pressure = SBP – DBP 40 mmHg • Mean arterial blood pressure (MAP) = DBP + 1/3 (SBP – DBP) 93 mmHg Note: MAP is NOT the simple mathematic average of diastolic and systolic pressures. WHY? A greater fraction of each cardiac cycle is spent in diastole thus, MAP gives more weight to diastolic pressure 29