CVS Lecture 1 2024 PDF

BIOM2012 - Systems Physiology Cardiovascular System Stephen Anderson School of Biomedical Sciences  CVS topics Circulation and Lymph Circulation Blood pressure drives blood flow Arteries and Arterioles Capillaries and fluid exchange...

BIOM2012 - Systems Physiology Cardiovascular System Stephen Anderson School of Biomedical Sciences  CVS topics Circulation and Lymph Circulation Blood pressure drives blood flow Arteries and Arterioles Capillaries and fluid exchange Lymphatics Venous System Blood Pressure Regulation Cardiac Output – heart rate Cardiac Output – stroke volume Baroreceptor and chemoreceptor reflexes Longer-term regulation of blood pressure Humoral control Pathophysiology - Hypertension  Cardiovascular System Definition: “The organs and tissues involved in circulating blood and lymph through the body”  Cardiovascular System There are 3 essential parts to the CVS: the heart (biological pump) blood and lymph (carrier) And vessels (transport paths) From a physiological perspective, all CVS components (heart, blood, and vessels) have built-in control systems that regulate the cardiovascular system - as a whole. For example, during exercise: Blood releases O2 upon sensing metabolic by-products in muscle Heart rate increases to provide more blood flow during exercise Vessels to the muscle dilate to provide more blood flow  The Circulation The CVS has two circuits in series – systemic and pulmonary circuits, united by the heart. The systemic circuit perfuses most of the tissues and organs with blood. It is high pressure circuit. In contrast, the pulmonary circuit takes blood to and from the lungs. It is a low pressure circuit.  The Circulation A key feature to appreciate is that the blood flow in both circuits over time should be matched, otherwise blood will pool in one of the circuits. In physiological terms, the cardiac output of the right side of the heart is linked/matched with the cardiac output on the left side of the heart. Recall the heart acts as a functional syncytium, with both the left and right ventricles contracting together to ensure coordinated blood flow in both circuits.  Distribution of Blood The blood volume in circulation is unevenly distributed. The vast majority of blood is in the systemic circuit. Systemic veins are essentially blood volume reservoirs. This 'reserve' can be utilised when needed.  Blood Pressure & Flow Pressure = Force Area Pressure (1) – Pressure (2) Flow = Resistance B&B Fig 17-2  Blood Pressure & Flow Resistance is a measure of the opposition to blood flow in a vessel. The viscous resistance reflects the frictional interaction between adjacent layers of fluid, each of which moves at a different velocity. B&B Fig 17-5 ‘concentric undisturbed laminae’ the velocities increase from the wall to the centre of the cylinder.  Blood Pressure & Flow The resistance depends upon multiple factors: geometry of blood vessels and type of flow blood viscosity vessel length and vessel width - radius Such understanding comes from Hagen-Poiseuille law, which describes the flow of Newtonian fluids in a cylindrical tube, in cases where there is no appreciable turbulence ie. flow is laminar. Think about the viscosity of blood as a measure of the internal ‘slipperiness’ between layers of fluid.  Blood Pressure & Flow Flow = Pressure Resistance The implications of Poiseuille’s law are as follows: Flow is directly proportional to the axial pressure difference, Δ P. Flow is directly proportional to the fourth power of vessel radius. Flow is inversely proportional to both the length of the vessel and the viscosity of the fluid.  Blood Pressure & Flow Assumptions implicit in Poiseuille’s equation: The fluid must be incompressible. blood ✅ The tube must be simple geometry - straight, rigid, cylindrical, and unbranched. generally ✅ except vessels not rigid The velocity of the thin fluid layer at the wall must be zero (no “slippage”). blood ✅ The flow must be laminar, not turbulent. generally ✅ The flow must be steady. Blood flow is pulsatile. The viscosity of the fluid must be constant. generally ✅ But it must be constant in a “newtonian” sense; that is, the viscosity must be independent of the magnitude of the shear stress and the shear rate. Blood is a non-Newtonian fluid, due to the elastic behaviour of red blood cells.  A pump is required to create pressure Requirement = 5L per minute (400 million L in a lifetime) it needs a biological pump, with no room for error! How could you describe this heart in words, without filling a whole book? A note written by Leonardo da Vinci with an anatomical drawing of the heart, 1513  How does the heart keep up with flow requirements? Cardiac Output Pulmonary Cardiac Output (CO) is the amount of blood the heart pumps through the O2 poor O2 rich circulatory system in a minute. CO2 rich CO2 poor blood Cardiac Output blood Stroke Volume Heart rate (HR) (SV) Number of Volume of blood contractions expelled from the per minute left ventricle with each contraction Systemic CO = SV × HR  Haemodynamics Change Total Cardiac in Peripheral Output Pressure Resistance Flow = Pressure CO = ΔP / TPR Resistance basic formula apply to CVS as a whole Mean Blood flow Arterial CVP = 2 mmHg (Systemic) Pressure MAP = 97 mmHg Veins Arteries Low P High P Low R Low R High V Low V Capillaries (Microcirculation) High R P à Pressure R à Resistance Low V V à Volume CO = (MAP – CVP) / TPR add pressure detail  Total Peripheral Resistance TPR = Rarteries + Rarterioles + Rcapillaries + Rvenules + Rveins in both circuits the overall resistance of the circulation reflects the contributions of the network of vessels in both the systemic and pulmonary circuits.  Haemodynamics Change Total Cardiac in Peripheral Flow = Pressure Output Pressure Resistance Resistance CO = ΔP / TPR CO = (MAP – CVP) / TPR But CVP is close to zero (so disregard here) CO = (MAP) / TPR rearrange formula MAP = CO x TPR  Mean Arterial Pressure add CO = SV x HR Total Cardiac Peripheral Output Resistance MAP = (SVCO × HR) × TPR Stroke Heart rate Volume Mean Arterial Pressure average pressure through one cardiac cycle ie. the pressure that drives blood flow but recall blood pressure is not constant – it is pulsatile!  Mean Arterial Pressure 120 Systolic Blood Pressure mmHg Systole MAP 80 mmHg Diastolic Blood Pressure 1 Cardiac Cycle (Systole and Diastole) Diastole MAP = Diastolic + 1/3 Pulse Pressure  Mean Arterial Pressure MAP = (SVCO × HR) × TPR Arteries To systemic circulation  Mean Arterial Pressure Example 1 Increased Cardiac Output MAP = (SVCO × HR) × TPR ꜛ ꜛ Arteries To systemic circulation Increased CO Increased MAP Remember that blood flow does not stop in diastole (hard to illustrate) There is continuous pulsatile flow.  Mean Arterial Pressure Example 2 Change in Total Peripheral Resistance MAP = (SVCO × HR) × TPR Arteries To systemic circulation  Mean Arterial Pressure Example 2 Change in Total Peripheral Resistance MAP = (SVCO × HR) × TPR ꜛ Arteries To systemic circulation Atherosclerosis or Vasoconstriction  Mean Arterial Pressure Example 2 Change in Total Peripheral Resistance MAP = (SVCO × HR) × TPR ꜛ ꜛ Arteries To systemic circulation Atherosclerosis or vasoconstriction Increased MAP  Mean Arterial Pressure Arterial Blood Cardiac Output Volume SV x HR Mean Arterial Peripheral Arterial Pressure Resistance Compliance Short term Longer term physiological factors physical factors we will return to the regulation of blood pressure later