Forces acting on blood during circulation Dynamics of Blood Flow 11.12.12 Forces acting on blood during circulation

Circulatory System A closed double-pump system: Left side of heart Lung Circulation Systemic Circulation Right side of heart

Circulatory System Branching of blood vessels Arteries branch into arterioles, veins into venules Arteries Arterioles Heart Capillaries Veins Venules

Forces acting on blood during circulation The main forces acting on blood during circulation Viscous force (Fv) Pressure gradient force FP (force produced by heart pump) Gravitational force FG According to Newton’s law of motion which also governs the motion of blood F = FV + FP+ FG

Viscous force for Newtonian Fluids Viscosity is the property of flowing fluid (liquid/gas) by virtue of which relative motion between layers in contact is opposed Relative motion causes internal friction between layers in contact. This internal friction is called viscous force For Newtonian fluids, the viscous force is proportional to the surface area (A) of the layer and the velocity gradient between layers (in the direction perpendicular to the layer)

Viscous force and viscosity of Newtonian Fluids η is the constant of proportionality and is the viscosity. Negative sign shows that F acts in a direction opposite to the one in which the layers move Viscosity can be expressed as the ratio of shear stress and time rate of shear strain (or shear rate)

Shear stress produced by a ball falling in a stationary liquid A spherical ball falling in a viscous fluid produces a shear stress It reaches a terminal speed when the sum of the forces acting on it is zero Laminar flow

Blood Pressure Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels The blood pressure in the circulation is principally due to the pumping action of the heart. Differences in blood pressure are responsible for blood flow from one location to another in the circulation

Cardiac Cycle - Filling of Heart Chambers 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

Cardiac Cycle - Filling of Heart Chambers

Mechanical systole and diastole

Systolic and Diastolic Pressure The force of contraction of the ventricles raises the pressure to about 120mmHg (systolic pressure) and the elastic recoil of the arteries maintain the pressure to about 80mmHg during ventricular diastole (diastolic pressure) This pressure is enough to keep the blood flowing continuously to all parts of the body

Mean arterial pressure It is a term used in medicine to describe an average blood pressure in an individual. It is defined as the average arterial pressure during a single cardiac cycle Mean arterial blood pressure is not the arithmetic mean of systolic and diastolic pressure but is instead about 93mmHg This is because the time the heart spends relaxing is longer than the time it spends contracting and ejecting blood into aorta

Blood Pressure Profile Blood pressure is highest in the arteries. It decreases as the circulating blood moves away from the heart through arterioles, capillaries and then to veins due to viscous losses of energy Although blood pressure drops over the whole circulation, most of the fall occurs along the arterioles

Reference points for measuring blood pressure While measuring pressures in cardiovascular system, ambient atmospheric pressure is used as zero reference point. Thus a blood pressure of 90mmHg means that pressure is 90mmHg above atmospheric pressure The second reference point for measuring blood pressure is anatomical and is the position of heart. For example, the usual convention is to measure blood pressure in the brachial artery above elbow i.e. approximately at hearts level when patient is seated If the blood pressure measurements are to be made in the legs, the patient is brought to lying down position. In this position vessel is approximately at cardiac level

Blood pressure measurement Direct method This is an invasive method in which artery or vein is cannulated or catheterised. Pressure measured by direct method is known as “end pressure” Here the kinetic energy of blood flow is measured in terms of pressure. Direct method is used in patients of ‘shock’ where indirect measurements may be inaccurate or indeed impossible indire

Indirect methods of blood pressure measurement Indirect method (non-invasive, measures lateral/side pressure) Auscultatory Oscillometric

Auscultatory Method The auscultatory method uses a stethoscope and a sphygmomanometer An inflatable cuff encircles the arm. Pressure in the cuff is transmitted through the tissue to compress brachial artery and can be viewed on a manometer A stethoscope is used to listen to sounds in the artery distal to the cuff. The sounds heard during measurement of blood pressure are not the same as the heart sounds 'lub' and 'dub' that are due to vibrations inside the ventricles that are associated with the snapping shut of the valves

Auscultatory Method If a stethoscope is placed over the brachial artery in a normal person no sound should be audible. As the heart beats, pulses (pressure waves) are transmitted smoothly via laminar (non-turbulent) blood flow throughout the arteries, and no sound is produced Similarly, if the cuff of a sphygmomanometer is placed around a patient's upper arm and inflated to a pressure above the patient's systolic blood pressure, there will be no sound audible. This is because the pressure in the cuff is high enough such that it completely occludes the blood flow

Oscillometric method The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure[ which are caused by the oscillations of blood flow, i.e. the pulse It uses a sphygmomanometer cuff, like the auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff.

Oscillometric method The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below diastolic pressure over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. It is essential that the cuff size is correct: undersized cuffs may yield too high a pressure; oversized cuffs yield too low a pressure

Oscillometric method When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The values of systolic and diastolic pressure are computed, results are displayed

Blood Pressure Effect of gravity on pressure Venous pressures -35 mm Hg 1 mm Hg 105 mm Hg Effect of gravity on pressure Distance heart-head~ 0.4 m Heart-feet ~ 1.4 m DP = rgh Arterial pressures 55 mm Hg 100 mm Hg 195 mm Hg 100 mmHg 95 mmHg 95 mm Hg

The pressure in any vessel above heart level is decreased by the effect of gravity The arterial pressure is increased by 0.77mmHg for every centimeter below the right atrium and similarly decreased for each cm above the right atrium