Work, Energy and Power RECENT

1. Work, Energy and Power kinetic energyis the energy that is produced when something moves MOHAMED ANWER RIFKY

2. 1-WORK: Work is done and energy is expended-- One joule of work is done when a force of one newton moves itspoint of application one metre in the direction of the force. A-An apple which has a mass of 102 grams is 1 newton>> raised 1 metre vertically against the force of gravity, 1 joule of work is performed as illustrated. Shorteningof the muscle multiplied by the mean force exerted>work. B-The mechanical energy of the apple is converted >>potential energy represented by additional height of the apple. 2-WORK OF A VENTILATOR DURING INSPIRATION: ). A constant-pressure generator type of ventilator: Work done = F x D Consequently the work done = PA x V/A (F = PA & Work done = (0.6 x 103 ) Pa x (0.5 x 10-3) m3 = 300 mJ= 0.3J indicates the work of the ventilator for one inspiration is still valid. The shaded area enclosed by the loop 3-ENERGY CHANGES: Energy cannot be lost,ininspiration half of the mechanical energy is stored (potentialenergy) in the elastic tissues of the lung and the chest wall.Thisenergy can be divided into two parts; 1-(stretching the elastic band), and 2-(in overcoming resistance) to airflow into the syringe model. a constant-pressure generator attached to the patient's lungs are represented by a syringe in which movement of the plunger is opposed by the stretching of an elastic band. In this model the plunger represents the diaphragm and the elastic band represents the elasticity of the lung tissues and chest wall.Airway resistance and the pressure within the syringe is indicated by PL.The end-inspiratory pressure in the syringereaches the ( PM ),pressure generated by the elastic band giveS a tracing C to D, the slopeof which represents ( compliance), stippled section(energyexpended in moving the gas against the airway resistance,PMis 0.6 kPa,with a volume of 0.5 litre ,and the energy expended in moving the gas is 1/2PV. 4-WORK OF EXPIRATION:The cycling device is open to the atmosphere >>pressure withinthe syringe then falls linearly to zero if plotted against volume expired.The work expended is consequently1/2 PVand is used in overcoming the airway resistance in the model. 5-WORK OF INSPIRATION IN SPONTANEOUS BREATHING: a-inspiration >>desent of the diaphragm 15 mm.(in the model). b-syringe now has two plungers with fluid between them(intrapleural)-right-hand plunger (diaphragm)and the left-hand plunger (lung tissues). c-A plot of volume against intrapleural pressure for a typical inspiration is shown.d-The stippled area on the graph gives the work expended in overcoming airway resistance and the triangular shaded area givesthe stored elastic energy which is used during expiration. e- In this model 0.5 litre of air is inspired when pressure is 0.6 kPa, then 300 mJ of work is done in inspiration. f-Chest expansion (not shown in the graph), also occurs(10 to 40%)of the inspired tidal volume in the respiratory ms.,10% of chemical E. >>mechanical E.,and 90%>>heat. . Simple syringe model syringe model 6-MEASURING THE WORK OF INSPIRATION: The total work of inspiration is more difficult to measure in a spontaneously breathing patient.the volume inspired may be measured by a pneumotachograph.Intrapleural pressure changes>> inhe loweroesophagus(a catheter with a balloon at its tip).The greater the expiratory resistance the greater the bowing of this line to the right( figure).Oesophageal pressure readings can’t give absolute accuracy(only one site and it varies accordingly).

3. 7-POWER OF BREATHING: 8-EFFICIENCY OF RESPIRATORY MUSCLES: Are only 10% efficient in producing mechanical energy, the rest >>heat. Thus, the actual energy requirements in the example above would be ten times greater than 80 Mw(i.e. about 800 mW). As the normal metabolic rate (80 W) so,energy requirements for breathing would be 1% of the total . Power is the rate of working and is measured in watts, 1 watt being 1 joule per second 9-THE EFFECT OF THE TYPE OF FLOW: : Where 9-EFFECT OF HYPERVENTILATION IN PATIENTS: The above calculations are simplified in that they do not allow for the kinetic energy in the flowing fluid, but in the flow normally present in therespiratory and circulatory systems the kinetic energy is very small. . The power of breathing increases rapidly at a rate approaching the cube of the gas flow>> increase O2 need,and try to avoid resp.stimulation. 10-LUNG COMPLIANCE AND AIRWAY RESISTANCE: The time-constant for the lungs =compliance X resistance (Patients with a low time-constant >>rapid respiratory rate .whereas,with a high constant >>slower rate. 11-WORK OF MYOCARDIAL CONTRACTION: A-left ventricular volume increased 60 ml during diastolic B-The pressure increased from 0 to 16 kPa (0-120 mmHg) during isovolumetric contraction. . C-Work done = (16 x 10 ) Pa x (60 x 10 ) m3 -6 3 = 960mJ D-If the heart rate is 60, then the power of the left ventricle of the heart is about 60 joules per minute, i.e. one watt. E-Continuous ventricular pressure>>intraventricular cardiac catheter, and volume measurements may be deduced >> ultrasonic flow measurement or estimated from echocardiography ,cineangiography or other imaging techniques. 12-POWER OF THE HEART: power as a product of pressure difference and fluid flow may also be applied to the cardiac output. A- C-The right heart can be similarly calculated>> = 0.2 W B- D-Total power of heart = 1.2 W E-If the efficiency of the heart were 15% in this instance, then the total energy requirements of 8 W would be 10% of a typical basal metabolism of 80W. F-Hypertension results in additional energy requirements even if the cardiac output is unchanged, Thyrotoxicosis, anaemia or exercise increases the work load of the heart, if maintained, may lead to heart failure. .