The physics of the lungs and breathing

Babylon University
college of dentistry / first stage / medical physics
Lecture four

Part one : The physics of the lungs and breathing

1-Function of the breathing system
2- The Airways
3- How Blood and Lungs Interact
4- Physics of The Alveoli
1-Function of the breathing system
The lungs ( pulmonary system ) serve as the supplier of O2 .The blood takes the O2 to the tissues , and removes the CO2 from the tissues , it must come in close contact with the air in the lungs in order to exchange its load of CO2 for a fresh load of O2 . The air we inspire is about 80 % N2 and 20 % O2 . Expired air is about 80 % N2 , 16% O2 and 4% CO2. The lungs perform other physiologic functions :
1- Exchange of O2 and CO2 .
2- Keeping the PH (acidity ) of the blood constant .
3- Play secondary roll in heat exchange and fluid balance of the body by warming the air we breathe in (inspire).
4- Controlling flow of air for talking , coughing , …., etc .
5- Voice production .
6- Removing the dust particles stuck to the moist lining of various airways .
2-The Airways
The principal air passages into the lungs are shown in Fig. Air normally enters the body through the nose where it is warmed (if necessary) ,filtered , and moisturized . The moist surfaces and the hairs in the nose trap dust particles. During heavy exercise , such as jogging , air is breathed in through the mouth and by passes this filter system .The air then passes through the windpipe (trachea) . The trachea divides in two (bifurcates) to furnish air to each lung through the bronchi. Each bronchus divides and redivides and about 15 more times, the resulting terminal bronchioles supply air to millions of small sacs called alveoli. The alveoli defined , which are like small interconnected bubbles ,are about 0.2 mm in diameter (a sheet of paper is 0.1mm thick) and have walls only 0.4 um thick . They expand and contract during breathing ,they are "where the action is " in the exchange of O2 and CO2.Each alveolus is surrounded by blood so that O2can diffuse from the alveolus into the red blood cells and CO2 can diffuse from the blood into the air in the alveolus.


3- How Blood and Lungs Interact
The primary purposes of breathing are to bring a fresh supply of O2 to the blood in the lungs and to dispose of the CO2 . Blood is pumped from the heart to the lungs under relatively low pressure. About (1 liter ) of blood supply in the lungs but only 70 ml is in the capillaries of the lungs getting O2 .The transfer of O2 and CO2 into and out of blood is controlled by law of diffusion . Molecules diffuse from region of higher concentration to lower concentration until concentration uniform. A molecule of O2 diffuses faster than CO2 because of its smaller mass.
Two general processes are involved in gas exchange in the lungs :
1- getting the blood to the pulmonary capillary bed ( perfusion )
2- getting the air to the alveolar surfaces ( ventilation ).
If either process fails the blood will not be properly oxygenated.
There are three types of ventilation perfusion areas in the lungs:
1- Area with good ventilation and good perfusion.
2- Area with good ventilation and poor perfusion.
3- Area with poor ventilation and good perfusion.
In a normal lung ,the first type accounts for over 90 % of the total volume .The second occur if the blood flow to part of a lung is blocked by a clot
That causes poor perfusion . In the third type the air passage in the lungs are obstructed
4- Physics of The Alveoli
The alveoli like millions of small interconnected bubbles they have tendency to get smaller due to surface tension of unique fluid lining . This lining called surfactant . The absence of surfactant in the lungs of some newborn infant is the cause of respiratory distress syndrome ( RDS ) called hyaline membrane disease causes death. To understand the physics of alveoli we have to understand the physics of bubbles . The pressure inside bubble is inversely proportional to radius and directly to surface tension. This relation called ( Laplace s Law) .
P= 4?/R where R radius , ? surface tension
Two forces keep lungs from collapsing :
1- Surface tension between lungs and chest wall .
2- Air pressure inside the lungs.

Part two : Application of electricity and magnetism in medicine
1- High- Frequency electricity In Medicine
2- Low-frequency electricity and magnetism in medicine
3- Electrocardiography
4- Electricity Waves – Energy Travel Over a Long Distance
1-High- Frequency electricity In Medicine
There are many method used in the thermal treatment include :
1- short wave
2- Ultrasound wave
3- microwave
4- infrared wave
5- electrical stimulation
Micro wave diathermy. Is different from short- wave diathermy, in short wave diathermy the tissue to be heated is part of resonance circuit where the tissue to be heated is placed between two capacitor plates that have an oscillating electric field across them. The changing electric field forces the ions in the tissue to move back and forth . they thus acquire kinetic energy , Part of kinetic energy dissipated when the ions collide with molecules in the tissues. While in micro wave diathermy the tissue absorbs electromagnetic waves that are incident up on it. microwave diathermy is used to heat joints , tendon sheaths , and muscles
The use of frequencies near 30 MHz for heating is called short-wave diathermy . Long - wave diathermy , at frequencies near 10 KHz , is no longer used . In short-wave diathermy two methods are used to get the electromagnetic energy into the body : the capacitance method and the inductance method . short-wave diathermy is used in the treatment of arthritis , traumatic injuries , strains , and sprains . However, it does have limitations . The amount of energy absorbed depends upon the frequency of the microwaves , the energy is absorbed best at frequencies near 20 GHz and poorly at lower frequencies near 100 MHz and at higher frequencies around 1000 GHz . Because the energy is deposited more effectively in tissue with high water content , microwave energy is absorbed better in muscle tissue than in fatty tissues , which have less water . And because of the large amount of energy deposited in surface fatty layers . For this reason microwave diathermy is frequently used.
2- Low-frequency electricity and magnetism in medicine
When electrical conductor is moved perpendicular to magnetic field , a voltage is induced in the conductor proportional to the product of the magnetic field and the velocity of the conductor (Faraday s Law). This Law also holds for conducting fluid moving perpendicular to the magnetic field . Blood acts as conducting fluid . If it passes with mean velocity (v) through magnetic field (B) as shown in ( fig. 11-9 page 245 ) . a voltage (V) is induced between the electrodes such that .
V = B d v
Where (d) is the diameter of the blood vessel . since (V, B, d ) can all be measured , the mean velocity can obtained . The volume flow of blood (Q ) through the vessel can then be calculated , since ( Q) is the product of the mean velocity times the area of the vessel ( ?d2/ 4 ) or
? d2 V
Q = x
4 B d
Example : A magnetic blood flow meter is positional across a blood vessel ( 5 * 10-3 m ) in diameter. With a magnetic field of (3*10-2 T) an induced voltage of ( 15*10-6 v ) is measured
a-Find the velocity in the vessel.

From V= B d v
V 1.5*10-5
v = = = 0.1 m/sec
B d (3*10-2) (5*10-3)
b- Assuming all the blood travels at the mean velocity, what is the volume flow rate ?

? d2 V
Q = x = 1.9 cm3/sec
4 B d
3- Electrocardiography (ECG ) is the process of recording the electrical activity of the heart over a period of time using electrodes placed on a patient s body. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle depolarizing during each heartbeat.
4- Electricity Waves - Energy Travel over a Long Distance
Electric force can transform into different types of energy waves, such as heat, radiation, radio and micro waves; and these energy waves can travel along distance. A changing magnetic field will induce a changing electric field and vice-versa, the two are linked. These changing fields form electromagnetic waves. Electromagnetic waves can travel not only through air and solid materials, but also through space.
The human nervous system can create electric energy waves that can be measured with scientific instruments. The human body produces infra-red radiation that, with night vision equipment, can be seen from miles away.