How much gas is exchanged in one human breath?

How much gas is exchanged in one human breath?

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When we breathe, our lungs absorb a portion of the oxygen in the air, and replace it with some amount of carbon dioxide and water vapor. Typically, how much $O_2$ (in grams, milliliters, or moles for instance) is absorbed and how much $CO_2$ and $H_2O$ are released in one breath of a healthy adult?

Of course, the exact amounts will vary from person to person and based on how deeply the person is breathing, lung health, etc. I'm just looking for a ballpark figure.

According to Wikipedia

"In a healthy, young adult, tidal volume is approximately 500 ml per inspiration… "

(tidal volume is the volume inspired/expired)

Using this figure, together with values for gas composition also taken from Wikipedia, I estimate that in each breath we take in 18 mg O2 (1.1 mmol) and we release 36 mg of CO2 (1.2 mmol) plus 20 mg H2O (1.1 mmol). These are, as you say, ballpark figures.

Sample calculation:

O2 inspired = 21% by volume; O2 expired = 16% by volume

O2 change = 5% by volume = 5*500/100 = 25 mL

1 mole gas = 22.4 L; 1 mmol gas = 22.4 mL

O2 change = 25/22.4 mmol = 1.1 mmol

MW O2 = 16

O2 change = 17.6 mg

The relative values are reassuringly close to what you might predict from the textbook equation for oxidation of carbohydrate: C6H12O6 + 6O2 -> 6CO2 + 6H2O

Gas Exchange

IVOX relies on the placement of a hollow-fiber oxygenator mounted on a double-lumen catheter. The fibers are 200 μm in diameter and contain micropores that facilitate gas exchange . Containing many thousands of fibers and extending around 40–50 cm in length once inserted via the femoral or internal jugular routes, the system extends from superior to inferior vena cava across the right atrium. Oxygen is supplied via negative pressure to allow gas exchange to occur between the intraluminal gas phase and the extraluminal blood phase by diffusion. Complications of use include those of central venous access, hemorrhage (in part due to the necessity for systemic anticoagulation), infection, deep venous thrombosis (due to a reduction in venous drainage from the lower limb following femoral venous placement), and reduction in the efficiency of the device due to thrombus formation within the fiber bundle.

IVOX was indicated for oxygenation of patients with severe acute respiratory distress syndrome (ARDS) to allow for less aggressive conventional ventilation in the hope of avoiding baro- and volutrauma to the lungs. However, partly because of an increase in the sophistication of current ventilators and a better understanding of the disease process, and partly because of the complexity and complication rate of IVOX, it has now fallen out of favor.

How much oxygen does the human lung consume?

Background: The amount of oxygen consumed by the lung itself is difficult to measure because it is included in whole-body gas exchange. It may be increased markedly under pathological conditions such as lung infection or adult respiratory distress syndrome. To estimate normal oxygen consumption of the human lung as a basis for further studies, respiratory gas analysis during total cardiopulmonary bypass may be a simple approach because the pulmonary circulation is separated from systemic blood flow during this period.

Methods: Lung oxygen consumption was determined in 16 patients undergoing cardiac surgery. During total cardiopulmonary bypass their lungs were ventilated with low minute volumes (tidal volume, 150 ml rate, 6 min-1 inspiratory oxygen fraction, 0.5 positive end-expiratory pressure, 3 mmHg). All expiratory gas was collected and analyzed by indirect calorimetry. As a reference value also, whole-body oxygen consumption of these patients was determined before total cardiopulmonary bypass. In a pilot study of eight additional patients (same ventilatory pattern), the contribution of systemic (bronchial) blood flow to pulmonary gas exchange during cardiopulmonary bypass was assessed. For this purpose, the amount of enflurance diffusing from the systemic blood into the bronchial system was measured.

Results: The human lung consumes about 5-6 ml oxygen per minute at an esophageal temperature of 28 degrees C. Prebypass whole-body oxygen consumption measured at nearly normothermic conditions was 198 +/- 28 ml/min. Mean lung and whole-body respiratory quotients were similar (0.84 and 0.77, respectively). Extrapolating lung oxygen consumption to 36 degrees C suggests that the lung consumes about 11 ml/min or about 5% of total body oxygen consumption. Because the amount of enflurane diffused from the systemic circulation into the bronchial system during cardiopulmonary bypass was less than 0.1%, the contribution of bronchial blood flow to lung gas exchange can be assumed to be negligible.

Conclusions: The lung consumes about 5% of whole-body oxygen uptake.

Exchanging Oxygen and Carbon Dioxide

The primary function of the respiratory system is to take in oxygen and eliminate carbon dioxide. Inhaled oxygen enters the lungs and reaches the alveoli. The layers of cells lining the alveoli and the surrounding capillaries are each only one cell thick and are in very close contact with each other. This barrier between air and blood averages about 1 micron ( 1 /10,000 of a centimeter, or 0.000039 inch) in thickness. Oxygen passes quickly through this air-blood barrier into the blood in the capillaries. Similarly, carbon dioxide passes from the blood into the alveoli and is then exhaled.

Oxygenated blood travels from the lungs through the pulmonary veins and into the left side of the heart, which pumps the blood to the rest of the body (see Function of the Heart). Oxygen-deficient, carbon dioxide-rich blood returns to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. Then the blood is pumped through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide.

The function of the respiratory system is to add oxygen to the blood and remove carbon dioxide. The microscopically thin walls of the alveoli allow inhaled oxygen to move quickly and easily from the lungs to the red blood cells in the surrounding capillaries. At the same time, carbon dioxide moves from the blood in the capillaries into the alveoli.

To support the absorption of oxygen and release of carbon dioxide, about 5 to 8 liters (about 1.3 to 2.1 gallons) of air per minute are brought in and out of the lungs, and about three tenths of a liter (about three tenths of a quart) of oxygen is transferred from the alveoli to the blood each minute, even when the person is at rest. At the same time, a similar volume of carbon dioxide moves from the blood to the alveoli and is exhaled. During exercise, it is possible to breathe in and out more than 100 liters (about 26 gallons) of air per minute and extract 3 liters (a little less than 1 gallon) of oxygen from this air per minute. The rate at which oxygen is used by the body is one measure of the rate of energy expended by the body. Breathing in and out is accomplished by respiratory muscles.

Gas Exchange Between Alveolar Spaces and Capillaries

The function of the respiratory system is to move two gases: oxygen and carbon dioxide. Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.

Anatomy and Physiology: Gas Laws and Breathing

Every chemistry student learns three basic gas laws: Charles's law, Boyle's law, and Dalton's law. In terms of respiration, Charles's law is the least applicable since body temperature rarely changes by much. Charles's law states the given constant pressure as the temperature of the gas increases so does the pressure. Boyle's and Dalton's laws, however, very much apply.

Contents Under Pressure

Boyle's law refers to a simple inverse relationship between volume and pressure. As thevolume of a container increases, the pressure of the gas within the container decreases.Conversely, a decrease in the size of a container will increase the pressure of the gas inside. In terms of containers, think about the thoracic cavity. The thoracic cavity is enclosed by the rib cage and by the diaphragm. Although the inside of the lungs is directly open to the outside environment, the area outside the lungs is not. As you will see, this is very important in your ability to breathe.

Flex Your Muscles

A great illustration of this is to take an empty 2-liter soda bottle with the cap off and squeeze it. It doesn't put up much resistance. Now put the cap on and try to squeezeit again. At this point, with the enclosed container, the gas molecules inside areaffected by the change in volume, increase the pressure, and push back against themuscles of your hand.

Figure 13.7 The control of gas pressure (through Boyle's law) is controlled by the contraction of the diaphragm and the rib cage.

Don't Stop Bellowing

Many muscles are involved in breathing. Some, such as the internal and external intercostal muscles, control the movement of the rib cage others control the movement of the abdomen. The most important muscle, however, in breathing is the diaphragm. The diaphragm is a dome-shaped muscle, and like every muscle, it moves by contracting. The dome shape is important because when the diaphragm contracts, the curve of the dome becomes flatter and shallow. As such, when the diaphragm contracts, the thoracic cavity gets bigger (see Figure 13.7).

The increase in the volume of the thoracic cavity means a slight drop in pressure. At this point the pressure of the outside air is slightly higher than the pressure inside the thoracic cavity. Since pressure moves, just like diffusion, from high to low, the outside air rushes in and fills the lungs. The dome shape kicks in again when the diaphragm relaxes, because relaxation causes the diaphragm to resume its dome shape in a process known as elastic rebound. This reduces the volume of the thoracic cavity, increases its pressure, and forces the air out. The change in pressure need not be large, however, for in normal breathing the pressure inside the thoracic cavity changes only slightly: air pressure at sea level = 760 mm Hg (mercury), normal inhalation = 759 mm Hg, normal exhalation = 761 mm Hg!

Crash Cart

One of the most common mistakes students make is to refer to ?sucking in air,? or ?sucking on a straw.? When people think of suction, they think that the air is being pulled into the straw, or into the vacuum cleaner. Nothing could be further from the truth! The lower pressure inside the mouth allows the higher pressure in the air to push down on the surface of the fluid you are drinking, thus pushing the fluid up the straw. There is no ?sucking force? in science, also known as ?Nothing Sucks in Science!?

Dalton's Law and Partial Pressure

Dalton's law states that each of the gases in a gas solution (such as air) exerts its own pressure based upon its concentration in the solution (see Figure 13.8). The air you breathe is made predominantly of two gases: nitrogen (78.6 percent) and oxygen (20.9 percent). The rest, only 0.5 percent, is mostly water, although in the summer on the East Coast it sure feels like more! Surprisingly, carbon dioxide is only 0.04 percent of the air!

Using ?P? or ?p? for partial pressure, air follows this formula for partial pressure:

with the percentages from above:

Figure 13.8 Each of the different gases in the air exerts a different partial pressure (Dalton's law). (Michael J. Vieira Lazaroff)

In the alveoli, the carbon dioxide reaches up to 5.2 percent. With a partial pressure of 40 mm Hg, this is 1,000 times higher than the pressure in the air! As you can imagine, you have no problem getting rid of carbon dioxide! On the other hand, the levels of oxygen drop as low as 13.2 percent, or a partial pressure of 100 mm Hg. Clearly you don't use all the oxygen with every breath?good news for people trapped with a limited supply of oxygen!

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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CBSE Class-11 Biology
Breathing and Exchange of Gases class 11 Notes Biology

The process of exchange of O2 from the atmosphere with CO2 produced by the cell is called breathing. It occurs in two stages of inspiration and expiration. During inspiration air enters the lungs from atmosphere and during expiration air leaves the lungs.

b. It is a physical process.

c. No energy is released.

b. It is a biochemical process.

c. Energy is released in form of ATP.

Respiratory Organs – Mechanism of breathing varies in different organism according to their body structure and habitat.

Respiratory OrgansOrganisms
Entire Body surfaceSponges, coelenterate, flatworms.
Tracheal systemInsects
GillsPisces, aquatic arthropods.
LungsAmphibians, mammals.

Human Respiratory System

  • Human respiratory system consists of a pair of nostrils, pharynx, larynx, bronchi and bronchioles that finally terminates into alveoli.
  • Nasal chamber open into pharynx that leads to larynx. Larynx contains voice box (sound box) that help in sound production.
  • The trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings to prevent collapsing in absence of air.
  • Each bronchiole terminates into an irregular walled, vascularized bag like structure called alveoli.
  • The branching network of bronchi, bronchioles and alveoli collectively form the lungs.
  • Two lungs are covered with double layered pleura having pleural fluid between them to reduce the friction on lung surface.
  • Conducting parts include nostrils, pharynx, larynx and trachea. Main functions include-
  1. Transport of atmospheric air to alveoli.
  2. Removing foreign particles from air, humidifying it and bringing it to body temperature.
  • The exchange parts are alveoli. It is the site of actual diffusion of and C between blood and atmospheric air.

Steps of Respiration

  1. Breathing in which Oxygen rich atmospheric air is diffused in and C rich alveolar air is diffused out.
  2. Diffusion of gases across alveolar membrane.
  3. Transport of gases by blood.
  4. Diffusion of and C between blood and tissues.
  5. Utilization of by cells to obtain energy and release of C (cellular respiration).

Mechanism of Breathing

  • Breathing involves inspiration and expiration. During inspiration atmospheric air is drawn in and during expiration, alveolar air is released out.
  • Movement of air in and out takes place due to difference in pressure gradient.
  • Inspiration occurs when pressure inside the lung is less and expiration occurs when pressure is more in lungs than outside.
  • The diaphragm and external and internal intercostal muscles between the ribs help in developing pressure gradient due to change in volume.
  • The contraction of intercostal muscles lifts the ribs and sternum causing an increase in volume of thoracic cavity that results in decrease in pressure than the atmospheric pressure. This causes inspiration.
  • Relaxation of the diaphragm and intercostal muscles reduce the thoracic volume and increase the pressure causing expiration.
  • The volume of air involved in breathing movements is estimated by using spirometer for clinical assessment of pulmonary functions.

Respiratory Volume and Capacities

Tidal volume (TV) – volume of air inspired or expired during a normal respiration. It is about 500mL in healthy man.

Inspiratory Reserve Volume (IRV) – additional volume of air a person can inspire by forceful inspiration. It is about 2500 mL to 3000mL.

Expiatory Reserve Volume (ERV) – additional volume of air a person can expire by forceful expiration. It is about 1000 mL to 1100mL.

Residual Volume (RV) – volume of air remaining in lungs even after a forcible expiration. It is about 1100mL to 1200mL.

Inspiratory Capacity (IC) – TV + IRV

Expiratory Capacity (EC) – TV + ERV

Functional Residual Capacity (FRC) – ERV + RV

Vital Capacity (VC) – maximum volume of air a person can breathe in after a forceful expiration. ERV+ TV+ IRV

Total Lung Capacity (TLC) – total volume of air accommodated in lung at the end of forced inspiration. RV+ ERV+ TV+ IRV or Vital capacity + Residual Volume.

Exchange of Gases

  • Exchange of gases takes place at two sites
  1. Alveoli to blood
  2. Between blood and tissues.
  • Exchanges of gases occur by simple diffusion due to pressure/ concentration gradient, solubility of the gases and thickness of membrane.
  • Pressure contributed by individual gas in a mixture of gas is called partial pressure represented by pC and p .
  • Partial pressure of Oxygen and carbon dioxide at different part involved in diffusion varies from one part to another and moves from higher partial pressure to lower partial pressure.
  • Solubility of C is 20-25 times more than solubility of , so C diffuse much faster through membrane.
  • Diffusion membrane is three layered thick, that is alveolar squamous epithelium, endothelium of alveolar capillaries and basement substance between them.

Transport of Gases

  • Blood is the medium of transport for C and . Most of oxygen (97%) is transported through RBC and remaining 3% by blood plasma.
  • 20-25% of C is transported by RBC, 70% as bicarbonate and rest 7% in dissolved state by blood plasma.

Transport of Oxygen

  • Haemoglobin in RBC combines with to form Oxyhaemoglobin. Each haemoglobin combine with four oxygen molecules.
  • Binding of is related with partial pressure of and , hydrogen ion concentration and temperature.
  • Percentage saturation of haemoglobin and partial pressure of oxygen forms sigmoid curve (oxygen dissociation curve).
  • In the alveoli, p is more and pC is less, less H+ ions concentration and lower temperature favour the binding of with hemoglobin. Where opposite condition in tissues favour the dissociation of Oxyhaemoglobin.

Transport of Carbon dioxide

  • Carbon dioxide is transported by haemoglobin as carbamino-haemoglobin. In tissues pC is high and p is less that favour the binding of carbon dioxide with haemoglobin. Opposite condition help in dissociation of carbamino- haemoglobin in alveoli.
  • Enzyme carbonic anhydrase help in formation of carbonate ions to transport carbon dioxide.

Regulation of Respiration

  • Human beings have ability to maintain and moderate the rate of respiration to fulfill the demand of body tissues by neural system.
  • Respiratory rhythm centre is located in medulla region of hind brain. Pneumotaxic centre in pons moderate the function of respiratory rhythm centre.
  • Chemo-sensitive area near rhythm centre is highly sensitive to C and H+ ions that ultimately control the respiratory rate. Oxygen do not play major role in controlling rate of respiration.

Functions of Respiration

  1. Energy production
  2. Maintenance of acid-base balance.
  3. Maintenance of temperature
  4. Return of blood and lymph.

Mountain Sickness is the condition characterised by the ill effect of hypoxia (shortage of oxygen) in the tissues at high altitude commonly to person going to high altitude for the first time.

  • Loss of appetite, nausea, and vomiting occurs due to expansion of gases in digestive system.
  • Breathlessness occurs because of pulmonary oedema.
  • Headache, depression, disorientation, lack of sleep, weakness and fatigue.

Disorder of Respiratory System

  1. Asthma– it is due to allergic reaction to foreign particles that affect the respiratory tract. The symptoms include coughing, wheezing and difficulty in breathing. This is due to excess of mucus in wall of respiratory tract.
  2. Emphysema– is the inflation or abnormal distension of the bronchioles or alveolar sacs of lungs. This occurs due to destroying of septa between alveoli because of smoking and inhalation of other smokes. The exhalation becomes difficult and lung remains inflated.
  3. Occupational Respiratory Disorders– occurs due to occupation of individual. This is caused by inhalation of gas, fumes or dust present in surrounding of work place. This includes Silicosis, Asbestoses due to exposer of silica and asbestos. The symptom includes proliferation of fibrous connective tissue of upper part of lung causing inflammation.
  4. Pneumonia– it is acute infection or inflammation of the alveoli of the lungs due to bacterium streptococcus pneumoniae. Alveoli become acutely inflamed and most of air space of the alveoli is filled with fluid and dead white blood corpuscles limiting gaseous exchange.


The main reason for exhalation is to rid the body of carbon dioxide, which is the waste product of gas exchange in humans. Air is brought in the body through inhalation. During this process air is taken in through the lungs. Diffusion in the alveoli allows for the exchange of O2 into the pulmonary capillaries and the removal of CO2 and other gases from the pulmonary capillaries to be exhaled. In order for the lungs to expel air the diaphragm relaxes, which pushes up on the lungs. The air then flows through the trachea then through the larynx and pharynx to the nasal cavity and oral cavity where it is expelled out of the body. [1] Exhalation takes longer than inhalation and it is believed to facilitate better exchange of gases. Parts of the nervous system help to regulate respiration in humans. The exhaled air isn't just carbon dioxide it contains a mixture of other gases. Human breath contains volatile organic compounds (VOCs). These compounds consist of methanol, isoprene, acetone, ethanol and other alcohols. The exhaled mixture also contains ketones, water and other hydrocarbons. [2] [3]

It is during exhalation that the olfaction contribution to flavor occurs in contrast to that of ordinary smell which occurs during the inhalation phase. [4]

Spirometry is the measure of lung function. The total lung capacity (TLC), functional residual capacity (FRC), residual volume (RV), and vital capacity (VC) are all values that can be tested using this method. Spirometry is used to help detect, but not diagnose, respiratory issues like COPD, and asthma. It is a simple and cost effective screening method. [5] Further evaluation of a person's respiratory function can be done by assessing the minute ventilation, forced vital capacity (FVC), and forced expiratory volume (FEV). These values differ in men and women because men tend to be larger than women.

TLC is the maximum amount of air in the lungs after maximum inhalation. In men the average TLC is 6000 ml, and in women it is 4200 ml. FRC is the amount of air left in the lungs after normal exhalation. Men leave about 2400 ml on average while women retain around 1800 ml. RV is the amount of air left in the lungs after a forced exhalation. The average RV in men is 1200 ml and women 1100 ml. VC is the maximum amount of air that can be exhaled after a maximum inhalation. Men tend to average 4800 ml and women 3100 ml. [ citation needed ]

Smokers, and those with Asthma and COPD, have reduced airflow ability. People who suffer from asthma and COPD show decreases in exhaled air due to inflammation of the airways. This inflammation causes narrowing of the airways which allows less air to be exhaled. Numerous things cause inflammation some examples are cigarette smoke and environmental interactions such as allergies, weather, and exercise. In smokers the inability to exhale fully is due to the loss of elasticity in the lungs. Smoke in the lungs causes them to harden and become less elastic, which prevents the lungs from expanding or shrinking as they normally would. [ citation needed ]

Dead space can be determined by two types of factors which are anatomical and physiological. Some physiological factors are having non-perfuse but ventilated alveoli, such as a pulmonary embolism or smoking, excessive ventilation of the alveoli, brought on in relation to perfusion, in people with chronic obstructive lung disease, and “shunt dead space,” which is a mistake between the left to right lung that moves the higher CO2 concentrations in the venous blood into the arterial side. [6] The anatomical factors are the size of the airway, the valves, and tubing of the respiratory system. [6] Physiological dead space of the lungs can affect the amount of dead space as well with factors including smoking, and diseases. Dead space is a key factor for the lungs to work because of the differences in pressures, but it can also hinder the person. [ citation needed ]

One of the reasons we can breathe is because of the elasticity of the lungs. The internal surface of the lungs on average in a non-emphysemic person is normally 63m2 and can hold about 5lts of air volume. [7] Both lungs together have the same amount of surface area as half of a tennis court. Disease such as, emphysema, tuberculosis, can reduce the amount of surface area and elasticity of the lungs. Another big factor in the elasticity of the lungs is smoking because of the residue left behind in the lungs from the smoking. The elasticity of the lungs can be trained to expand further. [ citation needed ]

Brain control of exhalation can be broken down into voluntary control and involuntary control. During voluntary exhalation, air is held in the lungs and released at a fixed rate. Examples of voluntary expiration include: singing, speaking, exercising, playing an instrument, and voluntary hyperpnea. Involuntary breathing includes metabolic and behavioral breathing. [ citation needed ]

Voluntary expiration Edit

The neurological pathway of voluntary exhalation is complex and not fully understood. However, a few basics are known. The motor cortex within the cerebral cortex of the brain is known to control voluntary respiration because the motor cortex controls voluntary muscle movement. [8] This is referred to as the corticospinal pathway or ascending respiratory pathway. [8] [9] The pathway of the electrical signal starts in the motor cortex, goes to the spinal cord, and then to the respiratory muscles. The spinal neurons connect directly to the respiratory muscles. Initiation of voluntary contraction and relaxation of the internal and external internal costals has been shown to take place in the superior portion of the primary motor cortex. [8] Posterior to the location of thoracic control (within the superior portion of the primary motor cortex) is the center for diaphragm control. [8] Studies indicate that there are numerous other sites within the brain that may be associated with voluntary expiration. The inferior portion of the primary motor cortex may be involved, specifically, in controlled exhalation. [8] Activity has also been seen within the supplementary motor area and the premotor cortex during voluntary respiration. This is most likely due to the focus and mental preparation of the voluntary muscular movement. [8]

Voluntary expiration is essential for many types of activities. Phonic respiration (speech generation) is a type of controlled expiration that is used every day. Speech generation is completely dependent on expiration, this can be seen by trying to talk while inhaling. [10] Using airflow from the lungs, one can control the duration, amplitude, and pitch. [11] While the air is expelled it flows through the glottis causing vibrations, which produces sound. Depending on the glottis movement the pitch of the voice changes and the intensity of the air through the glottis change the volume of the sound produced by the glottis. [ citation needed ]

Involuntary expiration Edit

Involuntary respiration is controlled by respiratory centers within the medulla oblongata and pons. The medullary respiratory center can be subdivided into anterior and posterior portions. They are called the ventral and dorsal respiratory groups respectively. The pontine respiratory group consists of two parts: the pneumotaxic center and the apneustic center. [9] All four of these centers are located in the brainstem and work together to control involuntary respiration. In our case, the ventral respiratory group (VRG) controls involuntary exhalation.

The neurological pathway for involuntary respiration is called the bulbospinal pathway. It is also referred to as the descending respiratory pathway. [9] “The pathway descends along the spinal ventralateral column. The descending tract for autonomic inspiration is located laterally, and the tract for autonomic expiration is located ventrally.” [12] Autonomic Inspiration is controlled by the pontine respiratory center and both medullary respiratory centers. In our case, the VRG controls autonomic exhalation. Signals from the VRG are sent along the spinal cord to several nerves. These nerves include the intercostals, phrenic, and abdominals. [9] These nerves lead to the specific muscles they control. The bulbospinal pathway descending from the VRG allows the respiratory centers to control muscle relaxation, which leads to exhalation.

Yawning Edit

Yawning is considered a non-respiratory gas movement. A non-respiratory gas movement is another process that moves air in and out of the lungs that don't include breathing. Yawning is a reflex that tends to disrupt the normal breathing rhythm and is believed to be contagious as well. [13] The reason why we yawn is unknown, but some think we yawn as a way to regulate the body's levels of O2 and CO2. Studies done in a controlled environment with different levels of O2 and CO2 have disproved that hypothesis. Although there isn't a concrete explanation as to why we yawn, others think people exhale as a cooling mechanism for our brains. Studies on animals have supported this idea and it is possible humans could be linked to it as well. [14] What is known is that yawning does ventilate all the alveoli in the lungs.

Receptors Edit

Several receptor groups in the body regulate metabolic breathing. These receptors signal the respiratory center to initiate inhalation or exhalation. Peripheral chemoreceptors are located in the aorta and carotid arteries. They respond to changing blood levels of oxygen, carbon dioxide, and H + by signaling the pons and medulla. [9] Irritant and stretch receptors in the lungs can directly cause exhalation. Both sense foreign particles and promote spontaneous coughing. They are also known as mechanoreceptors because they recognize physical changes not chemical changes. [9] Central chemoreceptors in the medulla also recognize chemical variations in H + . Specifically, they monitor pH change within the medullary interstitial fluid and cerebral spinal fluid. [9]

Yogis such as B. K. S. Iyengar advocate both inhaling and exhaling through the nose in the practice of yoga, rather than inhaling through the nose and exhaling through the mouth. [15] [16] [17] They tell their students that the "nose is for breathing, the mouth is for eating." [16] [18] [19] [15]

6.4.3 Describe the features of alveoli that adapt them to gas exchange.

Even though alveoli are so small there are huge numbers of them which results in a large surface area for gas exchange. Also the wall of the alveoli is made up of a single layer of thin cells and so are the capillaries, this creates a short diffusion distance for the gases. Therefore this allows rapid gas exchange. The alveoli are covered by a dense network of blood capillaries which have a low oxygen and high carbon dioxide concentrations. This allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood. Finally, there are cells in the alveolar walls which secrete a fluid that keeps the inner surface of the alveoli moist, allowing gases to dissolve. This fluid also contains a natural detergent that prevents the sides of the alveoli from sticking together.

  1. Great numbers increase the surface area for gas exchange.
  2. Wall made up of single layer of cells and so are the walls of the capillaries so diffusion distance is small allowing rapid gas exchange.
  3. Covered by a dense network of capillaries which have low oxygen and high carbon dioxide concentrations. This allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood.
  4. Some cells in the walls secret fluid allowing gases to dissolve. Fluid also prevents the sides of alveoli from sticking together.

How much gas is exchanged in one human breath? - Biology

  • Primary function is to obtain oxygen for use by body's cells & eliminate carbon dioxide that cells produce
  • Includes respiratory airways leading into (& out of) lungs plus the lungs themselves
  • Pathway of air: nasal cavities (or oral cavity) > pharynx > trachea > primary bronchi (right & left) > secondary bronchi > tertiary bronchi > bronchioles > alveoli (site of gas exchange)

The exchange of gases (O 2 & CO 2 ) between the alveoli & the blood occurs by simple diffusion: O 2 diffusing from the alveoli into the blood & CO 2 from the blood into the alveoli. Diffusion requires a concentration gradient. So, the concentration (or pressure) of O 2 in the alveoli must be kept at a higher level than in the blood & the concentration (or pressure) of CO 2 in the alveoli must be kept at a lower lever than in the blood. We do this, of course, by breathing - continuously bringing fresh air (with lots of O 2 & little CO 2 ) into the lungs & the alveoli.

Breathing is an active process - requiring the contraction of skeletal muscles. The primary muscles of respiration include the external intercostal muscles (located between the ribs) and the diaphragm (a sheet of muscle located between the thoracic & abdominal cavities).

  • Contraction of external intercostal muscles > elevation of ribs & sternum > increased front- to-back dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs
  • Contraction of diaphragm > diaphragm moves downward > increases vertical dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs:

  • relaxation of external intercostal muscles & diaphragm > return of diaphragm, ribs, & sternum to resting position > restores thoracic cavity to preinspiratory volume > increases pressure in lungs > air is exhaled

As the external intercostals & diaphragm contract, the lungs expand. The expansion of the lungs causes the pressure in the lungs (and alveoli) to become slightly negative relative to atmospheric pressure. As a result, air moves from an area of higher pressure (the air) to an area of lower pressure (our lungs & alveoli). During expiration, the respiration muscles relax & lung volume descreases. This causes pressure in the lungs (and alveoli) to become slight positive relative to atmospheric pressure. As a result, air leaves the lungs (check this animation by McGraw-Hill).

The walls of alveoli are coated with a thin film of water & this creates a potential problem. Water molecules, including those on the alveolar walls, are more attracted to each other than to air, and this attraction creates a force called surface tension. This surface tension increases as water molecules come closer together, which is what happens when we exhale & our alveoli become smaller (like air leaving a balloon). Potentially, surface tension could cause alveoli to collapse and, in addition, would make it more difficult to 're-expand' the alveoli (when you inhaled). Both of these would represent serious problems: if alveoli collapsed they would contain no air & no oxygen to diffuse into the blood &, if 're-expansion' was more difficult, inhalation would be very, very difficult if not impossible. Fortunately, our alveoli do not collapse & inhalation is relatively easy because the lungs produce a substance called surfactant that reduces surface tension.

  • Surfactant decreases surface tension which:
    • increases pulmonary compliance (reducing the effort needed to expand the lungs)
    • reduces tendency for alveoli to collapse
    • External respiration:
      • exchange of O 2 & CO 2 between external environment & the cells of the body
      • efficient because alveoli and capillaries have very thin walls & are very abundant (your lungs have about 300 million alveoli with a total surface area of about 75 square meters)
      • it's the individual pressure exerted independently by a particular gas within a mixture of gasses. The air we breath is a mixture of gasses: primarily nitrogen, oxygen, & carbon dioxide. So, the air you blow into a balloon creates pressure that causes the balloon to expand (& this pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about & collide with the walls of the balloon). However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide. That part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure, therefore, is a measure of how much of that gas is present (e.g., in the blood or alveoli).
      • the partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture. So, given that total atmospheric pressure (at sea level) is about 760 mm Hg and, further, that air is about 21% oxygen, then the partial pressure of oxygen in the air is 0.21 times 760 mm Hg or 160 mm Hg.
      • Alveoli
        • PO 2 = 100 mm Hg
        • PCO 2 = 40 mm Hg
        • Entering the alveolar capillaries
          • PO 2 = 40 mm Hg (relatively low because this blood has just returned from the systemic circulation & has lost much of its oxygen)
          • PCO 2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide)

            • Leaving the alveolar capillaries
              • PO 2 = 100 mm Hg
              • PCO 2 = 40 mm Hg
                • Entering the systemic capillaries
                  • PO 2 = 100 mm Hg
                  • PCO 2 = 40 mm Hg
                  • PO 2 = 40 mm Hg
                  • PCO 2 = 45 mm Hg
                    • Leaving the systemic capillaries
                      • PO 2 = 40 mm Hg
                      • PCO 2 = 45 mm Hg

                      Because almost all oxygen in the blood is transported by hemoglobin, the relationship between the concentration (partial pressure) of oxygen and hemoglobin saturation (the % of hemoglobin molecules carrying oxygen) is an important one.

                      • extent to which the hemoglobin in blood is combined with O 2
                      • depends on PO 2 of the blood:

                      The relationship between oxygen levels and hemoglobin saturation is indicated by the oxygen-hemoglobin dissociation (saturation) curve (in the graph above). You can see that at high partial pressures of O 2 (above about 40 mm Hg), hemoglobin saturation remains rather high (typically about 75 - 80%). This rather flat section of the oxygen-hemoglobin dissociation curve is called the 'plateau.'

                      Recall that 40 mm Hg is the typical partial pressure of oxygen in the cells of the body. Examination of the oxygen-hemoglobin dissociation curve reveals that, under resting conditions, only about 20 - 25% of hemoglobin molecules give up oxygen in the systemic capillaries. This is significant (in other words, the 'plateau' is significant) because it means that you have a substantial reserve of oxygen. In other words, if you become more active, & your cells need more oxygen, the blood (hemoglobin molecules) has lots of oxygen to provide

                      When you do become more active, partial pressures of oxygen in your (active) cells may drop well below 40 mm Hg. A look at the oxygen-hemoglobin dissociation curve reveals that as oxygen levels decline, hemoglobin saturation also declines - and declines precipitously. This means that the blood (hemoglobin) 'unloads' lots of oxygen to active cells - cells that, of course, need more oxygen.

                      Factors that affect the Oxygen-Hemoglobin Dissociation Curve:

                      • lower pH
                      • increased temperature
                      • more 2,3-diphosphoglycerate (DPG)
                      • increased levels of CO 2

                      CO 2 + H 2 0 -----> H 2 CO 3 -----> HCO 3 - + H +

                      & more hydrogen ions = a lower (more acidic) pH. So, in active tissues, there are higher levels of CO2, a lower pH, and higher temperatures. In addition, at lower PO 2 levels, red blood cells increase production of a substance called 2,3-diphosphoglycerate. These changing conditions (more CO 2 , lower pH, higher temperature, & more 2,3-diphosphoglycerate) in active tissues cause an alteration in the structure of hemoglobin, which, in turn, causes hemoglobin to give up its oxygen. In other words, in active tissues, more hemoglobin molecules give up their oxygen. Another way of saying this is that the oxygen-hemoglobin dissociation curve 'shifts to the right' (as shown with the light blue curve in the graph below). This means that at a given partial pressure of oxygen, the percent saturation for hemoglobin with be lower. For example, in the graph below, extrapolate up to the 'normal' curve (green curve) from a PO 2 of 40, then over, & the hemoglobin saturation is about 75%. Then, extrapolate up to the 'right-shifted' (light blue) curve from a PO 2 of 40, then over, & the hemoglobin saturation is about 60%. So, a 'shift to the right' in the oxygen-hemoglobin dissociation curve (shown above) means that more oxygen is being released by hemoglobin - just what's needed by the cells in an active tissue!

                        1 - bicarbonate (HCO 3 ) - 60%
                        • formed when CO 2 (released by cells making ATP) combines with H 2 O (due to the enzyme in red blood cells called carbonic anhydrase) as shown in the diagram below
                        • formed when CO 2 combines with hemoglobin (hemoglobin molecules that have given up their oxygen)

                        Control of Respiration

                        Your respiratory rate changes. When active, for example, your respiratory rate goes up when less active, or sleeping, the rate goes down. Also, even though the respiratory muscles are voluntary, you can't consciously control them when you're sleeping. So, how is respiratory rate altered & how is respiration controlled when you're not consciously thinking about respiration?

                        • controls automatic breathing
                        • consists of interacting neurons that fire either during inspiration (I neurons) or expiration (E neurons)
                          • I neurons - stimulate neurons that innervate respiratory muscles (to bring about inspiration)
                          • E neurons - inhibit I neurons (to 'shut down' the I neurons & bring about expiration)

                          Pneumotaxic center (also located in the pons) - inhibits apneustic center & inhibits inspiration


                          Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified in the nasal cavity. Air then travels down the pharynx, through the trachea, and into the lungs. In the lungs, air passes through the branching bronchi, reaching the respiratory bronchioles, which house the first site of gas exchange. The respiratory bronchioles open into the alveolar ducts, alveolar sacs, and alveoli. Because there are so many alveoli and alveolar sacs in the lung, the surface area for gas exchange is very large. Several protective mechanisms are in place to prevent damage or infection. These include the hair and mucus in the nasal cavity that trap dust, dirt, and other particulate matter before they can enter the system. In the lungs, particles are trapped in a mucus layer and transported via cilia up to the esophageal opening at the top of the trachea to be swallowed.

                          Concluding remarks

                          In conclusion, the lung is an elegant gatekeeper between environmental hypoxia and physical performance at high altitude. Because of the necessity of moving large quantities of air during exercise at altitude, the success of this task requires intact and functional lung mechanics, which are driven by central respiratory drive. An impairment of flow and a mechanical limitation may both be encountered, especially at extreme altitude. This process is facilitated by ongoing ventilatory acclimation, which is secondary to progressively increasing carotid body sensitivity to hypoxia. In spite of impressive lung mechanics and air flow, total body function is further impaired by arterial oxygen desaturation with increasing exercise and altitude, which is secondary both to the ventilation/perfusion heterogeneity and to the diffusion limitation of oxygen from the air to the blood. Further limitation is encountered from an extreme sense of dyspnea as well as depression of central nervous system output resulting from brain hypoxia.

                          Data from the 1981 American Medical Research Expedition to Everest showing that maximal exercise ventilation in liters per minute ( btps ) (dashed line) increased as the inspired partial pressure of oxygen decreased from sea-level values (150mmHg) to approximately 60mmHg (at an altitude of approximately 6300m), but decreased as climbers approach the extreme altitude of the summit of Mount Everest, where the inspired partial pressure of oxygen was 42mmHg. The increase in ventilation is secondary to the hypoxic stimulation of exercise hyperpnea and the level of exercise, which was approximately 200W of work at 6300m, while the hypoxic stimulus was greater at 8848m, but the work capacity was greatly reduced (West et al., 1983) (with permission). 1mmHg=0.133kPa.

                          Data from the 1981 American Medical Research Expedition to Everest showing that maximal exercise ventilation in liters per minute ( btps ) (dashed line) increased as the inspired partial pressure of oxygen decreased from sea-level values (150mmHg) to approximately 60mmHg (at an altitude of approximately 6300m), but decreased as climbers approach the extreme altitude of the summit of Mount Everest, where the inspired partial pressure of oxygen was 42mmHg. The increase in ventilation is secondary to the hypoxic stimulation of exercise hyperpnea and the level of exercise, which was approximately 200W of work at 6300m, while the hypoxic stimulus was greater at 8848m, but the work capacity was greatly reduced (West et al., 1983) (with permission). 1mmHg=0.133kPa.