Breathing is central to life, as it allows the human body to obtain the energy it needs to sustain itself and its activities. But how does it work?
Abstract
Breathing uses chemical and mechanical processes to bring oxygen to every cell of the body and to get rid of carbon dioxide. Our body needs oxygen to obtain energy to fuel all our living processes. Carbon dioxide is a waste product of that process. The respiratory system, with its conduction and respiratory zones, brings
air from the environment to the lungs and facilitates gas exchange both in the lungs and within the cells. Nurses need a solid understanding of how breathing works, and of vital signs of breathing and breathing patterns, to be able to care for patients with respiratory problems and potentially save lives in acute situations.
Citation: Cedar SH [2018] Every breath you take: the process of breathing explained. Nursing Times [online]; 114: 1, 47-50.
Author: SH Cedar is associate
professor and reader in human biology at the School of Health and Social Care, London South Bank University, and author of Biology for Health: Applying the Activities of Daily Living.
This article has been double-blind peer reviewed
Scroll down to read the article or download a print-friendly PDF here
Understanding:
• Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood
flowing in adjacent capillaries
Physiological
respiration involves the transport of oxygen to cells within the tissues, where energy production occurs
- It is comprised of three distinct processes and is not to be confused with cellular respiration [a single component of the activity]
The processes involved in physiological respiration are:
- Ventilation: The exchange of air between the atmosphere and the lungs – achieved by the physical act of breathing
- Gas Exchange: The exchange of oxygen and carbon dioxide between the alveoli and bloodstream [via passive diffusion]
- Cell Respiration: The release of energy [ATP] from organic molecules – it is enhanced by the presence of oxygen [aerobic]
Purpose of Ventilation
Because gas exchange is a passive process, a ventilation system is needed to maintain a concentration gradient in alveoli
- Oxygen is consumed by cells during cellular respiration and carbon dioxide is produced as a waste product
- This means O2 is constantly being removed from the alveoli into the bloodstream [and CO2 is continually being released]
The lungs function as a ventilation system by continually cycling fresh air into the alveoli from the atmosphere
- This means O2 levels stay high in alveoli [and diffuse into the blood] and CO2 levels stay low [and diffuse from the blood]
- The lungs are also structured to have a very large surface area, so as to increase the overall rate of gas exchange
Ventilation System
⇒ Click to view the monkey’s summary!
Physics of Gas Diffusion
The movement of gases in a contained space [in this case, the lungs] is random. However, overall diffusion results in movement from areas of high concentration to those of low concentration. The rate of diffusion of a gas is primarily affected by
- Concentration gradient: The greater the gradient, the faster the rate
- Surface area for diffusion: The greater the surface area, the faster the rate
- Length of the diffusion pathway: The greater the length of the pathway, the slower the rate
Collision of the molecules of gas with the sides of the container results in pressure. This is defined by the ideal gas law, given in the following equation:
[n represents the number of moles, R the gas constant [8.314], T the absolute temperature and V the volume of the container]
Fig 1 – Equation to calculate pressure of a gas in a container
Diffusion of Gases Through Gases
When gases are diffusing through other gases [such as in the alveoli], their rate of diffusion can be defined by Graham’s Law:
“The rate of diffusion is inversely proportional to the square root of its molar mass at identical pressure and temperature”
In other words, the smaller the mass of a gas, the more rapidly it will diffuse.
Diffusion of Gases Through Liquids
When gases are diffusing through liquids, for example across the alveolar membrane and into capillary blood, the solubility of the gases is important. The more soluble a gas is, the faster it will diffuse.
The solubility of a gas is defined by Henry’s law, which states that:
“The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid”.
If we assume that the conditions of temperature and pressure for all gases remain fixed [as they roughly do in the alveoli] then it is the inherent differences between different gases that determine their solubility.
Carbon dioxide is inherently more soluble than oxygen, and thus diffuses much faster than oxygen into liquid.
Fick’s Law
Fick’s law gives us a number of factors that affect the rate of diffusion of a gas through fluid:
- The partial pressure difference across the diffusion barrier
- The solubility of the gas
- The cross-sectional area of the fluid
- The distance molecules need to diffuse
- The molecular weight of the gas
- The temperature of the fluid – not important within the lungs and can be assumed to be 37oC
In the lungs, whilst oxygen is smaller than carbon dioxide, the difference in solubility means that carbon dioxide diffuses roughly 20 times faster than oxygen.
This difference between the rate of diffusion of the individual molecules is compensated for by the large difference in partial pressures of oxygen, creating a larger diffusion gradient than that of carbon dioxide.
However, this means that in disease states which impair the ability of the lungs to adequately ventilate with oxygen, oxygen exchange is often compromised before that of carbon dioxide.
Diffusion of Oxygen
The partial pressure of oxygen is low in the alveoli compared to the external environment. This is due to continuous diffusion of oxygen across the alveolar membrane and the diluting effect of carbon dioxide entering the alveoli to leave the body.
Despite this, the partial pressure is still higher in the alveoli than the capillaries, resulting in a net diffusion into the blood. Once it has diffused across the alveolar and capillary membranes, it combines with haemoglobin. This forms oxyhaemoglobin which transports the oxygen to respiring tissues via the bloodstream.
Further information on the transport of oxygen within the blood can be found here.
During exercise, blood spends up to half the normal time [one second at rest] in the pulmonary capillaries due to the increase in cardiac output moving blood around the body more quickly. However, diffusion of oxygen is complete within half a second of the blood cell arriving in the capillary. This means that exercise is not limited by gas exchange.
By CNX OpenStax [CC BY 4.0 [//creativecommons.org/licenses/by/4.0]], via Wikimedia Commons
Fig 2 – Diagram showing the partial pressures of oxygen and carbon dioxide in the respiratory system
Diffusion of Carbon Dioxide
The partial pressure of carbon dioxide in the capillaries is much higher than that in the alveoli. This means that net diffusion occurs into the alveoli from capillaries. The carbon dioxide is then exhaled as the partial pressure in the alveoli is higher than the partial pressure in the external environment.
Carbon dioxide is transported in the blood in multiple ways; including dissolved, associated with proteins and as bicarbonate ions. Further information on transport of carbon dioxide in the blood can be found here.
Diffusion Barrier
The diffusion barrier in the lungs consists of the following layers:
- Alveolar epithelium
- Tissue fluid
- Capillary endothelium
- Plasma
- Red cell membrane
By Cruithne9 [Own work] [CC BY-SA 4.0 [//creativecommons.org/licenses/by-sa/4.0]], via Wikimedia Commons
Fig 3 – Diagram showing the layers making up the diffusion barrier in the lungs
Factors That Affect The Rate of Diffusion
There are many properties which can affect the rate of diffusion in the lungs. The main factors include:
- Membrane thickness – the thinner the membrane, the faster the rate of diffusion. The diffusion barrier in the lungs is extremely thin, however some conditions cause thickening of the barrier, thereby impairing diffusion. Examples include:
- Fluid in the interstitial space [pulmonary oedema]
- Thickening of the alveolar membrane [pulmonary fibrosis]
- Membrane surface
area – the larger the surface area, the faster the rate of diffusion. The lungs normally have a very large surface area for gas exchange due to the alveoli.
- Diseases such as emphysema lead to the destruction of the alveolar architecture. This causes the formation of large air-filled spaces known as bullae. This reduces the surface area available and slows the rate of gas exchange
- Pressure difference across the membrane
- Diffusion coefficient of the gas
Clinical Relevance – Emphysema
Emphysema is a chronic, progressive disease that results in destruction of the alveoli in the lungs. This results in a greatly reduced surface area for gas exchange in the lungs, which typically leads to hypoxia [Type 1 respiratory failure].
The main symptom is of emphysema is shortness of breath, however patients may also experience wheezing, a persistent cough or chest tightness. Emphysema, alongside chronic bronchitis are the conditions that make up Chronic Obstructive Pulmonary Disease [COPD]. Whilst smoking is the most common cause, other risk factors include exposure to second-hand smoke, exposure to occupational fumes or dust and living in areas with high levels of pollution.
Treatment depends on the stage of the condition [i.e. the degree of symptoms and airway obstruction] but typically includes:
- Smoking cessation
- Bronchodilators to reduce bronchial constriction
- Inhaled corticosteroids to reduce airway inflammation
- Antibiotics and oral steroids to treat infectious exacerbations of the disease
- Long-term Oxygen Therapy [LTOT] in severe progressive disease
By Yale Rosen from USA [CC BY-SA 2.0 [//creativecommons.org/licenses/by-sa/2.0]], via Wikimedia Commons
Fig 4 – Emphysematous lungs