Table of Contents
Overview
Gas exchange in animals with lungs is a two-step process:
- External respiration – exchange of gases between air in the lungs and blood in the pulmonary capillaries.
- Internal respiration – exchange of gases between blood in systemic capillaries and body tissues.
The heart and circulatory system transport gases between these two exchange sites. Here, the focus is on how and why gases move, and what structural features of lungs and tissues make this possible.
Physical Principles of Gas Exchange
Partial Pressures and Diffusion
Gases move by diffusion from regions of higher partial pressure to regions of lower partial pressure.
- The partial pressure of a gas (e.g. oxygen, $p\mathrm{O_2}$) in a mixture is the pressure it would exert if it alone occupied the volume.
- In the lungs and tissues, oxygen and carbon dioxide diffusion are driven by partial pressure gradients across thin membranes.
Key points:
- $p\mathrm{O_2}$ is highest in fresh inhaled air, lower in blood returning from the body, and lowest in oxygen-consuming tissues.
- $p\mathrm{CO_2}$ is highest in actively respiring tissues, lower in venous blood entering the lungs, and lowest in the air within the alveoli.
Fick’s law (conceptual form) describes diffusion rate $J$ across a surface:
$$ J \propto \frac{A \cdot (P_1 - P_2)}{d} $$
where:
- $A$ = surface area for exchange
- $(P_1 - P_2)$ = partial pressure difference
- $d$ = thickness of the diffusion barrier
Gas exchange is efficient when the exchange surface has:
- Very large area $A$
- Very thin barrier $d$
- Steep partial pressure gradients $(P_1 - P_2)$
Lungs and capillary beds in tissues are built to maximize these factors.
External Respiration: Gas Exchange in the Lungs
Structure Relevant for Gas Exchange
Only structures specific to gas exchange matter here:
- Alveoli: tiny air sacs at the end of the bronchioles; millions of them create a very large surface area.
- Alveolar epithelium: a single cell layer.
- Capillary endothelium: also a single cell layer.
- Respiratory membrane: the combined thin barrier between air in alveoli and blood in capillaries (alveolar epithelium, fused basement membranes, capillary endothelium).
Typical properties:
- Thickness: about $0.2$–$0.6\,\mu\mathrm{m}$ (very thin)
- Total surface area in adult humans: roughly $70\,\mathrm{m}^2$ (comparable to a tennis court)
Partial Pressure Relationships in the Lungs
In the human lung (values at sea level, approximate):
- Alveolar air: $p\mathrm{O_2} \approx 100\,\mathrm{mmHg}$, $p\mathrm{CO_2} \approx 40\,\mathrm{mmHg}$
- Venous blood arriving at lungs (pulmonary artery):
$p\mathrm{O_2} \approx 40\,\mathrm{mmHg}$, $p\mathrm{CO_2} \approx 46\,\mathrm{mmHg}$ - Arterial blood leaving lungs (pulmonary vein):
equilibrates to near-alveolar values: $p\mathrm{O_2} \approx 100\,\mathrm{mmHg}$, $p\mathrm{CO_2} \approx 40\,\mathrm{mmHg}$
Consequences:
- Oxygen diffusion: from alveolar air (higher $p\mathrm{O_2}$) into blood (lower $p\mathrm{O_2}$).
- Carbon dioxide diffusion: from blood (higher $p\mathrm{CO_2}$) into alveolar air (lower $p\mathrm{CO_2}$).
Diffusion continues until the partial pressures in blood and alveolar air become equal, or blood is swept away by flow.
Role of Ventilation and Perfusion
Two processes must be matched:
- Ventilation – movement of air into and out of alveoli.
- Perfusion – blood flow through pulmonary capillaries.
If ventilation and perfusion are well matched:
- Fresh air is brought to exchange surfaces.
- Blood with “used” gas composition is replaced with “fresh” blood.
If either is disturbed:
- Low ventilation, normal perfusion (e.g. airway obstruction): blood leaves the lung with reduced $p\mathrm{O_2}$ and elevated $p\mathrm{CO_2}$.
- Normal ventilation, low perfusion (e.g. pulmonary embolism): some ventilated alveoli receive little or no blood, so their capacity for gas exchange is wasted.
Local regulatory mechanisms (e.g. constriction or dilation of small airways and blood vessels) help improve matching of ventilation to perfusion.
Time Course of Gas Exchange in Pulmonary Capillaries
In humans at rest:
- Blood spends about $0.75\,\mathrm{s}$ in pulmonary capillaries.
- Equilibration of $p\mathrm{O_2}$ and $p\mathrm{CO_2}$ between alveolar air and blood is usually complete in about $0.25\,\mathrm{s}$.
This safety margin allows adequate gas exchange even when blood flow speeds up during moderate exercise.
Internal Respiration: Gas Exchange in the Tissues
Capillary-Tissue Interface
In systemic tissues, capillaries ramify into dense networks:
- Capillary walls: single layer of endothelial cells.
- Surrounding tissue cells: use oxygen and produce carbon dioxide.
The diffusion distance from capillary blood to cell surfaces is usually only a few micrometers, ensuring:
- Short path length for O$_2$ and CO$_2$ diffusion.
- Efficient exchange even with modest partial pressure gradients.
Partial Pressure Relationships in Tissues
Typical partial pressures (resting conditions, human):
- Arterial blood arriving at tissues:
$p\mathrm{O_2} \approx 100\,\mathrm{mmHg}$, $p\mathrm{CO_2} \approx 40\,\mathrm{mmHg}$ - Venous blood leaving tissues:
$p\mathrm{O_2} \approx 40\,\mathrm{mmHg}$, $p\mathrm{CO_2} \approx 46\,\mathrm{mmHg}$ - Cells and interstitial fluid in active tissues:
$p\mathrm{O_2}$ can be well below \,\mathrm{mmHg}$,
$p\mathrm{CO_2}$ above \,\mathrm{mmHg}$ due to metabolism.
Gas flows:
- O$_2$ diffuses from blood (high $p\mathrm{O_2}$) to cells (low $p\mathrm{O_2}$).
- CO$_2$ diffuses from cells (high $p\mathrm{CO_2}$) to blood (low $p\mathrm{CO_2}$).
In more active tissues:
- O$_2$ consumption increases, dropping local $p\mathrm{O_2}$.
- CO$_2$ production increases, elevating local $p\mathrm{CO_2}$.
- Steeper gradients enhance diffusion rates and promote greater O$_2$ unloading and CO$_2$ uptake from those tissues.
Influence of Blood Flow and Capillary Recruitment
At rest, not all capillaries in a tissue bed are fully perfused at all times.
- When tissue activity rises:
- Local signals (such as CO$_2$, H$^+$, adenosine) cause vasodilation and recruitment of additional capillaries.
- Slower blood velocity and a larger exchange surface area improve O$_2$ delivery and CO$_2$ removal.
Thus, the rate of internal gas exchange is controlled not only by diffusion distances and gradients, but also by dynamic adjustment of local blood flow.
Coordination of Lung and Tissue Gas Exchange
Circulatory Transport Between Exchange Sites
The heart and vessels form the bridge between external and internal gas exchange:
- Pulmonary circulation:
- Brings deoxygenated blood from the right ventricle to the lungs.
- Returns oxygenated blood to the left atrium.
- Systemic circulation:
- Distributes oxygen-rich blood from the left ventricle to tissues.
- Returns oxygen-poor, CO$_2$-rich blood to the right atrium.
The continuous pumping action maintains:
- A high $p\mathrm{O_2}$ in arterial blood reaching tissues.
- A low $p\mathrm{O_2}$ and high $p\mathrm{CO_2}$ in venous blood entering the lungs, which favors efficient gas exchange at both ends.
Matching Supply to Demand
Gas exchange must adjust to changing metabolic needs:
- During exercise:
- Increased ventilation raises alveolar $p\mathrm{O_2}$ and lowers $p\mathrm{CO_2}$.
- Increased cardiac output delivers more blood to both lungs and tissues.
- Capillary recruitment and dilation in muscles improve O$_2$ unloading and CO$_2$ removal.
- At rest:
- Lower metabolic demands require less O$_2$ and produce less CO$_2$.
- Ventilation and cardiac output decrease, but partial pressure gradients remain adequate.
Nervous and chemical control systems (addressed elsewhere) regulate:
- Breathing rate and depth.
- Heart rate and stroke volume.
- Distribution of blood flow among organs.
These controls ensure that gas exchange in the lungs and tissues remains balanced and appropriate for the organism’s current activity level.
Summary of Key Features
- Gas exchange in both lungs and tissues relies on diffusion down partial pressure gradients.
- External respiration at the alveolar–capillary membrane:
- O$_2$ enters blood, CO$_2$ leaves blood.
- Requires thin barriers, large surface area, and matched ventilation–perfusion.
- Internal respiration at the tissue–capillary interface:
- O$_2$ leaves blood for cells, CO$_2$ enters blood from cells.
- Driven by cellular metabolism and regulated by local blood flow.
- The heart and circulatory system link these two exchange zones, maintaining appropriate gas compositions of blood to sustain cellular respiration throughout the body.