Adaptations and Limitations of the Human Lung System

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Learning Intentions

By the end of this topic, you should be able to:

Discuss specific adaptations humans have for each of the four characteristics of an efficient gas exchange system.
Discuss specific limitations of the human lung system, and how they limit the efficiency of gas exchange in humans.

Glossary

Key terms and definitions.

alveoli: Millions of tiny air-filled sacs in the lungs where gas exchange occurs in mammals.

breathing / ventilation: Movement of air/water in and out of the gas exchange system.

bronchioles: Many narrow, highly branched airways that connect the two bronchi to millions of alveoli.

capillaries: Tiny blood vessels that form a network surrounding the alveoli, transporting oxygenated blood away from the lungs and deoxygenated blood toward the lungs.

cartilage rings: Rigid C-shaped structures that prevent the trachea and bronchi from collapsing during inhalation.

cilia: Finger-like projections on cells that line the trachea. They move in a wave-like motion to sweep mucus and trapped dust and debris up the trachea.

concentration gradient: A difference in the concentration of a substance between two areas. This characteristic is the sole driver of diffusion.

dead space: Describes the 30% of air that enters the system but remains in the airways and never makes it to the specialised respiratory surface.

desiccation: Drying out

diaphragm: A large, strong sheet of muscle below the lungs that cause ventilation in mammals.

large SA : V: Characteristic that increases the rate of diffusion because there are more sites for gases to enter and exit the respiratory surface.

intercostal muscles: Muscles between the ribs that cause ventilation in mammals.

moist respiratory surface: Characteristic that increases the rate of diffusion because gases must first dissolve before they can diffuse.

mucociliary escalator: The process of cilia beating collectively and in a coordinated way to transport a sheet of mucus, debris and pathogens back up towards the throat to be swallowed.

mucus: A slimy substance produced in the nasal cavity and trachea to moisten and protect them.

residual volume: Air that is always left behind in the lungs, even after forcefully exhaling; it is oxygen-poor.

thin respiratory surface: Characteristic that increases the rate of diffusion because the diffusion distance for O₂ / CO₂ is short.

tidal ventilation: Describes how air enters and exists the system through the same way, causing new oxygen-rich air breathed in to mix with oxygen-poor residual air trapped in the system.

Adaptations for Ventilation and Maintaining a Steep Concentration Gradient

Diaphragm and intercostal muscles

So what specific adaptations are crucial for ventilating the lungs? The first are the muscles, the diaphragm and intercostal muscles which drive the process of ventilation. Without these muscles, there's nothing causing oxygen poor air to go out and oxygen rich air to come into the lungs.

Cartilage rings around the trachea and bronchi

Trachea and bronchi are held open by C-shaped cartilage rings. They’re like the rings around a vacuum cleaner hose that keeps the tube open when you’re running the vacuum cleaner. Why do they need to be held open? Because during inhalation, the suction is force is so large that without strong and rigid cartilage rings, the airways would collapse while breathing in. It’s like if the vacuum cleaner hose was made of soft plastic, the hose would collapse once you turned on the vacuum cleaner.

The concentration gradient of oxygen and carbon dioxide across the specialised respiratory surface is what drives diffusion and therefore, gas exchange. The steeper the concentration gradient across the alveoli, the more efficient gas exchange is. The human lung system has 2 adaptations for maintaining a steep concentration gradient across the alveoli.

Ventilation

The diaphragm and intercostal muscles work together, contract rhythmically to breathe in new oxygen-rich air to the alveoli, and breathe out old oxygen-poor air from the alveoli. Ventilation ensures that the alveoli always get a supply of new oxygen-rich air, so that the concentration of oxygen is always highest in air in the alveoli.

The opposite happens for carbon dioxide.

Circulatory system

The heart contracts rhythmically to pump keep blood flowing through the capillaries in the alveoli. Oxygen-rich blood is constantly being pumped away from the alveoli, and oxygen-poor blood is constantly being pumped towards the alveoli. The circulatory system ensures that the alveoli always get a supply of oxygen-poor blood, so that the concentration of oxygen is always lowest in the blood.

The opposite happens for carbon dioxide.

The concentration of oxygen is always highest in the air inside the alveoli, and lowest in blood inside the capillaries - so a steep concentration gradient is maintained. Because of ventilation and the circulatory system, oxygen will always diffuse from air to blood. The opposite happens for carbon dioxide.

Adaptations for a Large SA : V

A large SA : V is a requirement for efficient gas exchange, because the more sites for gases to enter and exit the alveoli, the faster the rate of diffusion. The human lung system has 3 adaptations to maximise the SA : V.

Extensive branching of airways / extensive folding of alveoli

There is extensive branching of the airways - as in 1 trachea branches into 2 bronchi, which branches into 60,000 bronchioles, before leading to 600,000,000 alveoli. If there was no branching of these airways, then there would be much less alveoli present. There would also be less alveoli present if there is less folding of the alveoli. If there were less alveoli, there would not be enough sites for oxygen and carbon dioxide to enter and exit the blood via diffusion.

Blocked airways and lung infections can reduce SA : V. So it is a problem that air contains debris and pathogens (e.g. viruses, bacteria, and fungi) because they can block airways or cause infections. in fact, infectious organisms thrive in the warm and moist environment of the lungs. Blocked airways prevent air from getting to alveoli, while infections destroy alveoli - significantly reducing the SA : V of alveoli. The next adaptations maintain a large SA : V by preventing airway blockages and lung infections.

Nose hairs

Small hairs inside the nostrils trap dirt, dust, debris, and pathogens (e.g. bacteria, viruses, fungi) floating in the air as you breathe in. There is also a thin layer of sticky mucus inside your nostrils that collect these unwanted particles, for you to blow out or swallow.

Mucociliary escalator

Muco = mucus that lines the trachea and bronchi.
Ciliary = cells with cilia (cells that have finger-like projections that can 'beat' or 'sway').
Escalator = refers to movement of unwanted particles up towards the throat.

The mucociliary escalator or mucociliary clearance describes the process of cilia 'beating' or 'swaying' in a coordinated way to transport a sheet of sticky mucus that has collected debris and pathogens from inhaled air. This 'beating' or 'swaying' transport back up towards the back of the throat to be swallowed. Once swallowed, these debris and pathogens will get destroyed by the stomach.

Adaptations for Moisture

Gases must first dissolve in water before they can diffuse across the specialised respiratory surface, therefore the alveoli must be kept moist. So it is a problem for humans that air is dry, because inhaling dry air could risk desiccating the alveoli. The human lung system has 3 adaptations to keep the alveoli moist.

Internal lungs

The lungs are internally located inside the chest cavity, to protect them from physical damage and desiccation. If the lungs were located outside of the body, they would dry out very quickly.

Mucus

There is mucus lining the nasal cavity, trachea, and bronchi. This mucus contains water, so as air passes through the airways, the air mixes with water vapour from the mucus, humidifying the air. So even though the air breathed in is dry, by the time inhaled air reaches the alveoli, the air should be adequately humidified and not be as dry.

Alveolar lining fluid and surfactant

The walls of the alveoli are also coated with a thin film of a water substance called alveolar lining fluid to keep the alveolus surface moist to allow oxygen to dissolve into it, in order to diffuse across the alveoli.

However, the water molecules in alveolar lining fluid create a strong attraction force called surface tension. This is a problem because cells of the alveoli are too thin to resist surface tension. So cells of the alveoli also produce 'surfactant', which is a substance that has both hydrophilic and hydrophobic parts) to lower the forces of surface tension, thin out the alveolar lining fluid and prevent the alveolus from collapsing.

Excellent explanation of surface tension and use of surfactants.

Watch up to 1 min and 30 second. For an in-depth explanation of surfactants.

Adaptations for a Short Diffusion Distance

Since diffusion is a passive process, it actually takes a relatively long time for gases to diffuse from one area to another. Diffusion of oxygen and carbon dioxide is only efficient when the diffusion distance is short. The human lung system has 2 adaptations to keep the diffusion distance short.

Thin respiratory surface

The specialised respiratory surface is only 2 cells thick - that's very thin! There are only two layers of cells that separate air from blood: one alveolar cell and one capillary cell. For oxygen to diffuse from air to blood, it only has to cross one alveolar cell, and one capillary cell (and vice versa for carbon dioxide).

Remember from Video 2: Characteristics of an Efficient Gas Exchange System, that as organisms get larger, the diffusion distance from the respiratory surface to cells of the body gets farther. Humans are actually very large animals. If the human lung system just relied on diffusion to move oxygen from alveoli to the cells in your feet, your toes will fall off because diffusion would take too long, and the cells in your toe would not get the oxygen they need for aerobic respiration.

How do humans reduce the diffusion distance from respiratory surface to cells?

Circulatory system

The circulatory system is made up of the:

Heart - a really strong muscle, that is constantly pumping.
Blood - contains red blood cells that carry oxygen and carbon dioxide.
Blood vessels like capillaries - tubes/pipes that blood flows through.

Blood inside the capillaries surrounding alveoli participate in gas exchange by releasing carbon dioxide waste, and absorbing oxygen. Then the heart pumps this oxygen-rich blood around the body. Oxygen is released to cells that need it for aerobic respiration. Carbon dioxide is picked up by blood, and the heart pumps this carbon dioxide-rich blood back to the lungs.

This way of transporting oxygen from alveoli to cells, and carbon dioxide from cells to alveoli is much much much faster than diffusion. If oxygen was not pumped to the rest of the body by the heart, the diffusion distance would be too far, and diffusion would take too much time. Cells would not get the oxygen they need for aerobic respiration.

With the circulatory system, the diffusion distance is greatly reduced because passive diffusion would only have to take place across the very thin alveoli (respiratory surface).

GCSE Biology - Gas Exchange and Lungs (Cognito). Nice video that gives a clear and simple description of gas exchange in lungs. Characteristics such as short diffusion distance, large surface area and concentration gradient is mentioned.

Limitations of the Lung System

There are 3 limitations of the lung system:

1) Incompatibility with water

One of the most obvious limitations of the lung system is that it cannot be used to "breathe under water". The lung system is incompatible with water for two reasons.

First, is that water is too dense/viscous to be ventilated by the diaphragm and intercostal muscles. These muscles are just not strong enough to move water in and out of the lung system. Without ventilation, gas exchange would stop because a concentration gradient of oxygen and carbon dioxide would not be maintained across the alveoli - there would be nothing driving the diffusion of oxygen and carbon dioxide.

Second, is that the lungs would not be able to exchange gases efficiently enough with water to meet the oxygen demands of aerobic respiration, and sustain life. This is because the SA : V of alveoli is not large enough to absorb enough oxygen from the 1% of oxygen available in water. The SA : V of the lung system is not compatible with the extremely low oxygen availability in water; it can only efficiently carry out gas exchange with air, which has 21% oxygen availability.

2) Tidal ventilation and residual volume

The term tidal ventilation describes how air enters and exits the lungs through the same way, just like an ocean tide comes and goes along the same beach. Air enters from the nasal cavity - trachea - bronchi - bronchioles - alveoli ... and exits through the same way from alveoli - bronchioles - bronchi - trachea - nasal cavity.

Tidal ventilation is a limitation of the lung system because the new oxygen-rich air breathed in mixes with old air trapped in the lungs. This old air is called the residual volume because it is the volume of air that is always left behind (residue) in the lungs, even after forcefully exhaling. Because it is left behind in the lungs, it is oxygen poor and carbon dioxide rich.

This mixing of new and old air during tidal ventilation lowers the oxygen concentration of the air that reaches the alveoli, thus reducing the concentration gradient across the alveoli.

3) Dead space

Dead space refers to the 30% of air that enters the lung system, but remains in the airways (nasal cavity, trachea, bronchi, and bronchioles)- so the air that never makes it to the alveoli to participate in gas exchange.

Dead space also refers to the air that makes it to the alveoli, but doesn’t participate in gas exchange. In fact, of the 70% of air that actually make it to the alveoli, only ¼ of oxygen in that air actually diffuses into the blood.

Dead space is a limitation because not all of the air inhaled gets to participate in gas exchange. Specifically, this is the limitation of the SA : V of the lung system. Not all all volume of air inhaled will have enough surface area on the alveoli to participate in gas exchange.