Gas exchange in Humans

2.38 understand the role of diffusion in gas exchange

In humans, gas exchange happens all the time in the lungs. The oxygen diffuses from the alveoli into the blood and carbon dioxide from the blood into the alveoli-the carbon dioxide is then exhaled.  This is actually a form of excretion would you believe it, as carbon dioxide is a metabolic waste product from respiration. 

2.44 describe the structure of the thorax, including the ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli and pleural membranes

So your thorax is the part of your body that lies between your neck and your abdomen (around your stomach), and it includes all of the above. They are vital for gas exchange.

So air usually enters your body through your nose-through your two external nostrils whose walls bear a fringe of hairs. The nostrils lead into two nasal passages which are lined with moist mucous membrane. Breathing through the nose has the following advantages:

• Dust and foreign particles, including bacteria in the air, are trapped by the hairs in the nostrils as well as by the mucus on the mucous membrane.
• As air passes through the nasal passages, it is warmed and moistened before it enters the lungs.
• Harmful chemicals may be detected by small sensory cells in the mucous membrane.

The air in your nasal passages enters the pharynx, then to the larynx, and then into your trachea. The trachea lies in front of your oesophagus. It extends downwards from the larynx into the chest cavity. The lower end of the trachea divides into two tubes, the bronchi (singular: bronchus), one to each lung. Each bronchus divides repeatedly and ends in very small, fine bronchioles. Each bronchiole ends in a cluster of air sacs called alveoli. 

Each lung lies in the pleural cavity, within which the lung expands. The pleural cavity is lined by two transparent elastic membranes called the pleura (singular: pleuron) or pleural membranes. The inner pleuron covers the lung. The outer pleuron is in contact with the walls of the thorax and the diaphragm. A thin layer of lubricating fluid between the pleura allows the membranes to glide over each other easily when the lungs expand and contract during breathing. 

Within the lungs, the bronchial tubes divide repeatedly, giving rise to smaller tubes called bronchioles as mentioned earlier. They each end in a cluster of air sacs or alveoli (singular: alveolus). Thousands of alveoli are found in the lungs, providing a very large surface area for gas exchange. 

Fun fact! 

The total surface area of the alveoli in both lungs has been estimated to be equal to the surface area of a tennis court! That is 50 times more than the whole area of the skin. 

Your chest wall is supported by the ribs. They are attached dorsally to the backbone in such a way that they can move up and down. The ribs are attached ventrally to the chest bone or sternum. Two sets of muscles, the external and internal intercostal muscles, can be found between the ribs. They are antagonistic muscles. When the external intercostal muscles contract, the internal intercostal muscles relax and vice versa.

The diaphragm, which is a dome-shaped sheet of muscle and elastic tissue, separates the thorax from the abdomen. When the diaphragm muscles contract, the diaphragm flattens downwards and whey they relax, the diaphragm arches upwards again. 

2.45 understand the role of intercostal muscles and the diaphragm in ventilation

During inhalation/inspiration:

• Your diaphragm contracts and flattens.
• Your external intercostal muscles contract while your internal intercostal muscles relax.
• Your ribs move upwards and outwards. Your sternum also moves up and forward.
• The volume of your thoracic cavity increases.
• Air pressure in your lungs causes them to expand to fill up the enlarged space in your thorax.
• Expansion of your lungs causes the air pressure inside them to decrease. 
• Atmospheric pressure (pressure of air outside) is now higher than the pressure within your lungs. This causes air to rush into your lungs from outside.

During exhalation/expiration:

• Your diaphragm relaxes and arches upwards. 
• Your internal intercostal muscles contract while your external intercostal muscles relax.
• Your ribs move downwards and inwards. Your sternum also moves down to its original position.
• The volume of your thoracic cavity decreases.
• Your lungs are compressed and air pressure inside them increases as the volume decreases. 
• Air pressure within the lungs is now higher than atmospheric pressure. The air is forced out of your lungs to the exterior. 

Here's my easy way of remembering what happens to your intercostal muscles when you are breathing:

RICE and ERIC

When you inhale, you...

Relax your 

Internal intercostal muscles and

Contract your 

External intercostal muscles

When you exhale, your...

External intercostal muscles

Relax and your 

Internal intercostal muscles

Contract

2.46 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and b