9. Human gas exchange

Table of Contents

9.1 Human gasexchange

Large surface Area: Millions of alveoli are present at the base of the bronchioles to provide a very large surface area
Thin surface: Diffusion occurs at a faster rate if there is a very short distance available. Henceforth, diffusion of gases in the lungs, there is only a 2-cell layer separating the alveoli from the blood capillaries. One cell layer is of the alveoliand the other cell layer is of the blood capillaries
Good blood supply: The alveoli are covered with blood capillaries that are constantly removing oxygen and bringing carbon dioxide from the respiring cells. This helps maintain a steep diffusion gradientallowing oxygen to diffuse in and carbon dioxide to diffuse out
Air Supply / Ventilation: Also helps maintain a steep diffusion gradient between the outside air, which has more oxygen, and the alveolar air. The respiratory surfaces of land-dwelling animals are moist because oxygen must dissolve in the thin layer of moisture before passing across the epithelium

Carbon dioxide is almost 10 times more in the expired air because respiring cells produce carbon dioxide as a metabolic waste product, which is excreted through the lungs
Oxygen is dissolved in the film of moisture in the alveoli and is absorbed by the bloodstream because oxygen is vital for our lives. The process of inhalation does not remove all oxygen from the atmosphere
Water Vapour: When we breathe in, the oxygen dissolves in a film of moisture in the alveoli. Some of the moisture is evaporated and saturates the air
Nitrogen remains the same

The lungs lie in the thorax. They have a spongy texture and are supported with horseshoe hoop-like cartridges to prevent the lung from collapsing when expanded. At the back of the mouth, the larynx joins the trachea [windpipe], which divides into two when it enters the lungs. The bronchi further divide into bronchioles. Each bronchiole ends with fine-walled pocket-like air sacs called the alveoli, which increase the surface area for absorption. A flap of tissue[epiglottis] helps prevent food from entering the windpipe during swallowing
Function of the rib cage: protects the lungs and the heart, provides ventilation for the lungs

Alveoli have thin elastic walls and have a thin epithelium [about one cell thick]. The alveoli are covered with a dense layer of blood capillaries. These blood capillaries bring deoxygenated blood from the right ventricle through the pulmonary artery, and after absorbing oxygen, they bring oxygenated blood to the left ventricle through the pulmonary vein. The left ventricle pumps the oxygenated blood throughout the heart through the aorta. They have a very large absorbing surface area to diffuse oxygen at the rate which is required by the body.
Ventilation means the movement of air into and out of the lungs. Gaseous exchange is the exchange of carbon dioxide and oxygen between the alveolar air and the blood vessels in the lungs

External Intercostal muscles [when contracted] - pull the rib upwards and outwards
Internal Intercostal muscles [when contracted] - pull the rib inwards and downwards
Diaphragm [when relaxed] - doomed upwards

Exercise increases the frequency and the depth of breathing
This is because muscles are working harder and aerobically respiring moreand they need more oxygen to be delivered to them (and carbon dioxide removed)to keep up with the energy demand.
If they cannot meet the energy demand, they will also respire anaerobically, producing lactic acid.
After exercise has finished, the lactic acid that has built up in muscles needs to be removed as it lowers the pH of cells and can denature enzymes catalysing cell reactions.
It can only be removed by combining it with oxygen – this is known as ‘repaying the oxygen debt’.
This can be tested by seeing how long it takes after exercise for the breathing rate and depth to return to normal – the longer it takes, the more lactic acid is produced during exercise, the greater the oxygen debt that needs to be repaid
As respiration rates increase, more carbon dioxide is produced and enters the blood.
Carbon dioxide is an acidic gas in solution, and so it can affect the working of enzymes in the cells and needs to be removed as quickly as possible.
As blood flows through the brain, the increase in carbon dioxide concentration stimulates receptor cells.
These send impulses to the muscles of the lungs, causing them to contract faster and more strongly.
This causes the frequency and depth of breathing to increase until the carbon dioxide concentration of the blood has lowered sufficiently

Goblet cells are found in the epithelium lining of the respiratory tract. They secrete mucus, which is a sticky liquid on the epithelium lining. Pathogens and other foreign particles stick onto the mucus and do not reach the alveoli, where they can cause infection or physical damage.
Ciliated cells are also found in the epithelium lining of the respiratory tract. They move in a flicking motion, sending the mucus secreted by the goblet cells up the trachea. At the top of the mucus passes down the gullet during normal swallowing

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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