6.4 Syllabus

U1. ​Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveolu and blood flowing in adjacent capillaries.

• Small single celled organisms can easily diffuse gas in and out of the cell as long as they are in an environment where concentration gradients exist for passive diffusion. For example, O2 in water can diffuse into a protist as long as the concentration of oxygen in the surrounding water is greater than the oxygen levels inside the protist cell
• On the other hand human bodies are surrounded and protected by layers of skin. The cells in the tissue that need oxygen for respiration are too far away, too protected, and too numerous to allow direct diffusion with their environment
• Therefore, humans need a system to keep a fresh supply of O2 and to get rid of excess CO2. 
• The ventilation system provides a fresh supply of O2 in the alveoli, allowing the oxygen to diffuse into the blood capillaries surrounding them 
• The oxygen is then transported to all the tissues in the body
• The CO2 in the tissues are transported by the blood to the lungs, where it diffuses into the alveoli and is exhaled into the surrounding atmosphere
• The fresh supply of O2 also makes sure that steep concentration gradient exists where gas is exchanged to allow efficient exchange of O2
• The blood arriving at the alveoli via the pulmonary arteries, arterioles and then capillaries, is rich in CO2 thus creating a concentration gradient between the capillaries and the alveoli, allowing CO2 to rapidly diffuse out of the alveoli into the blood in the capillaries. 

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U2. Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.

• The walls of the alveoli are predominately made from a single layer of epithelial cells called Type I pneumocytes
• These are flattened cells that are approximately 0.15um thick
• Since the alveoli are surrounded by capillaries that are also only one cell thick, oxygen and carbon dioxide have a very short distance to diffuse into the blood from the alveoli and out of the blood into the alveoli respectively
• This adaptation allows for a rapid rate of gas exchange

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U3. ​Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.

• About 5% of the inner surface of the alveoli consists of Type II pneumocytes
• These cells secrete a liquid made of proteins and lipids called surfactant
• This liquid allows oxygen to dissolve into the surfactant and then diffuse into the blood
• It also provides a medium for carbon dioxide to evaporate into the air inside the alveoli in order to be exhaled. 

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U4. ​Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

• Air enters the respiratory system through the nose or mouth and travels through the pharynx and then the trachea 
• The trachea divides into two bronchi (left and right) 
• Inside each lung the bronchi divide into many smaller tubes called bronchioles
• These numerous bronchioles form a tree root like structure that spreads throughout the lungs
• Each bronchiole ends in a cluster of air sacs called alveoli 

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U5. ​Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Ventilation consists of inhalation (inspiration) and exhalation (expiration) 

-Inhalation 

• External intercostal muscles contract pulling the ribs upwards and outwards
• The diaphragm which is a flat sheet of muscle extending across the bottom of the rib cage contracts and flattens out
• These two actions enlarge the thoracic cavity surrounded the lungs, thereby increasing the volume of the lungs
• When the volume of the lungs increases, the pressure inside the lungs decreases and becomes lower that the pressure in the surrounding atmosphere
• Since gas moves from higher pressure to lower pressure, air rushes into the lungs from the surrounding atmosphere to equalize the pressure

-Exhalation

• The external intercostal muscles relax and the diaphragm snaps back to its original shape (domed shape)
• This moves the ribs back down and inwards and decreases the volume of the thoracic cavity and the lungs
• This decrease in volume increases the pressure inside the lungs
• Since the pressure inside the lungs is now greater than the atmospheric pressure, and gas moves from high pressure to low pressure, air rushes out of the lungs into the surrounding environment. 

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U6. ​Different muscles are required for inspiration and expiration because muscles only do work when they contract.

• When different muscles work together to perform opposite movements, they do so in antagonistic fashion ; when one muscle contracts the other will relax
• When muscle contract and shorten, they exert a pulling force that causes movement
• The antagonistic muscle will relax and lengthen because of the pulling force of the other muscle; therefore no work is done
• For example, when one breaths in air, the external intercostal muscles contract, moving the ribcage up and out and the internal intercostal muscles relax. The opposite occurs during expiration
• Muscles therefore only cause movement in one direct while contracted. Movement in the other direction occurs when the other muscle of the pair contracts and first muscle relaxes. 

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A1. Causes and consequences of lung cancer.

-Smoking

• Is the number one cause of lung cancer
• There is an extremely high correlation with the number of cigarettes an individual smokes in a day and the incidence of lung cancer

-Air pollution

• Air pollution from exhaust fumes containing nitrogen oxides, fumes from diesel engines and smoke from burning carbon compounds such as coal are a minor cause of lung cancer. This depends on were in the world you live and the air quality. 

-Lung cancer is a very serious disease and the consequences can be sever, especially if the cancer is not recognized early on

-If the tumour is large when it is discovered, metastasis might have occurred. In these cases, mortality rates are very high. If the tumour is found early on, parts of the affected lung with the tumour can be removed and chemotherapy can be used to help kill the rest of the cancer cells. Re-occurrence of the disease is quite common.