Breathing and Gas Exchange

Breathing : Movement of air in and out of lungs / ventilation of lungs

Respiration : Release of energy in cells

Structure of the respiratory system :

·                The thoracic cavity is an airtight cavity.

·                It is bordered by the ribs on the sides, the sternum at the front, the thoracic vertebrae at the back and the diaphragm at the bottom.

·                The ribcage contains the heart and lungs.

·                The lungs are elastic in nature.

·                The trachea is in the upper thorax in front of the oesophagus.

·                It links the throat to the bronchi, allowing air to pass to / from lungs.

·                The rings of cartilage on trachea prevent it from collapsing when pressure in thorax is lower than atmospheric pressure.

·                The rings of cartilage allow the bolus to pass down the oesophagus.

Cleaning inhaled air :

·                The trachea has a ciliated epithelium in its lining. The lining cells secrete mucus.

·                Cilia on the free surface beat in sequence, forcing a current of mucus to move away from the lungs.

·                Bacteria, pollen and dust particles are trapped in the mucus and they move towards the oesophagus.

·                The mucus is then swallowed / coughed out.

·                Mucus passes into the stomach, where bacteria are destroyed by hydrochloric acid.

·                This prevents lung infection.

·                The air is thus cleaned of dust and bacteria.

·                The air is warmed as it passes over the epithelium by radiation. The air is surrounded by tissues at higher temperature.

·                The air is moistened by evaporation from mucus in the lining of the trachea.  

Inhaling :

·                The intercostal muscles contract, pulling the ribs up and outwards.

·                The diaphragm flattens / less dome shaped as the diaphragm muscles contract.

·                The volume of the thoracic cavity increases.

·                Pressure inside the thoracic cavity falls below atmospheric pressure.

·                Air is forced into the lungs to equalise the pressure.

·                Air travels via trachea, bronchus, bronchioles and enters the alveolus of the lungs.

Note : If the chest wall is damaged by a stab wound, inhalation becomes difficult. Air enters thoracic cavity via wound, increasing pressure in the thoracic cavity. Less difference between lung pressure and atmospheric pressure. It gets worse with each breath as air does not go into the lungs.

Exhaling :

·                Carbon dioxide produced in respiration in body cells diffuses down concentration gradient from blood into the cavity of the alveolus.

·                It is transported to heart as bicarbonate / hydrogencarbonate ions in plasma.

·                It is then transported to the lungs.

·                Carbon dioxide passes through alveolar walls.

·                The intercostal muscles relax.

·                The ribs move downwards and inwards.

·                The diaphragm muscles relax.

·                The diaphragm is pushed upwards by abdominal organs, such as, the liver.

·                The diaphragm becomes more dome shaped (curved).

·                The volume of the thoracic cavity decreases.

·                Pressure inside lungs increases above atmospheric pressure.

·                Air inside lungs if forced out to the air to equalise pressure.

Differences between inhaled and exhaled air :

·                Exhaled air is warmer

·                Exhaled air is more moist

·                Exhaled air has high concentration of carbon dioxide and low concentration of oxygen

·                Exhaled air has less dust particles

Alveoli :

Features of alveoli :

·                Alveolar wall is one cell thick – very thin.

·                The alveolar wall is moist due to presence of film of water – oxygen dissolves for diffusion

·                Capillary runs over the surface of the alveolus.

·                Large surface area for rapid diffusion provided by large number of alveoli.

Gaseous Exchange :

·                Concentration of oxygen is less in blood than in the alveolus.

·                Oxygen dissolves in film of moisture lining the alveolus and then diffuses across cell membranes into blood in the capillaries.

·                Oxygen diffuses across the thin alveolar wall from the alveolus to the red blood cells in the plasma in the capillary down concentration gradient, across the thin capillary walls.

·                Oxygen combines with haemoglobin to form oxyhaemoglobin.

·                Carbon dioxide is lost by diffusion to the alveolar air.

·                Temperature falls in the alveolus as air is at lower temperature than blood. So, water from water film evaporates.

·                The water lost is replaced by movement of water out of blood.

·                Glucose is used by alveolar cells in respiration.

·                Capillaries form pulmonary vein.

·                The oxygenated blood is circulated to the left atrium and then to the left ventricle.

·                Left ventricle forces blood out of the heart via the aorta and is pumped via arteries to the respiring cells and tissues.

·                Oxygen is in high concentration in the capillary and is in low concentration in the cells.

·                Oxyhaemoglobin breaks down to liberate oxygen which diffuses via tissue fluid and via cell membrane to the cells.

·                Oxygen is used in aerobic respiration.

Carbon dioxide is carried in blood

·                as hydrogencarbonate ion

·                in solution in blood plasma

·                combined with haemoglobin as carbamino haemoglobin

Carbon dioxide diffuses via cytoplasm through the partially permeable cell membrane down concentration gradient to tissue fluid.

Differences between blood in pulmonary artery and pulmonary vein :

Pulmonary Artery	Pulmonary Vein
More carbon dioxide as lungs remove carbon dioxide for excretion.	Less carbon dioxide
Deoxygenated as oxygen passes from lung into capillaries	Oxygenated
More glucose for respiration of cells of lung tissue	Less glucose

Why does breathing change when you climb a mountain and how?

·                The rate and depth of breathing increase.

·                More air needs to be inhaled into the alveoli of the lungs at each breath as each breath contains less oxygen at high altitude.

·                Pressure difference between internal and external air is less, hence, breathing needs to be done more frequently.

·                Breathing gets deeper as the intercostal muscles and the diaphragm muscles need to work harder / contract more forcefully to maintain steep concentration gradient at the alveolar surface.

Smoking :

·                Bronchitis : Smoke contains harmful chemicals, like nicotine that paralyse and destroy cilia. Dust particles, pathogenic bacteria and tar are not removed from the respiratory tract. More mucus is produced due to dust particles in smoke. Mucus / Phlegm accumulates in the airways. Mucus is warm and moist and provides ideal conditions for bacterial growth. Rapid increase in bacterial numbers, causing infection of lung tissue. This is called bronchitis. Coughing occurs to remove mucus.

·                Emphysema : Smoke particles irritate the respiratory surface. Coughing damages alveoli, leading to emphysema. The alveoli are damaged. They have less surface area. Less gaseous exchange occurs. Less oxygen diffuses into blood.

·                Lung Cancer : Tar containing carcinogens, causes cancer of the airways / lungs / throats as tar contains harmful carcinogens.

·                Nicotine increases heart rate and raises blood pressure. Artherosclerosis occurs – fat build up in arteries, especially, coronary artery. Less oxygen and glucose is pumped to the heart muscle. Lactic acid accumulation causes angina. Thrombosis / blood clot forms causing heart attacks. Nicotine diffuses across placenta.

·                Mother’s blood has higher carbon monoxide content than normal. Poisonous carbon monoxide is passed across the placenta into fetal blood. It combines with haemoglobin to form carboxyhaemoglobin, which is a stable and irreversible compound. It reduces oxygen transported by the red blood cells of mother’s blood. Oxygen uptake by fetus is reduced. Less oxygen is available for cell respiration. So, less respiration occurs, reducing growth. This may show brain damage symptoms. It can cause premature birth. Fetuses are born underweight.

Note :

·                Governments place warning notices on the side of cigarette packets so as to educate people of the dangers of smoking and to discourage them, so that they give up smoking.

·                Smokers find it difficult to give up smoking as nicotine is an addictive drug.

Vital Capacity : Smokers have a lower vital capacity than non – smokers as smoking damages the lungs and causes emphysema.

Functions of alveolus :

·                Oxygen diffuses into blood

·                Carbon dioxide diffuses out of blood

·                Gaseous exchange of oxygen and carbon dioxide