Respiratory System

• Breathing is the transport of oxygen from the outside air to the cells, and carbon dioxide from the cells to the outside air. This is not the same as cellular respiration, which is the process by which an organism breaks down food molecules to release energy for life processes.

• The human respiratory system consists of :

  (a) Nasal passages – Passages leading from the nostrils lined with a moist mucous membrane

  (b) Pharynx – Common passage for the opening of the oesophagus and the trachea

  (c) Larynx – Voice box containing vocal cords

  (d) Trachea – A tube supported by C-shaped cartilage connecting the larynx and the lungs. The C-shaped cartilage prevents the trachea from collapsing as the air pressure in the lungs changes. It branches into two bronchi, one to each lung.

  (e) Bronchi–Branches repeatedly within the lungs to produce numerous finer tubes called bronchioles. The bronchioles at the end of the branching terminate in clusters of air sacs called alveoli. The epithelial lining of the bronchi and trachea are covered with a thin film of mucus and cilia, which are hair-like structures that can move. The mucus traps dust, pollen and other particles and the cilia sweeps it upwards into the pharynx to be swallowed into the oesophagus.

  (f) Lungs – Located in the pleural cavity, they are enclosed by the pleura, a two-layered membrane structure. The inner layer is in contact with the lungs while the other layer adheres to the wall of the chest cavity. The space between the two membranes is known as the pleural space, and it contains a small amount of pleural fluid, which acts as a lubricant when the lungs expand and contract during breathing.

  (g) Related muscles, ribs and diaphragm.

The thoracic cavity

• The lungs are protected by the ribs which extend from the backbone to the sternum (breast bone).

• Two sets of muscles attached to the ribs are involved in breathing. These are the external and internal intercostal muscles. When one set contracts, the other set relaxes.

• The diaphragm is a sheet of skeletal muscle that forms the bottom wall of the thoracic cavity. When the diaphragm muscles contract, the diaphragm moves downwards. When they relax, the diaphragm moves up again.

• The intercostal muscles and the diaphragm work together to change the volume of the chest cavity (thoracic cavity).

Inhalation

• During inhalation, the diaphragm contracts, flattens and moves downwards.

• The external intercostal muscles contract while the internal intercostal muscles relax. The ribs move upwards and outwards.

• The thoracic cavity increases in volume.

• This causes the air pressure of the lungs to fall below that of the atmosphere.

• Air rushes into the lungs.

• During inhalation, air passes through the respiratory passage in the order: nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.

Exhalation

• During exhalation, the diaphragm relaxes and arches upwards.

• The internal intercostal muscles contract while the external intercostal muscles relax, moving the ribs downwards and inwards.

• The thoracic cavity decreases in volume.

• Air pressure in the lungs is now higher than that of the atmosphere.

• Air flows out of the lungs until the air pressure in the lungs reaches equilibrium with atmospheric air pressure.

The alveoli

• The alveoli are the sites of gas exchange in the lungs.

• They are present in large quantities, providing a huge surface area for gas exchange.

• The walls of the alveoli are one-cell thick, resulting in a small distance for diffusion.

• They are covered with a thin film of water to allow oxygen to dissolve and subsequently diffuse in solution across the cell surface membranes.

• They are well-supplied with blood capillaries which transport away diffused oxygen and supply carbon dioxide for excretion. The continuous removal of oxygen and the supply of carbon dioxide maintain the respective concentration gradients of these gases.

• The exchange surface of the alveoli is the thin moist epithelium of the inner surfaces.

• Capillaries branching from the pulmonary artery supply oxygen-poor blood to the alveoli.

• Oxygen from the air in the alveoli taken in during inhalation dissolves in the moisture on the lining.

• The dissolved oxygen diffuses down the concentration gradient across the alveolar wall and the endothelium of the blood capillaries into the oxygen-poor blood.

• The oxygenated blood leaves the capillaries and enters the pulmonary veins to be carried back to the heart.

Removal of carbon dioxide

• 7% of carbon dioxide released during respiration is transported as dissolved carbon dioxide in blood plasma. 23% is transported bound to haemoglobin in red blood cells. 70% is transported as bicarbonate ions in the blood.

• Mechanism of conversion of carbon dioxide into bicarbonate ions: (a) Carbon dioxide from respiring cells diffuses into blood plasma and then into red blood cells. (b) An enzyme, carbonic anhydrase, is present in red blood cells. It catalyses the interconversion of carbon dioxide with water to give carbonic acid, which dissociates into bicarbonate ions and hydrogen ions.

  (c) The hydrogen carbonate ions diffuse into plasma.

• In the lungs: (a) Hydrogen carbonate ions diffuse back into red blood cells where they combine with hydrogen ions released from haemoglobin to form carbonic acid. (b) Carbonic acid forms water and carbon dioxide. (c) The carbon dioxide diffuses out of the blood into the alveolar space where it is expelled during exhalation.

Effects of tobacco smoke on health

• Harmful components of tobacco smoke are:

  • Nicotine

    (i) Addictive stimulant that stimulates adrenaline release

    (ii) Increases heart rate and blood pressure

    (iii) Increases risk of stroke, heart attack and impotence

  • Carbon monoxide

    (i) Poisonous gas that combines irreversibly with haemoglobin to form carboxyhaemoglobin

    (ii) Reduces efficiency of blood to transport oxygen

    (iii) Increases risk of atherosclerosis

    (iv) Increases risk of thrombosis

  • Tar

    (i) Carcinogenic

    (ii) Paralyses cilia lining air passages, reducing effectiveness of dust and irritant removal

  • Irritants

    (i) Paralyse cilia lining air passages

    (ii) Increase risk of chronic bronchitis and emphysema

Chronic bronchitis

• Chronic bronchitis is caused by irritation to the respiratory lining of the airways, resulting in inflammation.

• There is increased production of mucus by the epithelium. Cilia on the epithelium become paralysed, unable to remove mucus and foreign particles.

• Airflow becomes blocked due to swelling and mucus.

• Symptoms are wheezing, shortness of breath and a persistent cough.

Emphysema

• Emphysema is caused by exposure to toxic chemicals, e.g. tobacco smoke.

• It is a lung disease characterised by the permanent enlargement of air spaces due to a destruction of alveolar walls. This decreases the gas exchange surface area.

• The lungs lose their elasticity and lose their ability to effectively expel air.

• Oxygen uptake and carbon dioxide removal is impaired and severe breathlessness is experienced.

Aerobic respiration

• Aerobic respiration is the oxidation of glucose molecules in the presence of oxygen to release a large amount of energy, with carbon dioxide and water as waste products.

• The overall equation is:

  C6H12O6 + 6O2 —> 6CO2 + 6H2O + energy

• Respiration is carried out in a complicated series of reactions involving enzymes.

• It occurs within the mitochondria of cells.

• Energy released from respiration is used for: (a) Synthesising complex molecules from simpler molecules i.e. proteins from amino acids, hormones, enzymes (b) Cell growth and division: synthesis of new protoplasm and genetic material (c) Muscular contraction, both voluntary (involving skeletal muscles) and involuntary (cardiac muscle and smooth muscle i.e. heartbeat and peristalsis) (d) Active transport (e) Transmission of nerve impulses

• Some energy is also released as heat during respiration.

Anaerobic respiration

• Anaerobic respiration is the breakdown of glucose molecules in the absence of oxygen. Waste products vary from organism to organism. Less energy is released compared to aerobic respiration.

• Anaerobic respiration in humans primarily occurs in the muscle cells.

• The preferred mode of respiration in muscle cells is aerobic. However, during periods of strenuous exercise, since there is a limit to the rate of breathing and heart rate, not enough oxygen is available to the muscle cells to sustain aerobic respiration.

• In such cases, muscle cells respire anaerobically for short durations in order to meet the energy demands of the activity.

• The equation for anaerobic respiration in humans is: C6H12O6 —> 2C3H6O3 (lactic acid) + energy

• The energy produced by anaerobic respiration supplements the energy produced by aerobic respiration.

• When anaerobic respiration occurs, there is a build up of lactic acid in the muscle cells.

• This causes fatigue. Anaerobic respiration in humans can only be sustained for a short time before the body needs to recover.

• During the recovery process, more oxygen needs to be taken in. This is evidenced by heavy panting after strenuous exercise.

• The oxygen taken in is used to restore the body to its resting state. This is done by transporting the lactic acid from the muscles to the liver, where some lactic acid is completely oxidised to carbon dioxide and water to produce energy to convert the remaining lactic acid into glucose.

• The amount of oxygen required for this process is called the oxygen debt.