%%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 %%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.
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.
Harmful components of tobacco smoke are:
(i) Addictive stimulant that stimulates adrenaline release
(ii) Increases heart rate and blood pressure
(iii) Increases risk of stroke, heart attack and impotence
(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
(i) Carcinogenic
(ii) Paralyses cilia lining air passages, reducing effectiveness of dust and irritant removal
(i) Paralyse cilia lining air passages
(ii) Increase risk of chronic bronchitis and emphysema
%%Aerobic respiratio%%n 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.
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