The five parts of gas exchange:
Breathing
consists of two phases, inspiration and expiration. During
inspiration, the diaphragm and the intercostal muscles contract.
The diaphragm moves downwards increasing the volume of the thoracic (chest)
cavity, and the intercostal muscles pull the ribs up expanding the rib cage
and further increasing this volume. This increase of volume lowers
the air pressure in the alveoli to below atmospheric pressure. Because
air always flows from a region of high pressure to a region of lower pressure,
it rushes in through the respiratory tract and into the alveoli. This
is called negative pressure breathing, changing the pressure inside
thelunsg relative to the pressure of the outside atmosphere. In contrast
to inspiration, during expiration the diaphragm and intercostal muscles relax.
This returns the thoracic cavity to it's original volume, increasing
the air pressure in the lungs, and forcing the air out.
When a breath
is taken, air passes in through the nostrils, through the nasal passages,
into the pharynx, through the larynx, down the trachea, into one of the main
bronchi, then into smaller broncial tubules, through even smaller broncioles,
and into a microscopic air sac called an alveolus. It is here that
external respiration occurs. Simply put, it is the exchange of oxygen
and carbon dioxide between the air and the blood in the lungs.
Blood enters the lungs via the pulomanory arteries. It
then proceeds through arterioles and into the alveolar capillaries. Oxygen
and carbon dioxide are exchanged between blood and the air. This blood
then flows out of the alveolar capillaries, through venuoles, and back to
the heart via the pulmanory veins. For an explanation as to why gasses
are exchanged here, see partial pressure.
If 100mL of plasma is exposed to an atmosphere with a pO2 of 100mm Hg, only 0.3mL of oxygen would be absorbed. However, if 100mL of blood is exposed to the same atmosphere, about 19mL of oxygen would be absorbed. This is due to the presence of haemoglobin, the main means of oxygen transport in the body. The respiratory pigment haemoglobin is made up of an iron-containing porphyron, haem, combined with the protein globin. Each iron atom in haem is attached to four pyrole groups by covalent bonds. A fifth covalent bond of the iron is attached to the globin part of the molecule and the sixth covalent bond is avaiable for combination with oxygen. There are four iron atoms in each heamoglobin molecule and therefore four heam groups.
Oxygen
Transport - In the loading and unloading of oxygen, there is a cooperation
between these four haem groups. When oxygen binds to one of the groups,
the others change shape slighty and their attraction to oxygen increases.
The loading of the first oxygen, results in the rapid loading of the
next three (forming oxyhaemoglobin). At the other end, when one group
unloads it's oxygen, the other three rapidly unload as their groups change
shape again having less attraction for oxygen. This method of cooperative
binding and release can be seen in the dissociation curve for haemoglobin.
Over the range of oxygen concentrations where the curve has a steep
slope, the slighest change in concentration will cause haemoglobin to load
or unload a substantial amount of oxygen. Notice that the steep part
of the curve corresponds to the range of oxygen concentrations found
in the tissues. When the cells in a particular location begin to work harder,
e.g. during exercise, oxygen cincentration dips in that location, as the
oxygen is used in cellular respiration. Because of the cooperation
between the haem groups, this slight change in concentration is enough to
cause a large increase in the amount of oxygen unloaded.
As with all proteins, haemoglobin's shape shift is sensitive to a variety of environmental conditions. A drop in pH lowers the attraction of haemoglobin to oxygen, an effect knownas the Bohr shift. Because carbon dioxide reacts with water to produce carbinic acid, an active tissue will lower the pH of it's surroundings and encourage haemoglobin to give up extra oxygen, to be used in cellular respiration. Haemoglobin a notable molecule for it's ability to tranport oxygens from regions of supply to regions of demand.
Carbon Dioxide
Transport - Out of the carbon dioxide released from
respiring cells, 7% dissolves into the plasma, 23% binds to the multiple
amino groups of haemoglobin (Caroxyhaemoglobin), and 70% is carried as
bicarbonate ions. Carbon dioxide created by respiring cells diffuses
into the blood plasma and then into the red blood cells, where most of it
is converted to bicarbonate ions. It first reacts with water forming
carbonic
acid, which then breaks down into H+ and
CO3-.
Most of the hydrogen ions that are produced attach to haemoglobin
or other proteins. In this
The body tissues need the oxygen and have to get rid of the carbon dioxide, so the blood carried throughout the body exchanges oxygen and carbon dioxide with the body's tissues. Internal respirtaion is basically the exchange of gasses between the blood in the capillaries and the body's cells.
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