BIOL10002 Lecture Notes - Lecture 17: Partial Pressure, Atmospheric Pressure, Hypercapnia
Lecture 17: Respiration
Animals rely mostly on aerobic metabolism to sustain energy
requirements
20x more ATP is synthesized by aerobic respiration compared to
anaerobic
Respiration: oxidation of the end-products of glycolysis (by O2 in
eukaryotes)
Ventilation: movement of the medium (air, water) over a gas exchange
surface
Gas exchange:
● lung & lung capillaries
● capillaries & tissues
bulk transport: CO2 and O2 via circulation
Oxygen in the environment:
Oxygen not equally available in all environments - adaptations
21% O2 in the atmosphere
Atmospheric pressure drops with increasing altitude, decreasing O2
molecules per litre of air
Hypoxia: low O2 in blood or environment
Anoxia: complete absence of O2 in environment
Hypercapnia: high CO2 levels in blood
Partial pressure: component of total pressure contributed by a single
(free) gas within a mixture of gases
Notation: PO2 of blood (mm Hg): partial pressure of oxygen in blood
Air is a better respiratory medium than water
oxygen does not dissolve easily in water
less effort involved moving air across a respiratory medium/gas
exchange site than water because water is more dense (800x & 50x
more viscous)
Air [O2] = 200ml/L
Water [O2] = <10 ml/L
Rate of diffusion (of O2) between air and body fluids depends on the
difference in it’s partial pressures; the higher the partial pressure, the
faster the diffusion due to a greater gradient
Diffusion is the only way respiratory gases are exchanged between the
internal body fluids of an animal and the outside medium
Diffusion: result of random thermal motion of molecules
although individual molecules move at random; diffusional transport of
O2 is only efficient if distance ≤1 mm
Aim of respiratory systems is to minimize diffusion distance and
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create a big surface area of which diffusion can occur
Respiration in small animals: can take up oxygen directly from the
environment through the body surface; simple diffusion distributes O2
throughout the body & into cells
Respiration in larger, more complex animals: greater demands for O2
to tissues, higher cellular metabolic rate; reduction in total surface area
available for gas exchange
large, multicellular organisms are too large for diffusion
thus, both aquatic and terrestrial animals have evolved respiratory
systems with a large SA to maintain respiratory supply of O2
Oxygen cascade: air to mitochondria - ever decreasing partial pressure
of oxygen as you go through the system to allow diffusion, but still need
enough for your cells to operate
Maximize gas exchange:
1. Increase area
2. Increase pressure difference
3. Decrease diffusion distance
Gas exchange: O2 ↔CO2
Gas exchange surfaces: evolved to increase rate of exchange
Fick’s law: describes the relationship between diffusion rate and influential factors
Diffusion coefficient: derived from graham’s law for solubility of gases
Components of a respiratory system
Alveoli: tiny air sacs in the lungs; highly vascularized; single, thin layer of
epithelial cells, lined with surfactant across which O2 is soluble
Surfactant: a fluid secreted by the cells of the alveoli that serves to
reduce the surface tension of pulmonary fluids; contributes to the
elastic properties of pulmonary tissue, preventing the alveoli from
collapsing
Exchange site: where respiratory gases diffuse across, includes
respiratory epithelium & capillary endothelium
large SA due to folding, small diffusion distance (i.e. thin
epithelium/endothelium), highly vascularised
ventilation system keeps a constant supply of medium moving across
the respiratory exchange site; improves rate of gas exchange; ensures
that fresh supplies of O2 rich medium are supplied to the exchange
surface
Hg = mercury
Internal transport (circulatory system):
● moves dissolved O2 from the exchange site to respiring
tissues
● transports CO2 away from the metabolising tissues
essential because diffusion is exceedingly slow over even moderate
distances
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Transport of O2 & CO2:
body fluids (blood plasma) can only hold approx. 0.4% of dissolved O2
oxygen carrying capacity of blood increased by respiratory pigments:
blood pigments: complexes of proteins & metal ions, have a
characteristic color that changes when it binds O2
O2 binds reversibly to a metal ion (Fe) associated with the protein chain
of the respiratory pigment
Hb: main vertebrate respiratory pigment
each heme group can combine with one O2 molecule →4 O2
molecules per Hb molecule
CO (carbon monoxide): stronger heme group binding affinity than
O2→poisonous/toxic
Sickle cell anaemia: abnormal form of hemoglobin produced - HbS molecules clump together, making RBC’s sticky and causing them to form into a
curved, sickle shape
● clog blood vessels/deprive the body’s tissues and organs of oxygen
● anaemia: when the body’s number of RBC’s (or amount of Hb) falls below normal
● anaemic people often feel weak & tired more easily
● carriers for sickle cell have more resistance to malaria
Positive cooperativity: binding of 1st O2 molecule increases binding
affinity of subsequent O2
Oxygen binding curve: describes the relation between the partial
pressure of oxygen (x axis) and the oxygen saturation (y axis);
Hemoglobin's affinity for oxygen increases as successive molecules of
oxygen bind
Affinity of respiratory pigments for O2 depends on different factors:
● type of respiratory pigment e.g. myoglobin, haemoglobin
● pH: blood circulating through active tissues has a lower pH
and H+ ions bind to the haemoglobin molecule in place of O2;
lowers the affinity for O2 (Bohr effect)
2,3-bisphosphoglyceric acid (BPG)
mammalian glycolysis metabolite/byproduct found in RBC’s
indicator of metabolic activity
reversibly combines with partly deoxygenated Hb and lowers its
affinity for O2, thus releasing remaining oxygen (O2 binding curve shifts
right)
level of BPG in RBC’s increases at high altitude or with increased
exercise
helps Hb deliver more O2 to tissues where it is most needed
BPG lost during blood storage (blood donations)
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Document Summary
Animals rely mostly on aerobic metabolism to sustain energy requirements. 20x more atp is synthesized by aerobic respiration compared to anaerobic. Respiration: oxidation of the end-products of glycolysis (by o2 in eukaryotes) Ventilation: movement of the medium (air, water) over a gas exchange surface. Gas exchange: lung & lung capillaries capillaries & tissues bulk transport: co2 and o2 via circulation. Oxygen not equally available in all environments - adaptations. Atmospheric pressure drops with increasing altitude, decreasing o2 molecules per litre of air. Partial pressure: component of total pressure contributed by a single (free) gas within a mixture of gases. Notation: po2 of blood (mm hg): partial pressure of oxygen in blood. Air is a better respiratory medium than water oxygen does not dissolve easily in water less effort involved moving air across a respiratory medium/gas exchange site than water because water is more dense (800x & 50x more viscous)
