BI0005 - Lecture 14 - Gas Transport
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What is perfusion?
- In physiology, perfusion is the process of a body delivering blood to a capillary bed in its biological tissue.
- The word is derived from the French verb "perfuser" meaning to "pour over or through."
What is ventilation?
- In respiratory physiology, ventilation is the movement of air between the environment and the lungs via inhalation and exhalation.
- Thus, for organisms with lungs, it is synonymous with breathing.
- Ventilation usually happens in a rhythmic pattern, and the frequency of that pattern is called the ventilation rate
How does gravity affect ventilation and perfusion in humans?
There is less perfusion and ventilation in the higher parts of the lungs.
When does gas exchange fail?
Gas exchange fails when ventilation and perfusion are mis-matched:
- • No ventilation, good perfusion
- - blood passing through the lung without coming into contact with alveolar air (right to left shunt)
- • Good ventilation, no perfusion
- - anatomical dead space, or ventilated alveoli that are not perfused
What is the VA:Q ratio?
In respiratory physiology, the ventilation/perfusion ratio (or V/Q ratio) is a measurement used to assess the efficiency and adequacy of the matching of two variables:
It is defined as: the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli.
"V" – ventilation – the air that reaches the alveoli
"Q" – perfusion – the blood that reaches the alveoli
What is VE?
Tidal volume - total volume of gas entering the lungs per minute
What is VA?
Respiratory rate (tidal volume - dead space)
The volume of gas per unit time that reaches the alveoli, the respiratory portions of the lungs where gas exchange occurs.
What is VD?
Dead space - The volume of gas per unit time that does not reach the alveoli, but instead remains in the airways (trachea, bronchi, etc).
What mechanisms defend the ventilation/perfusion ratio?
Principally achieved by modulation of blood flow, rather than ventilation.
- Vasoconstriction by low PO2 (hypoxia)
- • Blood is directed away from poorly-ventilated areas.
- • Response is very non-linear
What is transfer of O2 further influenced by?
- • Diffusion across the red blood cell membrane
- • Combination with haemoglobin
How is oxygen carried?
Oxygen is carried by hemoglobin in red blood cells.
How does gas exchange take place in the lungs?
The partial pressures of O2 and CO2 in the blood vary at different points in the circulatory system.
Blood arriving at the lungs via the pulmonary arteries has a lower PO2 and a higher PCO2 than the air in the alveoli.
As blood enters the capillaries, CO2 diffuses from the blood to the air in the alveoli.
Meanwhile, O2 in the air dissolves in the fluid that coats the alveolar epithelium and diffuses into the blood.
By the time the blood leaves the lungs in the pulmonary veins, its Po2 has been raised and its PCO2 has been lwoered.
After returning to the heart, this blood is pumped through the systemic circuit.
What are respiratory pigments?
Animals transport most of their O2 bound to certain proteins called respiratory pigments.
Respiratroy pigments circulate within specialised cells.
The pigments greatly increase the amount of O2 that can be carried in the circulatory fluid (to about 200 mL of O2 per liter in mammalian blood)
Human pigment is hemoglobin
What is hemoglobin?
Vertebrate hemoglobin consists of four subunits (poly-peptide chains), each with a cofactor called a heme group that has an iron atom at its center.
Each iron atom binds one molecule of O2; hence, a single hemoglobin molecule can carry four molecules of O2.
Like all respiratory pigments, hemoglobin binds O2 reversibly, loading O3 in the lungs or gills and unloading it in other parts of the body.
What is the pressure of O2 inside the alveolus and inside the body tissue?
100mm Hg inside alveolus
less than or equal to 40 mm Hg inside body tissue
What is the pressure of CO2 inside the alveolus and inside the body tissue?
40 mm Hg inside alveolus
greater than or equal to 46 mm Hg inside body tissue
What is the oxygen dissociation shift?
Cooperativity in O2 binding and release is evident in the dissociation curve for hemoglobin.
Ove the range of PO2 where the dissociation curve has a steep slope, even a slight change in PO" causes hemoglobin to load or unload a substantial amount of O2.
Notice that the steep part of the curve corresponds to the range of PO2 found in body tissues.
when cells in a particular location begin working harder - during exercise, for instance - PO2 dips in their vicinity as the O2 is consumed in cellular respiration.
Because of the effect of subunit cooperativity, a slight drop in PO2 causes a relatively large increase in the amount of O2 the blood unloads.
What is the Bohr shift?
The production of CO2 during cellular respiration promotes the unloading of O2 by hemoglobin in active tissues.
As we have seen, CO2 reacts with water, forming carbonic acid, which lowers the pH of its surroundings.
Low pH, in turn, decreases the affinity of hemoglobin for O2, an effect called the Bohr shift.
Thus, where CO2 production is greater, hemoglobin releases more O2, which can then be used to support more cellular respiration.
How is Carbon Dioxide transported?
In addition to its role in O2 transport, hemoglobin helps transport CO2 and assists in buffering the blood - that is, preventing harmful changes in pH.
Only about 7% of the CO2 released by respiring cells is transported in solution in blood plasma.
Another 23% binds to the amino ends of hemoglobin polypeptide chains, and about 70% is transported in the blood in the form of bicarbonate ions (HCO3-)
Carbon dioxide from respiring cells diffuses into the blood plasma and then into enthrocytes.
There the CO2 reacts with water (assisted by the enzyme carbonic anhydrase) and foms H2CO3, which dissociates into H+ and HCO3-.
Most of the H+ binds to hemoglobin and other proteins, minimizing the change in blood pH.
The HCO3 diffuses into the plasma.
When blood flows through the lungs, the relative partial presures of CO2 favor the diffusion of CO2 out of the blood
As CO2 diffuses into alveoli, the amount of CO2 in the blood decreases.
This decrease shifts the chemical equilibrium in favour of the conversion of HCO3- to CO2 enabling further net diffusion of CO2 into alveoli
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