Voxx Blood Gases.1

Blood GASES In Vitro & In Vivo Blood Gases provide info. on pt. acid-base balance ventilatory & oxygen status

Pressure
Gas molecules collide with surfaces and other molecules creating pressure.
P=Force/Area
Units
PSI
Millimeters of mercury- mm Hg
Torr- named after Torricelli
Centimeters of water- cm H2O
Atmospheric Pressure- gases & gravity

Dalton’s Law
The sum of the partial pressures of a gas mixture equals the total pressure of the system. Also the partial pressure of any gas within a gas mixture is proportional to its percentage of the mixture.
Example partial pressure O 2 ambient air
PO 2 =(760) x (0.21)
PO 2 = 159.6 mmHg

What is Partial Pressure?
The pressure exerted independently by each gas within a mixture of gases. The air is a mixture of gases: primarily nitrogen, oxygen, & carbon dioxide.
When you blow into a balloon the balloon expands. Pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about and collide with the walls of the balloon. The total pressure generated by the air is due to each gas. The part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure is determined by how much of that gas is present.
The partial pressure exerted by each gas in a mixture equals the total pressure times the percentage of the gas in the mixture. So, given that total atmospheric pressure (at sea level) is about 760 mm Hg and, further, that air is about 21% oxygen, then the partial pressure of oxygen in the air is 0.21 times 760 mm Hg or 159 mm Hg.
The PaO2 can go much higher than this if the patient is breathing supplemental oxygen and or under increased barometric pressure such as a hyperbaric chamber.

While in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli.
Leaving the alveolar capillaries
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs.
Entering the systemic capillaries
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
Body cells (resting conditions)
PO2 = 40 mm Hg
PCO2 = 45 mm Hg
Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood.
Leaving the systemic capillaries
PO2 = 40 mm Hg
PCO2 = 45 mm Hg
Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins). This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.

Partial Pressures
PCO 2 can vary from 10 – 100mm Hg
Normal 35-45 mm Hg
Regulated by ventilation (primarily)
PO 2 can range from 25 mmHg – 600 mm Hg depending on F I O 2.
Normal @ sea level 80-100mm Hg

Respiratory Gases
Two primary means
Convection
Air in & out of the lungs
Blood circulation
Diffusion
Across membranes
Driven by the difference in partial pressures
Partial press. Directly proportional to its concentration
Speed = cardiac output

Blood as the Vehicle
O 2 & CO 2 are poorly soluble in blood
Red Blood Cells (RBC)
Most O 2 bound to hemoglobin
Plasma (essential to CO 2 exchange)
<2% O 2 is transported dissolved in plasma
The extent to which Hb in the blood is combined (saturation) with O2 is reflexed in PaO2 measurement and shown on the oxyhemoglobin dissociation curve.

Hemoglobin
Main component of Red Blood Cells
1 RBC contains about 280 million molecules of hemoglobin.
Ea. molecule has 4 atoms of Iron - ⁭ ability to carry O 2
The chemistry of hemoglobin allows a reversible bond with O 2 & CO 2 molecules.
Oxyhemoglobin (HbO 2 ) makes blood scarlet vs reduced hemoglobin (Hb) being purplish.

Hemoglobin
4 iron-containing rings (hemes) chemically bonded to the protein globin.
Each iron atom is able to bind & release an O 2 molecule.
O 2 bound to hemoglobin is dependent on the partial press. of the O 2 which the blood is exposed.
Binding of 1st O 2 molecule ⁭ the affinity of Hb for O 2 making it easier for the next to bind.

Oxygen Affinity
O 2 delivery depends on the affinity .
Major factors
Heme-heme interaction
Allosteric interactions (activity of an enzyme )
Intra-red blood cell enzyme systems
CO 2 tension
Temperature

Oxygen Dissociation Curve

Hemoglobin Dissociation Curve
Exposing blood to a partial pressure of O 2 results in O 2 molecules diffusing into the blood. (Henry’s Law-next slide)
O 2 diffuses across the alveolar capillary membrane until point of equilibrium for the given O 2 tension. Max saturation (diffusion)
A sigmoid curve is seen at O 2 tensions between 0-100 mm Hg.

Henry’s Law
The solubility of a gas in a liquid depends on temperature & the partial pressure of the gas over the liquid.

The Curve
Shows content of O 2 in the blood at various partial pressures of oxygen.
The pressure exerted by a particular component of a mixture of gases
Gets its curve due to the binding of O 2 to one iron atom in turn influence the ability of O 2 to bind to other iron sites.
@ sea level can bind to all sites

Not all O 2 is Transferred
Only 70-75%
Reserve is used during ↑ demands
On the curve 10-40 mm Hg a small decline in the partial pressure releases a large amount of O 2 .
Hemoglobin binds with other substances
CO 2
CO

What do the shifts mean to the physiology?
H+ (determines pH of blood) ⁭ H+=↓pH (shift R)
CO 2 ( ⁭ shift R )
2,3-DPG ( ⁭ shift R )
↓ affinity of Hb to O 2 and shifts curve to the right
Of benefit when tissues need O 2 (extreme exercise)
↑ affinity for O2 shift curve to left
Of benefit when O2 availability is ↓ (extreme altitude)
↑ body temp shifts curve to the right

Uploading
If we look back at the dissociation curve , we see that in order to unload 25% of the oxygen from Hb, the PO 2 had to drop from 100 to 40 mm Hg. This puts us now right at the steepest part of the curve and due to the sigmoidal nature of the curve.
The next 25% unloading only requires a drop in PO 2 of 40 mmHg to 25 mm Hg and the next 25% requires even a lesser drop in PO 2 ( from 25 to 15 mm Hg). The unloading of Hb becomes progressively easier as the oxygen tension drops.
This becomes important when for example exercising. The harder our muscles exercise, the more oxygen they use. The more oxygen they use, the more diffuses into the tissues and the lower the oxygen tension in the capillaries. The lower the PO 2 in the capillaries, the easier the Hb unloads its oxygen.

RBC Enzyme Systems
Intracellular organic phosphate compounds e.g. 2,3-diphosphoglycerate.
Effects the release & binding of O2 from Hg.
Not fully understood.

Measurements
pH
Partial Pressure of gases in the blood
Aid in the diagnosis & treatment of patients
In lab non-invasive means
Pulse oximetry

pH
Method of expressing hydrogen ion (H+) activity of solutions
A way of discussing a pt. acid-base balance.
pH ↓ as H+ concentration ↑
Scale of 1-14 with 7.0 neutral,<7.0 is acid & >7.0 is alkaline
Normal arterial blood is 7.35-7.45

As pH drops, the affinity of Hb for oxygen drops, as seen by the "shift to the right" of the oxygen dissociation curve. Normal blood pH lies between 7.35 and 7.45. In the tissues, the pH is closer to 7.2. When blood arrives at these areas, hemoglobin more readily releases its oxygen to the tissues than it would at a higher pH. According to the graph, Hb is only 50% saturated at pH 7.2 when p02 is 40 mm Hg. This phenomenon is called the Bohr effect , in which Hb gives up oxygen more easily at low pH, areas of high cell metabolism.

Partial Pressures
PCO 2 can vary from 10 – 100mm Hg
Normal 35-45 mm Hg
Regulated by ventilation (primarily)
PO 2 can range from 25 mmHg – 600 mm Hg depending on F I O 2 .
Normal @ sea level 80-100mm Hg

Oxygen Saturation
% of Hb available that is saturated with O 2
Relation to partial pressure shown on curve.
P 50 is a a means of describing Hb affinity for O 2 .
Gives PO 2 when Hb is 50% saturated with O 2
Can be altered by pH, Temp. PCO 2 & 2,3 diphosphoglycerate concentration

CO
Binding with Hg forms carboxyhemoglobin
(HbCO)
Hg becomes incapable of binding with O 2
Hg affinity for CO is >200 times that of O 2
The greater # bond to CO the greater the affinity for O 2
The greater the partial pressure of either gas the greater the affinity.

CO 2 Transport
Like O 2, CO 2 transported by direct chemical combination with Hb.
RBC is also capable of rapidly producing bicarbonate ion is a 2 nd major transport system.

O 2 & CO 2 Interaction
Bohr effect: the addition of CO 2 to the blood enhances O 2 release from hemoglobin.
Haldane effect: the addition of O 2 to the blood enhances CO 2 release from hemoglobin.

CO 2
Most physiologically reflective parameter
Reflects adequacy of alveolar ventilation
Most carried through buffering systems (95%)
the bicarbonate ion, HCO3-
Sm portion dissolved in plasma (partial pressure)
Sm amount binds with hemoglobin to form carboxyhemoglobin
Homeostasis-The metabolic rate of the whole organism against the effectiveness of ventilation
CO 2 tensions are = in alveolar & arterial blood.

Cardiopulmonary Homeostasis Involves
Two distinct sets of capillary beds
The pulmonary system
The cardiovascular system
They maintain adequate respiration and blood gases reflect the homeostasis.

Systemic Capillary Bed
Gas exchange between blood & tissue (internal respiration)

Factors Effecting Gas Exchange
Values of gases entering.
Blood distribution
Arterial-to-venous shunt
Rate of blood flow
Cardiac output
Autonomic nervous system
Tissue O 2 tension
Tissue CO 2 tension

Pulmonary Capillary System
Gas exchange between blood & atmosphere.
External respiration

Alveoli
PO2 = 100 mm Hg
PCO2 = 40 mm Hg
Alveolar capillaries
Entering the alveolar capillaries
PO2 = 40 mm Hg (relatively low because this blood has just returned from the systemic circulation & has lost much of its oxygen)
PCO2 = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide)

Factors Involved in Gas Exchange
Distribution of blood flow.
Intrapulmonary shunting
Cardiac output
Total ventilation
Alveoli ventilation
Diffusion
Content of inspired air

Arterial Blood Gases
Mixed venous blood enters ventricles
Blood leaving is a reflection of total body gas exchange.
Arterial blood represents blood leaving ventricles.
Clinical blood gases sampling

Cardiopulmonary Homeostatic Schema Perturbations Result In
ABGs changing
An organ system ⁭⁭ its work
Various combinations of the former

Two Truths
Normal ABGs do not mean an absence of cardiopulmonary disease.
Disease may be present but compensated
Abnormal ABGs mean an acute uncompensated disease is present.
May be life-threatening

Ventilatory Status
Pt’s ventilatory status can be assessed by looking @ the PaCO 2.
⁭ in PaCO 2 is associated with hypoventilation & a resp. acidosis.
PaCO 2 > 50 mm Hg is called ventilatory failure.
Acute=Mechanical ventilation

O 2 Status
PaO 2 of 60-80 mmHg-mild hypoxemia
PaO 2 of 40-60 mmHg-moderate hypoxemia
PaO 2 of < 40 mm Hg-severe hypoxemia

Hypoxia
Tissue hypoxia
Cellular O 2 tensions are inadequate
Hypoxemic hypoxia- ↓ arterial tension
Anemic hypoxia- ↓ hemoglobin or ability to carry O 2
Circulatory hypoxia- stagnation, AV shunting
Histotoxic hypoxia- cyanide poisoning

Shunting & Deadspace
Normal unit- ventilation to perfusion =
Deadspace unit- alveolus is ventilated, no perfusion
Shunt unit- no ventilation but has perfusion
Silent unit- nether air or blood

Acid-Base Status
Acute ↓ in HCO 3 & ⁭ in PaCO 2 are associated with ↓ in pH & metabolic & respiratory acidosis respectively.
When only one parameter changes the problem is an acute acid-base disorder
(uncompensated)

Voxx Blood Gases.1

by weckhardt

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