Blood GASES

In Vitro & In VivoBlood Gases

provide info. on pt. acid-base balanceventilatory & 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 O2 ambient air● PO2=(760) x (0.21)● PO2= 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

• PCO2 can vary from 10 – 100mm Hg– Normal 35-45 mm Hg– Regulated by ventilation (primarily)

• PO2 can range from 25 mmHg – 600 mm Hg depending on FIO2.

– 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

• O2 & CO2 are poorly soluble in blood

• Red Blood Cells (RBC) Most O2 bound to hemoglobin

• Plasma (essential to CO2 exchange) <2% O2 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 O2

• The chemistry of hemoglobin allows a reversible bond with O2 & CO2 molecules.

• Oxyhemoglobin (HbO2) 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 O2 molecule.

• O2 bound to hemoglobin is dependent on the partial press. of the O2 which the blood is exposed.

• Binding of 1st O2 molecule the affinity of Hb for O2 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– CO2 tension

– Temperature

Oxygen Dissociation Curve

Hemoglobin Dissociation Curve

• Exposing blood to a partial pressure of O2 results in O2 molecules diffusing into the blood. (Henry’s Law-next slide)

• O2 diffuses across the alveolar capillary membrane until point of equilibrium for the given O2 tension. Max saturation (diffusion)

• A sigmoid curve is seen at O2 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 O2 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 O2 to one iron atom in turn influence the ability of O2 to bind to other iron sites.

• @ sea level can bind to all sites

Not all O2 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 O2.

• Hemoglobin binds with other substances– CO2

– CO

What do the shifts mean to the physiology?

• H+ (determines pH of blood) H+=↓pH (shift R)

– CO2 ( shift R )

– 2,3-DPG ( shift R )

– ↓ affinity of Hb to O2 and shifts curve to the right• Of benefit when tissues need O2 (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 PO2 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 PO2 of 40 mmHg to 25 mm Hg and the next 25% requires even a lesser drop in PO2 ( 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 PO2 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

• PCO2 can vary from 10 – 100mm Hg– Normal 35-45 mm Hg– Regulated by ventilation (primarily)

• PO2 can range from 25 mmHg – 600 mm Hg depending on FIO2.

– Normal @ sea level 80-100mm Hg

Oxygen Saturation

• % of Hb available that is saturated with O2

• Relation to partial pressure shown on curve.

• P50 is a a means of describing Hb affinity for O2.– Gives PO2 when Hb is 50% saturated with O2

– Can be altered by pH, Temp. PCO2 & 2,3 diphosphoglycerate concentration

CO

• Binding with Hg forms carboxyhemoglobin (HbCO)

• Hg becomes incapable of binding with O2

• Hg affinity for CO is >200 times that of O2

• The greater # bond to CO the greater the affinity for O2

• The greater the partial pressure of either gas the greater the affinity.

CO2 Transport

• Like O2, CO2 transported by direct chemical combination with Hb.

• RBC is also capable of rapidly producing bicarbonate ion is a 2nd major transport system.

O2 & CO2 Interaction

• Bohr effect: the addition of CO2 to the blood enhances O2 release from hemoglobin.

• Haldane effect: the addition of O2 to the blood enhances CO2 release from hemoglobin.

CO2

• 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

• CO2 tensions are = in alveolar & arterial blood.

Cardiopulmonary HomeostasisInvolves

• 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 O2 tension

• Tissue CO2 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 PaCO2.

• in PaCO2 is associated with hypoventilation & a resp. acidosis.

• PaCO2 > 50 mm Hg is called ventilatory failure.– Acute=Mechanical ventilation

O2 Status

• PaO2 of 60-80 mmHg-mild hypoxemia

• PaO2 of 40-60 mmHg-moderate hypoxemia

• PaO2 of < 40 mm Hg-severe hypoxemia

Hypoxia

• Tissue hypoxia– Cellular O2 tensions are inadequate

• Hypoxemic hypoxia-↓ arterial tension

• Anemic hypoxia-↓ hemoglobin or ability to carry O2

• 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 HCO3 & in PaCO2 are associated with ↓ in pH & metabolic & respiratory acidosis respectively.

• When only one parameter changes the problem is an acute acid-base disorder

(uncompensated)