Massachusetts General Hospital: Cabg Surgery (a)

Topics: Hemoglobin, Oxygen, Carbon dioxide Pages: 6 (1161 words) Published: June 21, 2013
TRANSPORT OF BLOOD GASES
From The Lungs To The Tissues & Back

Dr. Sally Osborne Department of Cellular & Physiological Sciences University of British Columbia Room 3602, D.H Copp Building 604 822-3421 sally.osborne@ubc.ca www.sallyosborne.com

Objectives
1. Specify in what forms O2 & CO2 are carried in the blood. 2. Describe the physiological significance of the oxyhemoglobin dissociation curve. 3. Relates shifts in the position of this curve to affinity of Hb & oxygen loading / unloading in the blood. 4. Describe the effect of anemia & carbon monoxide poisoning on tissue oxygenation. 5. Identify the color of hemoglobin in the following forms: 1) oxyhemoglobin, 2) deoxyhemoglobin 3) caboxyhemoglobin. 6. Specify the significance of carbonic anhydrase in transport of CO2 7. Describe the “Bohr & Haldane Effects” illustrating the effect O2 & CO2 on the transport of each other.

1

TRANSPORT OF OXYGEN BY BLOOD
Oxygen Is Transported In Two Forms In The Blood—

1. physically dissolved [2 %] 2. chemically bound to the Hemoglobin, Hb, molecule [98 %]

OXYGEN PHYSICALLY DISSOLVED IN BLOOD Compared to carbon dioxide, oxygen is relatively insoluble in the blood-at PO2 = 100 mmHg, 100 ml blood contains 0.3 ml of O2

TRANSPORT OF OXYGEN BY BLOOD
Chemically Bound to the Hemoglobin Molecule

• •

Hb can combine rapidly & reversibly with O2. The reversibility of this chemical reaction allows O2 to be released to the tissues. Hb + deoxyhemoglobin a.k.a reduced hemoglobin O2 oxygen ⇔ O2Hb oxyhemoglobin

2

THE STRUCTURE OF THE HEMOGLOBIN MOLECULE

4 SUBUNITS EACH MADE OF – • a globular protein (soluble in aqueous solution) with an embedded heme group • Heme group contains an iron atom responsible for the binding of oxygen

PULSE OXIMETER
A Non-invasive Device Measuring Percentage of Oxyhemoglobin in the arterial blood [Hb Saturation/SaO2/SpO2] • colour of blood varies depending on how much oxygen it contains. • pulse oximeter shines 2 beams of light through a finger/earlobe, one red (λ of 660 nm which you can see), one infrared light (λ of 940 nm which you don't see). • absorption at these wavelengths differs between oxy & deoxy Hb form & their ratios allows calculation of % oxyHb in arterial blood, SaO2. normal range = 95-100% Other components of tissue absorb light as well. Pulse oximeter measures the max & min absorptions of a pulse, eliminating contribution of components other than Hb.

3

The Colors Of Hemoglobin • Oxygenated Hb, HbO2 - bright red [e.g. normal arterial blood] • Deoxygenated Hb, Hb- blue [same as the color of systemic veins carrying venous blood] • Carboxyhemoglobin, COHb – cherry red [e.g. patient with CO poisoning]

Cyanosis = arterial blood with > 5g Hb/100ml in deoxygenated state bluish / purple discoloration of nail beds and mucous membranes • Presence-- is a sign of poor transport of oxygen • whereas absence-- does not indicate normal oxygen transport e.g. an anemic patient with low oxygen in the blood may not appear cyanotic because he may not have sufficient deoxygenated Hb to appear cyanotic. Also patients with abnormally high levels of Hb, such as those with polycythemia may appear cyanotic.

Sampling of Arterial Blood Gases

• radial • brachial • femoral

4

OXYGEN CONTENT OF BLOOD
TOTAL AMOUNT OF O2 IN THE BLOOD PHYSICALLY DISSOLVED & CHEMICALLY BOUND TO HB

units = ml O2 per 100 ml blood (volume %) CaO2 = 20 vol% CvO2 = 15 vol%

Each time blood circulates through the systemic circulation 5 vol% O2 diffuses to the tissues

DIFFUSION OF OXYGEN CONTINUES AS LONG AS THERE IS A PARTIAL PRESSURE GRADIENT FOR OXYGEN BETWEEN THE ALVEOLI & PLASMA oxygen bound to Hb does not participate in diffusion

= O2 molecule = Partially saturated hemoglobin molecules = Fully saturated hemoglobin molecules

5

THE OXYHEMOGLOBIN DISSOCIATION CURVE
Loading Oxygen in the Lungs

•mixed venous blood enters the capillary at PO2=40; sat =75%. It equilibrates with alveolar...
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