Transport of Oxygen and Carbon Dioxide How Gases Are Transported

Introduction

Goals For Learning To explore how O2 is transported in the blood. To explore how Co2 is transported in the blood. This will include understanding the oxygen dissociation curve. What you need to know Definition of partial pressure Processes of external respiration and internal respiration.

Oxygen Transport O2 is transported by the blood either, Only about 3 ml of O2 are dissolved in each litre of plasma. Assuming we have a total plasma volume of 3 to 5 litres, only about 9 – 15 ml of O2 can be carried in the dissolved state. O2 is transported by the blood either, Combined with haemoglobin (Hb) in the red blood cells (>98%) or, Dissolved in the blood plasma (<2%).

Oxygen Transport The resting body requires 250ml of O2 per minute. We have four to six billion haemoglobin containing red blood cells. The haemoglobin allows nearly 70 times more O2 than dissolved in plasma. This is not enough to supply even the resting body, which requires 250ml per minute. Fortunately we have four to six billion haemoglobin containing red blood cells. The haemoglobin allows nearly 70 times more O2 than dissolved in plasma.

Haemoglobin Haemoglobin molecules can transport up to four O2’s Co-operative binding: haemoglobin’s affinity for O2 increases as its saturation increases. When 4 O2’s are bound to haemoglobin, it is 100% saturated, with fewer O2’s it is partially saturated. Oxygen binding occurs in response to the high PO2 in the lungs

Lets Now Look at Haemoglobin Saturation Haemoglobin saturation is the amount of oxygen bound by each molecule of haemoglobin Each molecule of haemoglobin can carry four molecules of O2. When oxygen binds to haemoglobin, it forms OXYHAEMOGLOBIN; Haemoglobin that is not bound to oxygen is referred to as DEOXYHAEMOGLOBIN.

Haemoglobin Saturation The binding of O2 to haemoglobin depends on the PO2 in the blood and the bonding strength, or affinity, between haemoglobin and oxygen. The graph on the following page shows an oxygen dissociation curve, which reveals the amount of haemoglobin saturation at different PO2 values.

The Oxygen Dissociation Curve Reveals the amount of haemoglobin saturation at different PO2 values.

The Oxygen Disassociation Curve Haemoglobin saturation is determined by the partial pressure of oxygen. When these values are graphed they produce the Oxygen Disassociation Curve In the lungs the partial pressure is approximately 100mm Hg at this Partial Pressure haemoglobin has a high affinity to 02 and is 98% saturated. In the tissues of other organs a typical PO2 is 40 mmHg here haemoglobin has a lower affinity for O2 and releases some but not all of its O2 to the tissues. When haemoglobin leaves the tissues it is still 75% saturated.

Haemoglobin Saturation at High Values Lungs at sea level: PO2 of 100mmHg haemoglobin is 98% SATURATED Lungs at high elevations: PO2 of 80mmHg, haemoglobin 95 % saturated When the PO2 in the lungs declines below typical sea level values, haemoglobin still has a high affinity for O2 and remains almost fully saturated. Even though PO2 differs by 20 mmHg there is almost no difference in haemoglobin saturation.

Haemoglobin Saturation at Low Values

Factors Altering Haemoglobin Saturation

Factors Altering Haemoglobin Saturation (Exercise)

Factors Affecting Haemoglobin Saturation Blood acidity… Blood temperature… Carbon Dioxide concentration

Factors affecting Disassociation Respiratory Response to Exercise Factors affecting Disassociation BLOOD TEMPERATURE increased blood temperature reduces haemoglobin affinity for O2 hence more O2 is delivered to warmed-up tissue BLOOD Ph lowering of blood pH (making blood more acidic) caused by presence of H+ ions from lactic acid or carbonic acid reduces affinity of Hb for O2 and more O2 is delivered to acidic sites which are working harder Factors Affecting Haemoglobin Saturation – Blood Acidity If the blood becomes more acidic the dissociation curve shifts right. This means that more oxygen is being uploaded from the haemoglobin at tissue level. See overhead. Factors Affecting Haemoglobin Saturation – Blood Acidity The rightward shift of the curve is due to a decline in pH. This is referred to as the BOHR effect. The pH in the lungs is generally high. So haemoglobin passing through the lungs has a strong affinity for oxygen, encouraging high saturation. At the tissue level, however the pH is lower, causing oxygen to dissociate from haemoglobin, thereby supplying oxygen to the tissues. With exercise, the ability to upload oxygen to the muscles increases as the muscle ph decreases. Factors Affecting Haemoglobin Saturation – Blood Temperature Increased blood temperature shifts the dissociation curve to the right, indicating that oxygen is uploaded more efficiently. Factors Affecting Haemoglobin Saturation – Blood Temperature Because of this, the haemoglobin will upload more oxygen when blood circulates through the metabolically heated active muscles. In the lungs, where the blood might be a bit cooler, haemoglobin’s affinity for oxygen is increased. This encourages oxygen binding. CARBON DIOXIDE CONCENTRATION the higher CO2 concentration in tissue the less the affinity of Hb for O2 so the harder the tissue is working, the more O2 is released

Key Point Increased temperature and hydrogen ion (H+) (pH) concentration in exercising muscle affect the oxygen dissociation curve, allowing more oxygen to be uploaded to supply the active muscles.

Carbon Dioxide Transport Carbon dioxide also relies on the blood fro transportation. Once carbon dioxide is released from the cells, it is carried in the blood primarily in three ways… Dissolved in plasma, As bicarbonate ions resulting from the dissociation of carbonic acid, Bound to haemoglobin. Dissolved Carbon Dioxide Part of the carbon dioxide released from the tissues is dissolved in plasma. But only a small amount, typically just 7 – 10%, is transported this way. This dissolved carbon dioxide comes out of solution where the PCO2 is low, such as in the lungs. There it diffuses out of the capillaries into the alveoli to be exhaled. Bicarbonate Ions The majority of carbon dioxide ions is carried in the form of bicarbonate ion. 60 - 70% of all carbon dioxide in the blood. The following bit is quite heavy just listen hard. Carbon Dioxide and water molecules combine to form carbonic acid (H2CO3). This acid is unstable and quickly dissociates, freeing a hydrogen ion (H+) and forming a bicarbonate ion (HCO3-): CO2 + H2O H2CO3 CO2 + H2O The H+ subsequently binds to haemoglobin and this binding triggers the BOHR effect (mentioned earlier). This shifts the oxygen-haemoglobin dissociation curve to the right. Thus formation of bicarbonate ion enhances oxygen uploading. This also plays a buffering as the H+ is neutralised therefore preventing any acidification of the blood. When blood enters the lungs, where the PCO2 is lower, the H+ and bicarbonate ions rejoin to form carbonic acid, which then splits into carbon dioxide and water. In other words the carbon dioxide is re-formed and can enter the alveoli and then be exhaled. Key Point The majority of carbon dioxide produced by the active muscles is transported back to the lungs in the form of bicarbonate ions. Carbaminohaemoglobin CO2 transport also can occur when the gas binds with haemoglobin, forming a compound called Carbaminohaemoglobin. It is named so because CO2binds with the amino acids in the globin part of the haemoglobin, rather than the haeme group oxygen does.

Dissolved Carbon Dioxide Part of the carbon dioxide released from the tissues is dissolved in plasma. But only a small amount, typically just 7 – 10%, is transported this way. This dissolved carbon dioxide comes out of solution where the PCO2 is low, such as in the lungs. There it diffuses out of the capillaries into the alveoli to be exhaled. Dissolved Carbon Dioxide Part of the carbon dioxide released from the tissues is dissolved in plasma. But only a small amount, typically just 7 – 10%, is transported this way. This dissolved carbon dioxide comes out of solution where the PCO2 is low, such as in the lungs. There it diffuses out of the capillaries into the alveoli to be exhaled.

In Review Oxygen is transported in the blood primarily bound to haemoglobin though a small amount is dissolved in blood plasma. Haemoglobin oxygen saturation decreases. When PO2 decreases. When pH decreases. When temperature increases.

In Review Each of these conditions can reflect increased local oxygen demand. They increase oxygen uploading in the needy area. 3) Haemoglobin is usually about 98% saturated with oxygen. This reflects a much higher oxygen content than our body requires, so the blood’s oxygen-carrying capacity seldom limits performance.

In Review 4) Carbon dioxide is transported in the blood primarily as bicarbonate ion. This prevents the formation of carbonic acid, which can cause H+ to accumulate, decreasing the pH. Smaller amounts of carbon dioxide are carried either dissolved in the plasma or bound to haemoglobin