Week 13- Biological Transportation of Oxygen, Carbon Dioxide and Arterial Blood Gases

Gas Exchange

• happens due to diffusion

• gases move from a high concentration to low concentration region

• oxygen diffuses from the air in the lungs to the bloodstream.

• At the same time, carbon dioxide diffuses from the blood to the air in the lungs

  • from where it is blown out of the body the next time a person exhales as shown in the diagram to the right.

Oxygen Transport

• the basis of the aerobic respiration

• oxygen molecules attach to red blood cells

  • which travel from the alveoli to the blood capillaries

    • therefore oxygen from the blood reaches the tissues.

• O₂ binds to hemoglobin which is present in red blood cells, and the molecule known as Oxyhemoglobin (O2Hb)

• oxygen is transported to the tissues for metabolic functions

Oxygen transport cycle/steps

1. Oxygen from pulmonary alveolus bind to hemoglobin

2. Erythrocyte carries oxygen from lungs to rest cell.

3. Oxygen is released to tissue cells.

hemoglobin

• can carry 4 oxygen molecules

• can be reused and they pick up more O₂ molecules.

Oxyhemoglobin

Hemoglobin bound to Oxygen

deoxygenated hemoglobin (HHb)

The hemoglobin molecule without any oxygen molecule

oxygenated blood

blood with oxygen

it is found in arteries, through which it reaches the tissues

Carbon Dioxide Transport

• carbon dioxide in the tissues

  • which is produced as a waste product of cellular respiration,

  • is carried through the blood capillaries and brought to the lungs.

This process is done in different ways.

• CO₂ diffuses in the red blood cells (RBC) and binds to the hemoglobin

• bicarbonate buffer system

  • majority of the CO2 molecules are carried this way

• CO₂ diffuses in the red blood cells (RBC) and binds to the hemoglobin

• carbon dioxide binds to the hemoglobin,

  • the molecule is referred to as carbinohemoglobin.

• The blood with carbon dioxide is referred to as the venous blood and is found in the veins.

• This blood is carried to the lungs, where, the carbon dioxide gets dissociated from the hemoglobin and is expelled from the body.

• carbinohemoglobin

• carbon dioxide binds to the hemoglobin,

• venous blood

• The blood with carbon dioxide

• carried to the lungs

  • where, the carbon dioxide gets dissociated from the hemoglobin and is expelled from the body.

bicarbonate buffer system for CO2 transport

• CO₂ reacts with water in the RBC to produce carbonic acid (H₂CO₃).

• Carbonic acid is very instable so it quickly dissociates into

  • bicarbonate ions (HCO₃^-) and Hydrogen (H⁺).

• The hemoglobin binds to the free H⁺ and limits the pH change of blood.

• The HCO₃^- is then transported out of the RBCs and released in the plasma, in exchange for a chloride ion, as seen in the following image.

• In this process, a chloride Cl^- ion comes into the RBC from the blood to balance the charge (known as chloride shift), and HCO₃^- is transported to the lungs. There, the HCO₃^- dissociates into H₂0 and CO₂ from where CO₂ is expelled out during exhalation.

• chloride shift

• a chloride Cl^- ion comes into the RBC from the blood to balance the charge in exchange for a HCO₃^-

  • which is transported out of the RBCs and released in the plasma

Arterial Blood Gas (ABG) Test

• measures the pH of blood from an artery

• the levels of oxygen and carbon dioxide in the blood from an artery.

• used to check how well the lungs are able to

  • move oxygen into the blood

  • remove carbon dioxide from the blood.

The ABG test is performed in the following cases:

• Check for severe breathing problems and lung diseases, such as

  • asthma

  • cystic fibrosis

  • chronic obstructive pulmonary disease (COPD).

• See how well treatment for lung diseases is working.

• Find out if extra oxygen or help with breathing (mechanical ventilation) is needed.

• Find out if the patient is receiving the right amount of oxygen when using oxygen in the hospital.

• Measure the acid-base level in the blood of people who have

  • heart failure

  • kidney failure

  • uncontrolled diabetes

  • sleep disorders

  • severe infections

  • after a drug overdose.

Partial pressure of oxygen (PO₂) (measurment of ABG)

• the pressure of oxygen dissolved in the blood

• how well oxygen is able to move from the airspace of the lungs into the blood.

Partial pressure of carbon dioxide (PCO₂) (measurment of ABG)

• the pressure of carbon dioxide dissolved in the blood

• how well carbon dioxide is able to move out of the body.

pH (measurment of ABG)

• hydrogen ions (H⁺) in blood.

• The pH of blood is usually between 7.35 and 7.45.

Bicarbonate (HCO₃) (measurment of ABG)

a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.

Oxygen content (O₂ CT) (measurment of ABG)

measures the amount of oxygen in the blood.

Oxygen saturation (O₂ Sat) values.(measurment of ABG)

measures how much of the hemoglobin in the red blood cells is carrying oxygen (O₂).

Specimens that can be used for ABG

• arterial blood specimen

  • The preferred specimen

  • is usually taken from an artery from the inside of the wrist (radial artery)

  • see image

• venous blood specimen

  • collected from the veins.

• capillary blood specimen

  • collected by dermal puncture to

    • heel (always heel for babies)

    • finger

    • toe

  • a lancet is used to make an incision of smaller than 2.0 mm

ABG sorting

• is always a STAT test

• is ideally to be analyzed within 15 minutes of collection.

• The sample of should be mixed well to avoid any air bubbles.

• The specimen must be tested immediately upon arrival in the lab.

Arteril blood for ABG test storaging/transporting to lab

• Specimen should arrive to the lab in a sealed heparinized syringe as shown in the image below.

• Traditionally specimen must be placed in ice slurry immediately (not frozen)

• If the test is performed within 15 minutes, ice may not be required.

  • It is important to follow the lab’s SOP (standard operating procedure).

• As per recent guidelines

  • if a plastic syringe is used, it should be kept at room temperature and not on ice slurry.

  • If glass syringe is used, it can be kept on ice slurry.

if a plastic syringe used for ABG collection

• is used, it should be kept at room temperature and not on ice slurry.

• only arterial blood in a syringe

If glass syringe is used for ABG

• it can be kept on ice slurry.

• only arterial blood in a syringe

limits of venous blood for ABG

• it cannot be used to test for PO₂ and O₂ saturation

  • as it does not allow for accurate measurement.

What ABG can be done on venous blood

Blood gas tests for samples taken from venous blood include

• pH

• pCO₂,

• bicarbonate (HCO₃).

ABG container for venous blood

dark green top lithium heparin vacutainer or blood tube

Venous blood is not placed on ice slurry.

Capillary sample container

heparinized capillary tubes with caps

a lancet is used to make an incision of smaller than 2.0 mm