Question

In healthy people, pulmonary venous blood has a PO2 about 5 mmHg lower than is seen in the alveoli, which is mainly caused by:

competition with CO2 diffusion.

asthma.

diffuse interstitial fibrosis.

ventilation-perfusion inequalities.

pulmonary edema.

Answer 1

1.ASTHMA is a disease characterized by recurrent attacks of breathlessness and wheezing due to inflammation of the air passages of the lungs.Due to the irritation, swelling occurs which narrows the bronchus and bronchioles thus restricting the flow of air and producing a sound called wheeze.

Normal saturation of oxygen in the Alveoli and thereby arterial blood should be 92%. Normal arterial oxygen pressure is 8kpa. Normal arterial Co2 saturation in a healthy person ranges from 5-6 kpa.So the pulmonary venous oxygen pressure is 3kpa which is 5 kpa lesser than that of arterial pa02.

In asthma, Pa02 goes below 91%.so ,the pulmonary venous oxygen pressure Will be less than 1-2kpa when compared to the arterial oxygen pressure of 5-6kpa.

2.DIFFUSE INTERSTITIAL FIBROSIS :

It is a condition of the lungs in which the healthy lung parenchyma tissues are replaced by fibrous tissues which makes the lungs to loose elasticity and become stiff ,thereby hindering with the exchange of oxygen in to the body and removal of carbon dioxide away from the body.DIF may be associated with auto-immune disorders or Rheumatoid arthritis.

Symptoms include shortness of breath,dry cough, pleuritic chest pain, cyanosis of skin,lips and nails.

In DIF, SPo2 drops below 80%. pulmonary venous oxygen pressure Will be less than 0.8kpa. pulmonary arterial oxygen pressure Will be 4kpa.

3. VENTILATION-PERFUSION INEQUALITIES:

It is defined as a mis-match between the air (02)that reaches the Alveoli of lungs and the blood that reaches the blood vessels or capillaries in the Alveoli of lungs.This mismatch results in Type 1 respiratory failure.

1 litre of blood can carry 200ml of O2.and 1 litre of Dry air can carry 210ml of 02.so,the normal ventilation-perfusion ratio(V/Q) is 0.95 or approximately 1.0. This is called ventilation-perfusion Equality or matching.

A low V/Q ratio is the cause for low Partial arterial pressure of oxygen (Pa02) below 90% and saturation below 4 kpa.so,the pulmonary venous pressure of oxygen will be less than 0.9kpa. Meanwhile, arterial Co2 saturation exceeds 10kpa.This is commonly seen in Bronchial asthma and pulmonary oedema.

A high V/Q ratio is the cause for high Partial arterial pressure of oxygen (Pa02) greater than 95% and saturation more than 10kpa.At the same time, pulmonary venous pressure of oxygen will be 6kpa.This phenomenon is seen in lung conditions like pulmonary embolism and Emphysema.

4. PULMONARY EDEMA:

It is a condition characterized by the accumulation of excessive fluids in the parenchyma tissues of lungs , Alveoli and other air passages,thus restricting the flow of air and causing breathlessness, dyspnea and wheezing.

Causes include Congestive heart failure, Chronic kidney disease, Systemic Hypertension and chronic Lung disorders.

A low V/Q ratio is noted in pulmonary oedema resulting in low partial arterial pressure of oxygen below 90% and saturation below 4kpa.

Pulmonary venous pressure of oxygen will be less than 0.8kpa. While Arterial Co2 saturation goes beyond 10kpa resulting in low pH level of blood.