Systematic:

-High Pressure

-High Resistance

-Long vessels supplying many organs

-Non-compliant arteries, compliant veins

-Extensive neural control of vascular resistance

-Dilated by hypoxia

Pulmonary:

-Low pressure

-Low resistance

-Short vessels supplying one organ

-Compliant arteries and veins

-Limited neural control of vascular resistance

-Constricted by hypoxia

Pulmonary vascular resistance decreases

Mechanisms:

1. Increase capillary recruitment(more resistors added in parallel decrease total resistance)

2. Increase capillary distension(Relaxation)

Effects:

1. decrease blood velocity→ maintain adequate time for gas exchange.

2. increase capillary surface area→ increase gas exchange.

3. prevent pulmonary edema

• High and Low lung volume both increase pulmonary vascular resistance.
• Lowest pulmonary vascular resistance occurs at FRC.

Contriction diverts blood flow towards ventilated aveoli.

• Blood is diverted to ventilated area.
• Minimal effect on pulmonary arterial pressure

• In high altitude and chronic diseases e.g. asthma, emphysema
• Increase pulmonary arterial pressure→ Pulmonary hypertension→ Hypertrophy of right heart(because of high afterload)
• Play a critical role in fetal circulation

1. Alveolar surface tension: pull fluid toward interstitial and aveoli.

2. Alveolar air pressure: push fluid to the interstitial space and inside the capillaries.

• Less Blood flow and Ventilation in Apex.
• More Blood flow and Ventilation in Base.

When lying down they are about equal.

Blood flow is higher than ventilation at the base. V_A/Q is low.

 Ventilation is higher than blood flow at the apex. V_A/Q is higher.

• Max blood flow an no ventilation= Pulmonary shunt.
• Max ventilation and no blood flow= dead space