3 The lungs together are the only organ to receive the total cardiac output, i.e.
y 5 l/min. The pulmonary circulation is in series with the systemic circulation. It is s t a low pressure, low resistance system with systolic/diastolic pressure of 25/10 mm
I Hg and a mean pressure of 15 mm Hg. Pulmonary vascular resistance is about
1/10 that of systemic vascular resistance.
The lungs contain 450 ml blood or 12% of the total blood volume, i.e. the pulmonary circulation has a high capacity. The lungs require 1%-2% of the total body consumption of oxygen for maintenance of activities. Ninety per cent of the cross-sectional area is located in the pre-capillary vessels of order 1. There are 17 orders of arterial vessels and 15 orders of veins in the pulmonary circulation.
The capillary network in the lungs is more dense than elsewhere in order to give a large interface for gas exchange. The capillaries form an extensive system of interconnected parallel networks. The hydrostatic pressure in lung capillaries is much lower than the oncotic pressure of the plasma proteins, but the interstitial pressure is also lower than elsewhere due to lung factors, so net filtration is quite high.
The pulmonary arterioles constrict to a fall in pO2 or a rise in pCO2. Blood is preferentially distributed to dependent parts.
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