Pulmonary Circulation of Blood: Features & Measurement | Humans | Biology

In this article we will discuss about:- 1. Characteristics of Pulmonary Circulation 2. Pressures in the Pulmonary System 3. Factors 4. Measurement.

Characteristics of Pulmonary Circulation:

1. The entire pulmonary vascular system is a distensible low pressure system.

2. Reservoir:

Because of their distensibility, the pulmonary veins act as an important blood reservoir. The volume of blood at any one time in the pulmonary vessels is 1 liter.

Posture:

But, when a person lies down the pulmonary blood volume increases by 400 ml and when the person stands up this blood is discharged into the general circulation. When a person blows air out like blowing a trumpet as much as 250 ml is expelled into systemic circulation. Loss of blood due to hemorrhage can be met with this automatic shift of blood.

3. Blood Flow:

The pulmonary vessels can be divided into three categories:

i. Alveolar.

ii. Extra alveolar.

iii. Microcirculation.

The extra alveolar vessels are affected by change in lung volume, alveolar vessels caliber depends on the alveolar pressure changes. Microcirculation participates in the liquid and the solute exchange, in the maintenance of fluid balance.

4. Distribution (Effect of Gravity):

In the upright position, the upper portion of the lungs is well above the level of the heart, and the bases are below it. Conse­quently there is marked pressure gradient in the pulmonary arteries from the top to the bottom of the lungs due to the effect of gravity. In a normal upright adult, the lowest point in the lungs is about 30 cm below the highest point.

This represents a 23 mm Hg pressure difference, about 15 mm Hg above the heart and 8 below. Such pressure difference has profound effect on the blood flow through different areas of the lungs. To explain such effect the lungs is divided into 3 zones based on the blood flow during the cardiac cycle (Fig. 7.8).

Zone 1:

No blood flow occurs during any phase of the cardiac cycle, because the local alveolar capillary pressure in this area never rises higher than the alveolar air pressure and no blood flows through.

Zone 2:

Intermittent blood flow. Flow occurs only during the systole when the capillary pressure exceeds the alveolar pressure. This is known as ‘Water fall effect’.

Zone 3:

Continuous blood flow during the entire cardiac cycle as the capillary pressure remains higher than the alveolar pressure throughout the cycle.

Normally the lungs have only two zones, 2 inter­mittent flows in the apices and 3 continuous flows in the lower areas. Zone 1 occurs only under abnormal conditions like positive pressure ventilation artificially and in severe blood loss.

5. Pulmonary Vascular Resistance is Low:

Low pressure pathway produces less net filtration than produced in the systemic capillaries. It is because of low vascular tone and increased compliance.

6. Auto-Regulation:

Pulmonary arterioles constrict when alveolar PO₂ decreases and diverts the blood flow to the ventilated parts so that it matches ventilation/perfusion ratio.

Pressures in the Pulmonary System:

Right Ventricle ― 25/0mm Hg

Pulmonary artery ― 25/8 mm Hg

Pulmonary capillary ― 7 mm Hg

Left atrium ― 1-5 mm Hg

Blood Volume of the Lungs:

The blood volume of the lungs is about 450 ml (9% of cardiac output) ―

70 ml ― Capillaries

190 ml ― Arteries

190 ml ― Veins

Lungs act as a blood reservoir. During emergency, there is shift of blood from lungs into systemic vessels.

Factors Affecting Pulmonary Circulation:

1. The blood flow through the lungs is equal to cardiac output.

2. Therefore, the factors that control cardiac output, mainly peripheral factors also control pulmonary blood flow.

3. Under most conditions, the pulmonary vessels act as passive, distensible tubes that enlarge with increase in pressure and narrow with decrease in pressure.

4. For adequate aeration of the blood, it is important for the blood to be distributed to those segments of the lungs where the alveoli are best oxygenated. Hypoxia causes vasoconstriction. So, there is shift of blood flow to better aerated area of the lungs which provides automatic control system for distributing blood flow to the different pulmonary areas in proportion to their degree of ventilation.

5. Autonomic nervous control of blood flow in the lungs.

Stimulation of sympathetic nerves: Slight increase in pulmonary vascular resistance. Stimulation of vagus: Slight decrease in pulmonary vascular resistance.

Stimulation of sympathetic nerves causes constric­tion of large pulmonary capacitance vessel (veins) which causes displacement of blood from lungs to other segments where it is needed to combat low pressure.

6. Chemicals:

7. Effect of Exercise:

During heavy exercise, the blood flow through the lungs increases 4 to 7 fold.

This extra flow is accommodated in the lungs in three ways:

i. By increasing the number of open capillaries: recruitment

ii. By distending all the capillaries—distension

iii. By increasing pulmonary arterial pressure.

In the normal person, the first 2 changes together decrease the pulmonary vascular resistance so much that the pulmonary arterial pressure rises very little even during maximum exercise.

Blood Flow:

Blood flow through the lungs depends upon the relationship between the pulmonary arterial pressure, pulmonary venous pressure and alveolar pressure.

The difference between the pulmonary arterial and pulmonary venous pressure is the driving pressure (force).

Secondly, pulmonary capillary pressure must be above the alveolar pressure for the blood flow to continue.

Gravity affects the regional distribution of blood flow through the lungs by altering the pulmonary vascular pressure.

Regional Blood Flow:

Zone 1, 2 and 3 of Pulmonary Blood Flow:

The capillaries in the alveolar walls are distended by the pressure inside them, but simultaneously, they are compressed by the alveolar pressure on their outside.

Under different normal and pathological lung conditions, one may find any one of three possible zones of pulmonary blood flow as follows:

Zone 1:

No blood flow during any part of the cardiac cycle because local capillary pressure in that area of the lung never rises higher than the alveolar pressure during any part of the cardiac cycle.

Zone 2:

Intermittent blood flow only during the pulmonary arterial pressure peaks because the systolic pressure is greater than the alveolar pressure but the diastolic pressure is less than alveolar pressure.

Zone 3:

Continuous blood flow ― because the capillary pressure remains greater than alveolar pressure during the entire cardiac cycle.

Normally, the lungs have only Zone 2 (apex) and Zone 3 (base) blood flow.

When a person is in upright position, the pulmonary arterial pressure at the lung apex is about 15 mm Hg less than the pressure at heart level. Therefore, apical systole pressure is only 10 mm Hg, i.e. 25 mm Hg at heart level – 15 mm Hg pressure difference.

This is more than zero alveolar pressure so that blood flows through the pulmonary apical blood vessels during systole.

On the other hand, during diastole, pulmonary arterial pressure is 8 mm Hg – 15 mm Hg = 7 mm Hg. So, no blood flows during diastole. Zone 2 extends from the level of lungs which is 10 cm above the level of the heart to the top.

In the lower regions of the lungs, from about 10 cm above the level of the heart to the bottom, the pulmonary arterial pressure during both systole and diastole remain more than zero alveolar pressure. Therefore, there is continuous flow. Also, when a person is in a lying position, no part of the lung is more than a few cm above the level of the heart. Blood flow then in a normal person is entirely Zone 3 in lying position.

Zone 1 blood flow occurs only under abnormal conditions, i.e. when alveolar pressure is too high or capillary pressure is too low.

For example (Fig. 6.46):

1. If an upright person is breathing against a positive air pressure so that intra-alveolar pressure is atleast 10 mm Hg more than normal, but the pulmonary capillary pressure is normal—zone 1 blood flow.

2. In hypovolemic states, pulmonary capillary pressure is too low—zone 1 bipod flow.

Measurement of Pulmonary Circulation:

Regional blood flow through the lungs has been assessed by using radioactive gases. In one method, a person takes a single breath of 15 CO₂, and holds the breath for 15 secs. Scintillation counters are placed in pairs at the front and back of the chest. The initial rise in radioactivity is due to the distribution of gas by ventilation (the greater the rise, better the ventilation in the region). The latter fall in radioactivity is due to removal of the gas by blood. Faster the fall in a region of the lung, higher the blood flows through the region.

Alternatively, radioactive xenon (¹³³Xe) may be dissolved in saline and injected into the superior vena cava. It soon travels to the pulmonary blood vessels. If counters are placed around the chest the relative distribution of radioactivity in different regions of the chest can be assessed. Greater the concentration of radioactivity in a region, higher the blood flow through the region. Both the radioactive CO₂ and ¹³³Xe methods have shown the blood flow to be higher at the base than at the apical region of the lungs.

Pulmonary Vascular Reflexes:

1. Stimulation of baroreceptor causes reflex dilatation of pulmonary vessels. While stimulation of chemoreceptor causes reflex pulmonary vaso­constriction.

2. In pulmonary trunk and its main branches, vagal mechanoreceptors are present. Therefore, increase in pulmonary arterial pressure leads to bradycardia and hypotension.

3. Stimulation of vagal mechanoreceptors at the junction of pulmonary vein with left atrium produces tachycardia and diuresis which helps in regulating blood volume.

4. Stimulation of vagal nerve endings in the pulmonary small vessels produces tachypnea.