Pulmonary Circulation & Innervation
Pulmonary circulation is characterized by low pressure, low resistance, and high compliance.
Low-oxygen blood returns from the systemic tissues via the venae cavae and enters the right atrium. This blood passes to the right ventricle, which pumps it into the pulmonary trunk and pulmonary arteries; these vessels deliver the low-oxygen blood to the lungs.
Blood passes through the arteries of the lungs, picking up oxygen from the alveoli and leaving behind carbon dioxide (which will be exhaled).
Oxygenated blood then exits the lungs via the pulmonary veins and is returned to the left atrium. This blood drains to the left ventricle, which pumps it into the aorta for transport to the systemic tissues, where internal respiration occurs.
Pulmonary arterial pressure is determined by the cardiac output of the right ventricle; normal mean pulmonary arterial pressure is approximately 15 mmHg.
This low pulmonary vascular resistance is the product of short, wide vessels with relatively little smooth muscle in their walls, which is very different from the systemic vasculature.
Pulmonary Hypertension:
Pulmonary Embolism:
Bronchial circulation is the blood that is delivered to the conducting zone of the respiratory tract; it is a very small portion of total blood flow.
Because of our upright position, gravity influences the blood flow in the lungs: indicate that it is lower at the apex (the top) of the lungs, and higher at the base (the bottom) of the lungs.
Innervation
The bronchial tubes have smooth muscle in their walls, which allows them to alter their diameter and, therefore, to alter air flow in response to changes in oxygen demand.
Of course, this requires nervous innervation, which is achieved via the Pulmonary Plexus, which comprises components of the autonomic nervous system and afferent fibers.
Sympathetic signals induce bronchodilation; think of sympathetic stimuli during exercise assisting in bronchodilation to bring in more oxygen. This mechanism underlies the use of beta-adrenergic inhalers in asthma patients as a way to dilate the airways.
Parasympathetic signals induce bronchoconstriction; think of restricting airways in response to exposure to allergens. Of course, this mechanism can be maladaptive, as we see in asthmatic patients, who suffer from pathologic bronchoconstriction.
Neural control of respiration - details:
Lastly, let's make a note of the
phrenic nerve, even though it doesn't innervate the lungs, themselves.
The phrenic nerve innervates the diaphragm, which is a sheet of muscle at the base of the lungs; when this muscle contracts, the thoracic volume expands, pressures decrease, and air is pulled into the lungs.
