The lungs re-oxygenating our blood is what pulmonary circulation is all about. We have a cardiovascular (heart and blood vessels) system composed of two closed circuits. The pulmonary circuit takes de-oxygenated blood, containing low levels of oxygen and large amounts of carbon dioxide, from the heart to the lungs. It then returns oxygenated blood, high in oxygen and low in carbon dioxide, from the lungs back to the heart. The heart then pumps oxygenated blood through the arteries of the systemic circuit to where it is needed throughout the body, at speed.
It is the "at speed" that is important. The two-circuit cardiovascular (CV) system is more efficient than the single circuit one of fish. The heart of a fish pumps de-oxygenated blood to the gills where it is re-oxygenated but slowed down before passing through the rest of the body and back to the heart. By being able to pump the oxygenated blood faster to where it is needed, for example muscle tissue, the two-circuit CV system enables fast responses with less damage to the muscle cells. The quicker arrival of oxygen when needed reduces the "wear and tear" on our bodies, which generally results in longer life. If people had only a single circuit like fish, they would either need to be more sedate or would have shorter lifespans.
While the heart is the pump at the center of the CV system, it is what occurs at the capillary webs or beds that gives the CV system its purpose. The heart has four chambers, the two upper chambers are called atria and they receive blood from veins; the two lower chambers are called ventricles and they pump the blood back out into arteries. The arteries simply carry the blood from the heart to the capillary webs and the veins return it from the capillary webs to the heart. The capillary webs of the pulmonary circuit are laced throughout the lungs; all other capillary webs are part of the systemic circuit.
Lungs are made up of lobes; in humans the left lung has two lobes and the right three. Dogs and cats have two and four lobes respectively. This is because the left lung is smaller to give room to the heart which is positioned slightly to the left of center in the thoracic cavity (chest). The functional part of the lobes are the alveoli, there are approximately 300 million alveoli in total.
The alveoli are formed in small clusters, like bunches of grapes. Each alveoli is a small air-sack connected to an alveolar duct through which air moves in and out. The alveolar ducts of each cluster merge into a respiratory bronchiole. Several respiratory bronchiole merge to form terminal bronchiole, these combine to make bronchiole which in turn merge to make tertiary bronchi. The tertiary bronchi in each lobe merge to make a secondary bronchi, there is one secondary bronchi per lobe. The secondary bronchi of the right lung merge to make the right bronchi and the two of the left lung join to form the left bronchi. The left and right bronchi join at the bottom of the trachea, commonly know as the windpipe. The top of the trachea connects to the oral cavity (mouth) and nasal passages through the pharynx. Basically, these are all pipes connecting the air of the outside right through to the alveoli. The air in the alveoli is refreshed by a process called pulmonary ventilation, more commonly known as breathing.
At the bottom of the thoracic cavity is a large muscle called the diaphragm. When we are breathing in we are contracting the diaphragm and other muscles in the thoracic cavity, when we breath out we are relaxing them. The contraction increases the volume of the cavity, lowering gaseous pressure outside the lungs to below air pressure. This results in air pushing into our lungs to equalize the pressure difference between the inside and outside of the lungs, even though to us it feels as though we are sucking it in. When we relax the muscles it reduces the volume of the thoracic cavity, increasing the pressure outside the lungs to greater than air pressure. This causes the lungs to compress, expelling some of the air.
External respiration is the name given to the movement of oxygen and carbon dioxide between the air in the alveoli and the blood in a capillary. Each cluster of alveoli has its associated capillary web with each alveoli having a closely associated capillary. The amount of carbon dioxide in the capillary is higher than in the alveoli; air currently has approximately 38 molecules of carbon dioxide per 1 million molecules on average. Through a process called diffusion, these amounts attempt to equalize. Carbon dioxide actually passes in both directions, from the blood in the capillary to the air in the alveoli and vice versa. But because there is a much higher concentration in the blood than the air, much more passes out of the blood than into it.
Some of the carbon dioxide is dissolved in the blood plasma, some is attached to hemoglobin proteins within erythrocytes (red blood cells) and some has reacted with water (H2O) to form carbonic anhydrase attached to the erythrocytes. As the dissolved carbon dioxide passes through the blood vessel wall, that attached to the hemoglobin detaches and that contained in the carbonic anhydrase splits into carbon dioxide and water, making it available to pass through into the air within the alveoli too.
The same principle applies to the flow of oxygen from within the alveoli to the blood stream. Air is approximately 20 to 21 percent oxygen. The oxygen level within the de-oxygenated blood entering the lungs is lower. Oxygen passes in both directions, but much more passes in than out. Some oxygen dissolves in the blood plasma, but most attaches to the hemoglobin proteins within the erythrocytes once the carbon dioxide that was attached to them has detached.
The above two processes are independent of each other, except that oxygen cannot attach to hemoglobin that still have carbon dioxide molecules attached. External respiration is sometimes called gas exchange, and indeed gases are being exchanged between the blood in the capillaries and the air in the alveoli, but oxygen and carbon dioxide are not being exchanged one for the other.
External respiration is the primary purpose of the pulmonary circuit, however, the lungs also play a role in the body's renin-angiotensin-aldosterone system (RAAS), a part of the endocrine (hormonal) system that regulates blood pressure and fluid balance. The lungs produce an enzyme called angiotensin-converting enzyme (ACE) that is central to RAAS. When blood pressure is low or there is a loss in blood volume, perhaps from a wound, the kidneys produce a hormone called renin. Renin cleaves (splits) an organic molecule called angiotensinogen produced by the liver. This produces a molecule called angiotensin I that travels within the blood system. When it reaches the lungs, ACE converts it to angiotensin II, which then triggers blood vessel constriction and the production of aldosterone by the adrenal glands, which work to maintain blood pressure. It is complex, but it means that the active substance, angiotensin II, effectively spreads out from the heart onwards.