As they function, our cells use oxygen and produce carbon dioxide. The respiratory system brings the needed oxygen into and eliminates carbon dioxide from the body by working closely with the cardiovascular system. The blood transports these gases, carrying oxygen to the tissues and carbon dioxide to the lungs. Your goals for learning are to review the major organs of the respiratory system, to examine the structures of the respiratory zone of the lungs, to explore the microscopic anatomy of an alveolus. Here's what you need to know - the location of pulmonary arteries and veins. Let's review the organs of the respiratory system by following the flow of air. Air enters the nose by passing through two openings called the Nares or nostrils. Within the nose, the air passes through the nasal cavity and then travels through the pharynx, a muscular tube which carries both food and air throughout most of its length. Air then enters the larynx. After passing through the larynx, air enters the trachea which is held open by incomplete rings of cartilage. The trachea divides into the right and the left main bronchus which carry the air into the lungs. Although not part of the respiratory system, the diaphragm and the intercostal muscles play important roles in breathing. Each lung is surrounded by two layers of serous membrane known as the pleurae. The relationship between the pleurae and the lungs can be demonstrated by pushing a fist into a water filled balloon. The balloon represents the pleurae and the fist represents the lung. As the fist pushes into the balloon, notice how the balloon wraps around it and the opposite surfaces of the balloon almost touch. The inner part of the balloon which wraps around the fist represents the visceral pleura. The visceral pleura is part of the pleura which covers the surface of the lungs. The outer part of the balloon represents the parietal pleura, which lines the mediastinum, the diaphragm and the thoracic wall. Notice that the visceral and parietal pleurae are actually a continuation of the same membrane. The water-filled space between the two layers represents the pleural cavity which contains pleural fluid. The size of the pleural cavity is exaggerated in this illustration. Now let’s see these structures as they actually appear in the body. The visceral pleura and the parietal pleura enclose each lung in a separate sac. The frosty layer you see here covering the lungs is the portion of the parietal pleura that lines the anterior thoracic wall. Let’s see what happens when the anterior part of the parietal pleura is removed. Now that the anterior portion of the parietal pleura has been removed, we can see the visceral pleura which covers the surface of the lungs and the cut edges of the parietal pleura. The pleural cavity is an extremely thin, slit-like space between the pleurae, separating them by a thin layer of pleural fluid. The pleural fluid assists in breathing movements by acting as a lubricant. Let’s see the posterior portion of the parietal pleura. In this view, the lung has been removed to show the posterior portion of the parietal pleura. Recall that the parietal pleura lines the mediastinum, the superior surface of the diaphragm, and the inner thoracic wall. Now let's continue to follow the airflow as it enters the lungs. The lungs contain many branching airways which collectively are known as the bronchial tree. Let’s look at an enlargement of part of the bronchial tree. Air enters the lungs through the main bronchi which branch into lobar bronchi, which in turn branch into segmental bronchi. The trachea and all the bronchi have supporting cartilage which keeps the airways open. Airflow is deeper into the lungs as the segmental bronchi branch repeatedly into smaller bronchi which eventually branch into bronchioles. Bronchioles lack cartilage and contain more smooth muscle in their walls than the bronchi. These features allow airflow regulation by altering the diameter of the bronchioles. Bronchioles branch further into terminal bronchioles. The airways from the nasal cavity through the terminal bronchioles are called the conducting zone. The air is moistened, warmed, and filtered as it flows through these passageways. Beyond the terminal bronchioles, the air enters the respiratory zone - the region of lung where gas exchange occurs. Let us now discuss the respiratory zone structures. Beyond the terminal bronchioles, lie the structures of the respiratory zone where we begin to find alveoli, tiny thin-walled sacs where gas exchange occurs. Respiratory bronchioles have scattered alveoli in their walls. They lead into alveolar ducts which are completely lined by alveoli. These ducts end in clusters of alveoli called alveolar sacs. Let’s see a photomicrograph of these structures. Compare the structures in the lung section shown in the photomicrograph to the surface view of the same structures in the illustration. Alveolar sac...alveolar duct...alveoli...respiratory bronchiole. Now that we've looked at the respiratory zone airways, let's look at the associated blood vessels. The pulmonary arteries carry blood which is low in oxygen from the heart to the lungs. These blood vessels branch repeatedly eventually forming dense networks of capillaries that completely surround each alveolus. This rich blood supply allows for the efficient exchange of oxygen and carbon dioxide between the air in the alveoli and the blood in the pulmonary capillaries. Blood leaves the capillaries via the pulmonary veins which transport the freshly oxygenated blood out of the lungs and back to the heart. Now let’s see this process again. Let's look closely at the inside of an individual alveolus. Alveoli contain three types of cells- simple squamous epithelium, alveolar macrophages, and surfactant-secreting cells. The wall of an alveolus is primarily composed of simple squamous epithelium or type I cells. Gas exchange occurs easily across this very thin epithelium. Let’s explore the role of a macrophage. The alveolar macrophages, or dust cells, creep along the inner surface of the alveoli removing debris and microbes. The alveolus also contained scattered surfactant-secreting, or type II, cells. Let’s explore the role of a surfactant-secreting cell. Here we see that the inside surface of the alveolus is lined with alveolar fluid. The water in the fluid creates a surface tension. Surface tension is due to the strong attraction between water molecules at the surface of a liquid which draws the water molecules closer together. As seen here, this force pulls the alveolus inward and reduces its size. If an alveolus were aligned with pure water it would collapse. Surfactant, which is a mixture of phospholipids and lipoproteins, lowers the surface tension of the fluid by interfering with the attraction between the water molecules preventing alveolar collapse. Without surfactant, alveoli would have to be completely reinflated between breaths which would take an enormous amount of energy. Now let's examine the wall of an alveolus and the wall of a capillary. Together these walls form the respiratory membrane where gas exchange occurs. The respiratory membrane is made up of two layers of simple squamous epithelium and their basement membranes. This membrane is extremely thin, averaging point five micro meters in width. Notice also that in many regions of the membrane, there is no interstitial fluid. This is because pulmonary blood pressure is so low that little fluid filters out of the capillaries into the interstitial space. Oxygen and carbon dioxide can diffuse easily across this thin respiratory membrane. Here's a summary of what we've covered. The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs. The visceral pleura covers the surface of the lungs. The parietal pleura covers the mediastinum and diaphragm and lines the thoracic wall. The lungs contain the bronchial tree, the branching airways from the main bronchi through the terminal bronchioles. The respiratory zone of the lungs is the region containing alveoli - tiny thin-wall sacs where gas exchange occurs. Oxygen and carbon dioxide diffuse between the alveoli and the pulmonary capillaries across the very thin respiratory membrane.