Your lungs lie on each side of your heart inside your chest cavity. The right lung is divided into three lobes (sections), and the left lung is divided into two lobes. Your left lung is slightly smaller than your right lung, since your heart takes up some space on the left side. When you breathe in, air enters your airways and travels down into the alveoli (air sacs) in your lungs. This is where gas exchange takes place.

The circulatory system, which is made up of the heart and blood vessels, supports the respiratory system by bringing blood to and from the lungs. The circulatory system helps to deliver nutrients and oxygen from the lungs to tissues and organs throughout the body and removes carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymphatic system, and immune system.

Airways

The airways are pipes that carry oxygen-rich air to the alveoli in your lungs. They also carry the waste gas carbon dioxide out of your lungs. The airways include these parts of your body:

• Mouth
• Nose and linked air passages called the nasal cavity and sinuses
• Larynx (voice box)
• Trachea (windpipe)
• Tubes called bronchial tubes, or bronchi, and their branches
• Small tubes called bronchioles that branch off of the bronchial tubes

Air comes into your body

Air first enters your body through your nose or mouth, which moistens and warms the air since cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. Rings of tough tissue, called cartilage, acts as a support to keep the bronchial tubes open.

Inside your lungs, the bronchial tubes branch into thousands of thinner tubes called bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli.

Air fills your lung’s air sacs

Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Alveoli are able to easily expand and contract, because their insides are coated with a substance called surfactant. Surfactant reduces the work it takes to breathe by helping the lungs inflate more easily when you breathe in and preventing the lungs from collapsing when you breath out.

Each of these alveoli is made up of a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.

Blood low in oxygen flows through the lungs

The pulmonary artery and its branches deliver blood to the capillaries that surround the alveoli. This blood is rich in carbon dioxide and low in oxygen.

Oxygen flows into your blood

Carbon dioxide moves from the blood into the air inside the alveoli. At the same time, oxygen moves from the air into the blood in the capillaries.

How does my body protect the airways from food or bacteria?

When you swallow, a thin flap of tissue called the epiglottis covers your windpipe. Along with coughing and gag reflexes, the epiglottis prevents food and drink from entering the airway. The epiglottis also helps direct food into your esophagus, which is the pipe that goes to your stomach.

Except for the mouth and some parts of the nose, the airways have cells that make mucus, a sticky substance that coats the walls of the airways. Other cells in the airways have hairlike structures called cilia. The cilia and mucus trap germs and other particles that enter your airways when you breathe in air. The cilia then sweep the mucus-coated germs up to the nose or mouth. From there, the germs are swallowed, coughed, or sneezed out of the body.

The pleura

The lungs are surrounded by the pleura, a membrane with two layers. The space between these two layers is called the pleural cavity. A slippery liquid called pleural fluid acts as a lubricant to reduce friction during breathing.

A major organ of the respiratory system, each lung houses structures of both the conducting and respiratory zones. The main function of the lungs is to perform the exchange of oxygen and carbon dioxide with air from the atmosphere. To this end, the lungs exchange respiratory gases across a very large epithelial surface area—about 70 square meters—that is highly permeable to gases.

Gross Anatomy of the Lungs

The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm. The diaphragm is the flat, dome-shaped muscle located at the base of the lungs and thoracic cavity. The lungs are enclosed by the pleurae, which are attached to the mediastinum. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right. The cardiac notch is an indentation on the surface of the left lung, and it allows space for the heart (image). The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm. The costal surface of the lung borders the ribs. The mediastinal surface faces the midline.

Blood Supply and Nervous Innervation of the Lungs

The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body. In addition, innervation by the both the parasympathetic and sympathetic nervous systems provides an important level of control through dilation and constriction of the airway.

Blood Supply

The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation. This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes, also known as red blood cells, pick up oxygen to be transported to tissues throughout the body. The pulmonary artery is an artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli. The pulmonary artery branches multiple times as it follows the bronchi, and each branch becomes progressively smaller in diameter. One arteriole and an accompanying venule supply and drain one pulmonary lobule. As they near the alveoli, the pulmonary arteries become the pulmonary capillary network. The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers. The capillaries branch and follow the bronchioles and structure of the alveoli. It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane. Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum.

Nervous Innervation

Dilation and constriction of the airway are achieved through nervous control by the parasympathetic and sympathetic nervous systems. The parasympathetic system causes bronchoconstriction, whereas the sympathetic nervous system stimulates bronchodilation. Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from this autonomic nervous system control. Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum. The nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels.

Recall that blood returning from the systemic circuit enters the right atrium (image) via the superior and inferior venae cavae and the coronary sinus, which drains the blood supply of the heart muscle. These vessels will be described more fully later in this section. This blood is relatively low in oxygen and relatively high in carbon dioxide, since much of the oxygen has been extracted for use by the tissues and the waste gas carbon dioxide was picked up to be transported to the lungs for elimination. From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the pulmonary circuit.

The single vessel exiting the right ventricle is the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve, which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it curves posteriorly and rapidly bifurcates (divides) into two branches, a left and a right pulmonary artery. To prevent confusion between these vessels, it is important to refer to the vessel exiting the heart as the pulmonary trunk, rather than also calling it a pulmonary artery. The pulmonary arteries in turn branch many times within the lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround lung structures known as alveoli that are the sites of oxygen and carbon dioxide exchange.

Once gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger pulmonary veins. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. image defines the major arteries and veins of the pulmonary circuit discussed in the text.

Each lung is enclosed within a cavity that is surrounded by the pleura. The pleura (plural = pleurae) is a serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum. The pleurae consist of two layers. The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures (image). In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm. The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers.

The body’s muscles and nervous system help control your breathing.

The pleura and the muscles used for breathing

The lungs are enclosed by the pleura, a membrane that has two layers. The space between these two layers is called the pleural cavity. The membrane’s cells create pleural fluid, which acts as a lubricant to reduce friction during breathing.

The lungs are like sponges; they cannot move on their own. Muscles in your chest and abdomen contract, or tighten, to create space in your lungs for air to flow in. The muscles then relax, causing the space in the chest to get smaller and squeeze the air back out.

These muscles include the:

• Diaphragm, which is a dome-shaped muscle below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.
• Intercostal muscles, which are located between your ribs. They also play a major role in helping you breathe.
• Abdominal muscles. They help you breathe out when you are breathing fast, such as during physical activity.
• Muscles of the face, mouth, and pharynx. The pharynx is the part of the throat right behind the mouth. These muscles control the lips, tongue, soft palate, and other structures to help with breathing. Problems with these muscles can cause sleep apnea.
• Muscles in the neck and collarbone area. These muscles help you breathe in when other muscles involved in breathing are not working well or when lung disease impairs your breathing.

Damage to the nerves in the upper spinal cord can interfere with the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This can happen due to a spinal cord injury, a stroke, or a degenerative disease such as muscular dystrophy. The damage can cause respiratory failure. Ventilator support or oxygen therapy may be necessary to maintain oxygen levels in the body and protect the organs from damage.

Role of the nervous system

Your breathing usually does not require any thought, because it is controlled by the autonomic nervous system, also called the involuntary nervous system. The autonomic nervous system has different effects on your breathing.

• The parasympathetic nervous system tells the diaphragm and intercostal muscles to tighten and relax more quickly or more slowly to adjust your breathing rate in response to carbon dioxide and oxygen levels in the brain. This system also causes your bronchial tubes to narrow and the pulmonary blood vessels to widen.
• The sympathetic nervous system increases your breathing rate through what is called the fight-or-flight response. When this system is activated, it releases a chemical signal called norepinephrine that makes your bronchial tubes widen and the pulmonary blood vessels narrow.

Your breathing changes depending on how active you are and the condition of the air around you. For example, you need to breathe more often when you do physical activity. In contrast, your body needs to restrict how much air you breathe if the air contains irritants or toxins. At times, you can control your breathing pattern, such as when you hold your breath or sing.

To help adjust your breathing to changing needs, your body has sensors that help your respiratory system provide enough oxygen to the body while removing carbon dioxide. The sensors transmit signals to the centers of the brain involved in breathing. These sensors include:

• Sensors in the airways that detect lung irritants. The sensors can trigger sneezing or coughing. In people who have asthma, the sensors may cause the muscles around the airways in the lungs to contract. This makes the airways smaller.
• Sensors in the brain and in two blood vessels, the aorta and the carotid artery in the neck, that detect carbon dioxide or oxygen levels in your blood and change your breathing rate as needed.
• Sensors in your joints and muscles that detect the movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you are physically active.

Some health conditions can interfere with brain signals to the airways and chest muscles, causing central sleep apnea. In central sleep apnea, the respiratory centers of the brain do not respond properly to rising carbon dioxide levels and do not properly control the diaphragm and intercostal muscles. As a result, breathing stops for a short period. Central sleep apnea can lead to stroke and other medical conditions.

Breathing involves two phases: breathing in and breathing out. If you have problems breathing, gas exchange may be impaired, which can be a serious health problem.

Breathing in

When you breathe in, or inhale, your diaphragm contracts and moves downward. This increases the space in your chest cavity, and your lungs expand into it. The muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.

As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs. After passing through your bronchial tubes, the air travels to the alveoli, or air sacs.

Gas exchange

Through the thin walls of the alveoli, oxygen from the air passes into your blood in the surrounding capillaries. At the same time, carbon dioxide moves from your blood into the air sacs. The oxygen in your blood is carried inside your red blood cells by a protein called hemoglobin.

The oxygen-rich blood from your lungs is carried to the left side of the heart through the pulmonary veins. The heart pumps the blood to the rest of the body, where oxygen in the red blood cells moves from blood vessels into your cells.

Your cells use oxygen to make energy so your body can work. During this process, your cells also make a waste gas called carbon dioxide. Carbon dioxide needs to be breathed out or it can damage your cells.

Carbon dioxide moves from the cells into the bloodstream, where it travels to the right side of your heart. The blood rich in carbon dioxide is then pumped from the heart through the pulmonary artery to the lungs, where it is breathed out.

Breathing out

When you breathe out, or exhale, your diaphragm and rib muscles relax, reducing the space in the chest cavity. As the chest cavity gets smaller, your lungs deflate, similar to releasing of air from a balloon. At the same time, carbon dioxide-rich air flows out of your lungs through the windpipe and then out of your nose or mouth.

Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you are physically active, your abdominal muscles contract and push your diaphragm against your lungs even more than usual. This rapidly pushes air out of your lungs.

Conditions that affect the respiratory system

Damage, infection, or inflammation in the lungs or airways or both can lead to the following conditions.

• Acute Respiratory Distress Syndrome
• Asbestos-related lung diseases
• Asthma
• Atelectasis
• Bronchitis
• COPD
• Hypersensitivity pneumonitis
• Idiopathic pulmonary fibrosis
• Pleural Disorders
• Pneumonia
• Primary ciliary dyskinesia
• Sarcoidosis
• Sleep Apnea

Exposure to cigarette smoke, air pollutants, or other substances can damage the airways, causing disease of the airways or making a disease more severe.

You can take these steps to help protect your lungs from injury or disease:

• Quit smoking or don’t start. For free help and support to quit smoking, you may call the National Cancer Institute’s Smoking Quitline at 1-877-44U-QUIT (1-877-448-7848).
• Avoid secondhand tobacco smoke by staying away from places where smoking is allowed. Ask friends and family members who smoke not to do it in the house or the car.
• Aim for a healthy weight. Unhealthy eating patterns and lack of physical activity can lead to overweight and obesity, which can lead to sleep apnea. Research has shown that losing weight can reduce sleep apnea in people who have also been diagnosed with obesity.
• Be physically active. Physical activity can help strengthen your heart and lungs so they work more efficiently. Physical activity may also reduce your risk of lung injury or disease. Before starting any exercise program, ask your healthcare provider what level of physical activity is right for you.
• Limit exposure to outdoor air pollution by checking the Air Quality Index before taking part in outdoor activities. Also, avoid heavy traffic when possible.
• Reduce indoor air pollution by making sure that the places where you live and work are well ventilated and cleaned regularly to prevent the buildup of allergens, dust, and mold. You can also remove products that create fumes, such as strong cleaning fluids and aerosols. Avoid burning solid fuels such as wood for heating and cooking.
• Take precautions against seasonal flu and pneumonia. Get a flu shot every year. You may also want to ask your healthcare provider about the pneumonia vaccine.
• Test your home for radon gas. Radon is a colorless, tasteless, and odorless gas that forms naturally. It can enter buildings through cracks in the wall and can cause lung cancer. Inexpensive testing kits are available from many hardware stores. If radon levels in your house are hazardous, take the recommended steps to reduce them. If you’re in an apartment, alert the property owners so they can fix it.
• Use protective gear if you work in an industry that involves exposure to dust, silica, allergens, chemical fumes, or other indoor or outdoor air pollution.