Respiratory Physiology - Anatomy

Unconscious breathing is controlled by the pons and the medulla oblongata, both of which are parts of the brain stem. This unconscious breathing can be consciously controlled by using the cerebral cortex, which manages most voluntary actions.

It helps to remember that the brain stem is responsible for unconscious control of the body: breathing, heart rate, blood pressure, etc. It is the addition of the cerebral cortex that allows humans to have conscious control over actions, such as breathing, and override the unconscious controls. For example, the cerebral cortex is used to consciously stop breathing when diving underwater.

At rest the diaphragm is slightly curved superiorly such that it makes this sort of shape: When it contracts, it flattens out, with the middle of the muscle being pulled down until the muscle is roughly horizontal. Remembering that the diaphragm separates the thoracic and abdominal cavities, if it contracts, it physically increases the volume of the thoracic cavity. Now, remembering your fluid physics, an increase in volume is accompanied with a decrease in pressure. We know that high pressure flows to low pressure spontaneously. The atmospheric pressure is now higher than the intrapleural (or thoracic cavity) pressure, causing air to flow into the lungs.

Note that the external intercostals aid in inspiration and the internal intercostals aid in expiration.

Tidal volume is, by definition, the amount of air inspired/expired during normal breathing. The maximum volume of air that can be inspired after a normal expiration is the inspiratory capacity. The maximum volume of air that can be expired after a maximal inspiration is the vital capacity. The volume of air still in the lungs after a maximal expiration is the residual volume. The maximum volume of air that can be inspired after a normal inspiration is the inspiratory reserve volume.

During a respiratory cycle, the diaphragm contracts and moves downward. When this occurs the pressure in the alveoli falls. This pulls air into the lungs. At the same time external intercostals muscles contract, raising ribs and sternum and enlarges the cavity even more. During exhalation the diaphragm relaxes (moves up) and air is foced out of the body. A hiccup is a muscular spasm of the respiratory muscles including the diaphragm. A pneumothorax is a "hole" in the lungs that causes air to accumulate in the pleural space.

When exhaling, the lungs elasticity compresses the walls increasing the pressure within so that it exceeds atmospheric pressure and forces air out. Humans, despite how it might feel, do not suck in air. Rather pressure differences allow air to rush in and out. During expiration, the diaphragm relaxes, bowing up into the thoracic cavity, thereby decreasing the volume of the thoracic cavity. This results in a corresponding increase in pressure (Boyle's law), and thus the movement of air from the lungs out of the body through the upper respiratory structures.

The phrenic nerve is the nerve responsible for innervation of the diaphragm. The term phrenic is often associated with descriptions of the diaphragm (i.e cardiophrenic ligament is a ligament associated with connecting the diaphragm to the pericardium of the heart)

During the normal inspiratory phase of breathing, in other words, when a human is "breathing in," the physiological mechanism used is called "negative-pressure" breathing. Negative-pressure refers to the pressure in the chest cavity as compared to the surrounding environment. The body generates negative-pressure in the chest cavity during breathing by the contraction of the diaphragm muscle (it pulls downward, expanding the thoracic cavity size and space for the lungs to fill), and the outward expansion of the ribcage (which also expands the thoracic cavity size and provides more space for the lungs to fill). With the increased volume of the thoracic cavity generated, this creates the negative pressure that is needed to draw air into the lungs down its gradient of higher pressure (outside the body/thorax) to lower pressure (into the lungs/thorax). Positive-pressure is an incorrect choice because it is the opposite of what occurs during normal human inspiration.

Positive-pressure is sometimes artificially used in the medical setting with machines in patients with obstructive sleep apnea, or in patients who cannot breathe on their own, but is not a part of standard physiological respiration. The Frank-Starling mechanism describes the mechanism by which the heart pumps blood, but does not describe respiration. Glomerular filtration describes the mechanism by which the glomeruli of the kidneys initially filter blood, but does not describe respiration. Idiopathic pulmonary fibrosis is a disease of the interstitium of lung tissue, but does not describe the physiological mechanism used during inspiration.

In this patient with Chronic Obstructive Pulmonary Disease (COPD), which is a disease that results in difficulty expiring air (and generally does not affect one's ability to inspire air), we would expect to see an increased level of in the air that he/she expires, as compared to someone without COPD. Although this is a medically-oriented question, this does not require you to know anything about COPD that is not already supplied in the question. By stating that the ability to expire air is impacted but that the ability to inspire is generally not affected, this calls upon your knowledge of pulmonary physiology, telling you that if inspiration is not affected, levels are probably not significantly affected, and that if expiration is affected, levels are probably affected.

Once you identify that levels are affected in COPD, the question now is, how exactly are they affected? In a COPD patient, it is stated in the question stem, that they have a decreased ability to expire air. When a healthy person expires air, they remove from the body. In a COPD patient, who therefore has a decreased ability to remove from the lungs, if we measured the amount of a in a single breath, we would expect it to be elevated as compared to a healthy individual. At first glance, this may seem counter-intuitive, since we are stating that COPD patients have trouble removing from the body. However, the air that they expire is the same air that is coming from their lungs, which contains the elevated levels of . Thus, to answer the question, we expect the reading for the expired breath to be elevated as compared to that of a healthy person.

During forced maximum expiration, the lungs are trying their best to push air out of the lungs with the most force. This cannot be accomplished by the lungs alone, so the additional contraction of the abdominal muscles aid to help push air out of the lungs with maximum force.

This is different from normal (quiet) expiration, where only the elastic recoil of the lungs are needed with no additional muscle contractions. Normal expiration does not require pushing air out of the lungs with force.

None of the volumes listed would be normal. The internal intercostal muscles contribute to forced expiration and would affect ERV, TLC and VC. The external intercostal muscles contribute to inspiration and would affect the IRV, TLC and VC. Thus, none of the above volumes could be normal in a man who has non-functioning intercostal muscles.

When exercising, the muscles of inspiration include: external intercostals, scalene muscles, sternocleidomastoids.

When exercising, the muscles of expiration include: rectus abdominis, internal and external obliques, transversus abdominis, internal intercostals.

The main inspiratory muscle involved in quiet breathing is the diaphragm, which is a dome shaped sheet of skeletal muscle that is attached to the ribs, sternum, and vertebral column. External intercostals are also involved in quiet breathing, but if these muscles were impaired, quiet breathing would still continue because the diaphragm is the main muscle involved. Accessory muscles are not involved in quiet breathing, but may become involved during exercise. The abdominal wall muscles and internal muscles are both involved in expiration.

The pharynx is located posterior to the nose and mouth and receives both inhaled air and masticated food before they are transferred to the trachea and esophagus, respectively. The pharynx is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx. Food and air first enter the nasopharynx and proceed to the oropharynx. The laryngopharynx is the last portion of the pharynx, and is superior to the larynx, which is the passageway for air. Food is not meant to pass through the laryngopharynx and will result in coughing if it does.

The pharynx is shared by the respiratory and digestive systems, and is separated into three sections. The nasopharynx is primarily used for respiration, while the laryngopharynx is primarily used for digestion; it is inferior to the epiglottis and connects to the esophagus. The oropharynx is shared by pathways for both respiration and digestion.

The trachea and alveoli are exclusively used for respiration, while the esophagus and pyloric sphincter are exclusively used for digestion. Alveoli are the site of gas exchange in the lungs. The pyloric sphincter connects the stomach to the small intestine.

If the volume of the lungs increases, the air pressure inside the lungs decreases. Boyle's Law can be used to describe the process of human breathing. It states that for fixed mass, pressure and volume are inversely proportional.

If the volume increases, then pressure must decrease in order for these equations to hold true. This is responsible for the mechanics of inspiration. As the diaphragm contracts, the volume of the lungs increases and creates a negative pressure differential with the environmental atmosphere. This pressure differential draws air into the lungs through the nose and mouth.

The inspiratory reserve volume can be thought of as the amount of air that can be brought into the lungs consciously after an unconscious inhalation. This value does not include the tidal volume that is brought into the body by normal breathing. The maximum volume of inhaled air from rest is equal to half the tidal volume plus the inspiratory reserve volume.

Total lung capacity is the sum of the tidal volume (normal breathing), inspiratory reserve volume (additional volume from forced inhalation), expiratory reserve volume (additional volume from forced exhalation), and residual volume (air that cannot be forcefully moved from the lungs).

Depending on physical ability and gender, the inspiratory reserve volume is between 1900 and 3000 milliliters of air. Women typically have a lower inspiratory reserve volume compared to men.

Tidal volume is typically 500 milliliters, and is the amount of air that is moved by normal respiration; however, some of this air does not make it to the alveoli in order to take part in gas exchange. This air may simply remain in the trachea, bronchi, and bronchioles, where gas exchange cannot take place. As a result, a portion of the tidal volume is considered "dead space" volume.

Dead space volume is a small fraction of the tidal volume, and is usually around 150 milliliters of air.

The vital capacity is defined as the maximum amount of air that a person can exhale after a maximum inspiration. This value does not incorporate the amount of air that remains in the lungs after the maximum expiration. This remaining volume of air is called the residual volume, and is included in the total lung capacity.

Tidal volume (TV) is the amount of air moved with each unconscious breath. Inspiratory reserve volume (IRV) is the volume of additional air that can be forcefully inhaled, and expiratory reserve volume (ERV) is the additional volume of air that can be forcefully exhaled. After a forced inhalation, the lungs contain a volume equal to the lung capacity. After a forced exhalation, a volume equal to the vital capacity has been exhaled and a volume equal to the residual volume (RV) remains in the lungs.

To sum it up using equations:

Hemoglobin's primary function is to transport oxygen from the lungs to the myoglobin in the tissues that need oxygen. Oxygen is required for aerobic cellular respiration, so the tissues that have high metabolisms require the most oxygen. The byproducts of metabolic processes include acid, heat, carbon dioxide, and 2,3-bisphosphoglycerate (BPG). It should make sense that the byproducts of metabolism (evidence that oxygen is being used) influence hemoglobin to drop off its oxygen. Remember, if we stabilize the deoxygenated form of hemoglobin, it is less reluctant to drop off its oxygen since the oxygenated form of hemoglobin is always more stable than deoxygenated.

For your reference, 2,3-BPG is an isomer of a glycolytic intermediate that sits in the central cavity of hemoglobin. 2,3-BPG carries a large negative charge, and interacts with the basic (positive) amino acid side chains facing the central cavity of the molecule. When positives and negatives are close together, the molecule is stable.