Respiratory Physiology - Anatomy

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)

Oxygen will attach to hemoglobin and be delivered to the body's tissues via gas exchange. In order to detach from hemoglobin, there needs to be a decrease in the partial pressure of oxygen. The oxygen pressure in the lungs is much higher than in the body. This is why oxygen attaches to hemoglobin in the lungs. As the hemoglobin goes to the body's tissues, oxygen pressure decreases. This causes the oxygen to detach and be diffused into the tissues.

There are a few factors which can decrease the affinity of hemoglobin for oxygen: increased acidity, increased temperature, and carbon dioxide pressure. As carbon dioxide levels increase, hemoglobin's affinity for oxygen decreases. This means that it will release oxygen more readily in areas of high carbon dioxide levels, such as in the body's tissues. Since carbon dioxide is a product of cell metabolism, regions with high carbon dioxide content are likely very active in metabolism. This metabolism requires oxygen for the electron transport chain; thus, it is beneficial for hemoglobin to release oxygen in these regions in order to promote further metabolism.

There are three main ways that carbon dioxide can be carried in the blood: it can be in solution independently, it can be turned into a bicarbonate ion via the enzyme carbonic anhydrase, or it can be attached to hemoglobin and other proteins. Carbon dioxide transport as bicarbonate ions is ten times more common than any other method.

In the lungs, the bicarbonate ion will undergo the reverse reaction experienced in the tissues, and dissociate into carbon dioxide and water. This allows the carbon dioxide gas to be exhaled.

Normal blood pH is about 7.4 in most tissues (it is a bit lower in veins since they carry waste products, which are acidic). To get back to the physiological set point of pH = 7.4, we want to remove the acid from the blood. The major blood buffer system is shown in the following equation:

As we know, carbon dioxide is one of the major byproducts of respiration, and is considered waste for our bodies. Combined with water and catalyzed by carbonic anhydrase, it is converted into carbonic acid. Carbonic acid is a weak acid and will partially dissociate into hydrogen ions and bicarbonate ions. Thus, overall, carbon dioxide and water yields acid (hydrogen ions). As a result, excess carbon dioxide in the blood will lower the pH.

In order to increase the pH, we must stop this equation from proceeding in the forward direction; thus, (remember Le Chatelier's principle) we must remove carbon dioxide from the left side. This will push the reaction in the reverse direction, quenching hydrogen ions (acid) and removing them from the blood, increasing blood pH back to normal.

Since we want to get rid of excess carbon dioxide, we breathe faster. Oxygen does not have any effect on blood pH. Furthermore, the atmospheric oxygen level (21%) is plenty for our bodies to utilize, as when we exhale there is about 15% oxygen left over, meaning we only use about 25% of the oxygen we breathe (this is why CPR works!).

Cyanosis in the body occurs due a reduced hemoglobin concentration that is at least 6-8 grams of hemoglobin per deciliter of blood lower than the normal hemoglobin range for men and women.

Hemoglobin is what carries oxygen in the blood. The blood then carries this oxygen to various tissues in the body. When hemoglobin is low, oxygen is not delivered fast and efficiently enough to the appropriate tissues of the body, thus turning them visibly blue (cyanosis).

The man has above normal residual lung volume , as the normal residual volume (RV) for an adult male of average size is 1.2 liters. Causes for such high residual lung volumes in a man can occur from lung diseases, such as emphysema, that cause obstruction of the lungs and trapping of air.

The partial pressure of carbon dioxide would be the highest in systemic venous blood. This is because the systemic venous blood contains both the carbon dioxide that was in the systemic arterial blood and that which is added to the blood by tissue metabolism as the blood passes through the systemic capillaries.

The normal oxyhemoglobin concentration in mixed systemic venous blood for a person at rest is 75%. Therefore, a person with a oxyhemoglobin concentration of 25% is much below normal.

The total lung capacity (TLC) = IRV (inspiratory reserve volume) + TV (tidal volume) + ERV (expiratory reserve volume) + RV (residual volume).

The total lung capacity (TLC ) is the maximum volume of gas present in the lungs after a maximal inspiration. It includes all of the possible lung volumes.

Surfactant coats each alveolus, and is a detergent that lowers surface tension that prevents the alveolus from collapsing on itself. Also, decreasing surface tension facilitates the diffusion of gasses across the alveolar epithelium.

Alveoli are the terminal point of the respiratory zone and closest to the blood vessels in the lung. Since gas exchange uses diffusion, using alveoli makes sense because they are closer to the blood vessels.

Normal inspiration typically involves the flattening (contraction) of the diaphragm in order to increase the volume of the thoracic cavity, and can be done unconsciously. In order to increase the amount of inhaled air, other muscles such as the external intercostals and the sternocleidomastoids are included by conscious control. Both of these muscles aim to raise and expand the thoracic cavity in order to assist in inhalation.

The rectus abdominis is involved in the opposite action of forced exhalation. The rectus abdominis aims to decrease the volume of the thoracic cavity by contracting. This assists in forced exhalation.

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.

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.