Chapter 11: Respiratory system (C5319619)

1 Respiratory system

Respiratory system works with the cardiovascular system to deliver oxygen to and remove carbon dioxide from tissue throughout the body, by interfacing with the cardiovascular system at the lungs.

As mentioned , breathing is controlled by the medulla oblongata [of the brain], which sends a signal down the phrenic nerve to the diaphragm. Diaphragm is skeletal muscle, which as mentioned , is under the control of the [voluntary] somatic/voluntary nervous system. Diaphragm is in the shape of an upside down “U”, located just below the lungs. On contraction, the diaphragm flattens, enlarging the volume of the chest. From the ideal gas law mentioned  ($PV=nRT$), pressure and volume are inversely proportional (again, expressed  as Boyle's law), meaning if [chest] volume increases, [chest] pressure decreases [with respect to atmospheric pressure]. As fluids flow from high to low pressure, air flows into the lungs, expanding the elastic lungs. In contrast, in relaxation, the diaphragm reshapes, thereby reducing chest volume, and by Boyle’s law, increases chest pressure. Air therefore flows out of the lungs, recoiling the elastic lungs [from stretch].

 Frequently asked questions What is the respiratory system?The breathing system, which helps bring oxygen into the body. It also helps get rid of carbon dioxide.I thought that's what the cardiovascular system did?It is. But the respiratory system is the portion that interfaces between the body, and the outside atmosphere.What controls breathing?The medulla oblongata in the brain.In the main, how is breathing controlled?The signal travels down from the brain, through the phrenic nerve, to contract the diaphragm.What's the diaphragm?Skeletal muscle, in the shape of an upside down "U" - sort of like a mountain - located just below the lungs.When the diaphragm contracts, why does it "flatten"? And... how does that help?Because of its shape, when it contracts, the "mountain" reduces in size. It helps because it enlarges the volume of the chest.What was the ideal gas law again? And how does that relate to breathing? Say, when you breathe in?The ideal gas law is $PV=nRT$, which states that pressure and volume are inversely related. That's relationship is Boyle's law. That shows that when the volume of the chest increases, pressure decreases. Thus, air will flow into the lungs, because it flows from high to low pressure.And when you breathe out, the opposite occurs?You got it !And when the lungs fill with air, they expand?Right!

Nasal cavity can receive, warm, moisten, and filter air. Nasal hair keeps macroscopic foreign particles from entering through the nasal cavity, and cilia traps and sweeps microscopic foreign particles towards the pharynx, pharynx located at the back of the throat. As mentioned , the cartilaginous flap known as the epiglottis closes the anterior windpipe, preventing food [and other particle] from entering into the windpipe, instead redirecting these materials to the [posterior] esophagus. On top of the windpipe is the larynx, which contains the vocal cords. Windpipe, also known as trachea, provides a passageway for air into the lungs. Like the nasal cavity, trachea is ciliated, sweeping trapped particles up towards the pharynx. Trachea divides into the right and left primary bronchi, which branches into various bronchioles, ending in alveolar ducts. Gas exchange occurs between the air in the alveoli and blood in the capillaries. Each alveolus has a wall that is one cell thick, and is wrapped with a mesh of capillaries. As expressed , the one cell thick capillary is traversed by RBC in single file, depositing carbon dioxide and withdrawing oxygen.

[Inhaled] air is composed of 79% nitrogen and 21% oxygen, and exhaled air is composed of 79% nitrogen, 16% oxygen and 4% carbon dioxide. Nitrogen doesn’t react as it is a very stable molecule. However, elevated oxygen in inhaled air, and elevated carbon dioxide in exhaled air, is reflective of the basis of breathing. Carbon dioxide reacts with water in the presence of the enzyme carbonic anhydrase, to form carbonic acid ($\ce{H2CO3}$), or alternatively, $\ce{CO2 + H2O \rightleftharpoons H2CO3}$. The enzyme permits an equilibrium to be reached rapidly inside a RBC. Thus, when carbon dioxide is removed [by alveoli] from blood, by Le Chatelier’s principle, the equation will move to counteract the change, thereby moving left to replace the [removed] carbon dioxide, thereby decreasing concentration of carbonic acid, and thus increasing blood pH (more basic). In contrast, when carbon dioxide is added [by tissue] to the blood, the opposing effect will occur, which is the decrease of blood pH (more acidic).

Each RBC contains approximately 270 million hemoglobin’s. Hemoglobin, found in RBC, is a protein which contains four polypeptide subunits. Each polypeptide contains a heme group, which contains one iron atom, which can bind to one oxygen molecule. Thus, each hemoglobin can carry four oxygen’s. The first oxygen’s binding to hemoglobin increase affinity of hemoglobin at the other [three] binding sites, known as cooperativity. Hemoglobin affinity for oxygen is also affected by temperature, partial pressure of carbon dioxide, pH, and presence of organic phosphates such as 2,3-biphosphoglycerate (aka 2,3-BPG). Decreased affinity for oxygen is caused by increased temperature, increased carbon dioxide, increased $\ce{H+}$ ions (acidity, which is decreased pH), and increased 2,3-BPG. This makes sense, because respiration likes to be aerobic, so oxygen needs to be dropped off at tissue, thereby requiring decreased affinity [for oxygen]. Therefore, decreased affinity for oxygen is driven by the products of respiration, namely, [with the exception of 2,3-BPG,] increased heat [generated from glycolysis], carbon dioxide [from various stages of cellular respiration], and acidity [from lactic acid fermentation; or carbon dioxide subsequently reacting with water to form carbonic acid]. In contrast, myoglobin, which stores oxygen in muscle cells, is analogous to a single hemoglobin subunit, meaning it doesn’t exhibit cooperativity. Additionally, myoglobin is unaffected by acidity, carbon dioxide, and BPG. Note BPG is found inside RBC, and not muscle cells.

Oxygen dissociation curve for hemoglobin plots percent oxygen saturation of hemoglobin, against the partial pressure of oxygen [also known as oxygen tension]. The oxygen dissociation curve for hemoglobin is sigmoidal, which is caused by the cooperativity of oxygen (as stated ) in combination with BPG, which decreases affinity for oxygen as there are increased oxygen’s. Decreased affinity for oxygen is represented by a shift of the curve to the right, as at any given pressure, the oxygen saturation is less, meaning the presence of acidity, carbon dioxide, heat, etc., will lead to a rightward shift.

Evidently, the oxygen dissociation curve for myoglobin is not sigmoidal, as it doesn’t display cooperativity. Additionally, the myoglobin curve is to the left of the hemoglobin curve, as myoglobin has greater affinity for oxygen than hemoglobin. This makes sense because hemoglobin in blood needs to [leave hemoglobin and] drop off oxygen for [and bind to] myoglobin in blood, so thus [hemoglobin] would have lesser affinity for oxygen. Analogously, fetal hemoglobin receives oxygen from maternal hemoglobin, meaning fetal hemoglobin must have greater affinity than maternal hemoglobin, meaning fetal hemoglobin must be to the left of maternal hemoglobin. However, fetal hemoglobin must deliver oxygen to its myoglobin, meaning myoglobin must have greater affinity [and therefore to the left] of fetal hemoglobin.

As stated , in aqueous blood, carbon dioxide converts into carbonic acid (mentioned ) or bicarbonate ion ($\ce{HCO3-}$), in accordance with the [expanded] formula $\ce{CO2 + H2O \rightleftharpoons HCO3- + H+}$. As the enzyme carbonic anhydrase is not present in blood plasma [only in RBC], carbonic acid and bicarbonate ion can only be appreciably generated within the RBC. Therefore, in deoxygenated blood (blood with carbon dioxide), as there is elevated carbon dioxide, Le Chatelier’s principle shifts the equation to the right, such that more bicarbonate ion will be produced [in the RBC] and shuttled out [into the plasma]. However, the RBC membrane is impermeable to hydrogen ions which are trapped within the cell, increasing the positive charge within the cell. To offset this, [negative] chloride ions move into the cell as [negative] bicarbonate ion moves out. Thus, it can be said that deoxygenated blood, expressed  as venous blood, has chloride ions moving into the cell. In contrast, in deoxygenated blood (blood with little carbon dioxide), as there is depressed carbon dioxide, Le Chatelier’s principle shifts the equation to the left, such that bicarbonate ions move into RBC[causing chloride ions to move out of the cell], and are converted into carbon dioxide and released into the blood as required. Thus, it can be said that oxygenated blood, expressed  as arterial blood, has chloride ions moving out of the cell. Note therefore, that chloride ions are greater in venous RBC than in arterial RBC, known as chloride shift.

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# Pre-med science (MED5118352)

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