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The Respiratory System

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The Respiratory System

Systems of Gas Exchange
Gas Exchange across Respiratory Surfaces
Breathing
Transport of Gases in Human Bodily Fluids

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Systems of Gas Exchange

•Introduction and Direct Diffusion
•Skin, Gills, and Tracheal Systems
•Mammalian Systems and Protective Mechanisms

Introduction and Direct Diffusion

•Respiration ensures that cells, tissues, and major organs of the body receive an adequate supply of oxygen and that the carbon dioxide, a waste product, is efficiently removed; the exchange of oxygen and carbon dioxide occurs via diffusion across cell membranes.
•The mechanisms, processes, and structures used for respiration are dictated by the type, size, and complexity of the organism.
•Direct diffusion of gases through the outer membranes can be used by organisms such as flatworms as a means of respiration due to their small size and simplicity.

Skin, Gills, and Tracheal Systems

•Some animals, such as amphibians and earthworms, can use their skin (integument) to exchange gases between the external environment and the circulatory system due to the network of capillaries that lie below the skin.
•Fish and other aquatic organisms use gills to take up oxygen dissolved in the water and diffuse carbon dioxide out of the bloodstream.
•Some insects utilize a tracheal system that transports oxygen from the external environment through openings called spiracles.

Mammalian Systems and Protective Mechanisms

•The air that moves from the external environment into the body must pass through the nasal cavity where it is warmed, humidified, and surveyed for particulates.
•As air moves out of the nasal cavity, it moves into the pharynx, larynx, trachea, the primary bronchi (right and left lung), secondary and tertiary bronchi, bronchioles, terminal then respiratory bronchioles, alveolar ducts then alveolar sacs where gas exchange occurs with the capillaries.
•Components in the respiratory system allow for protection from foreign material; these include mucus production in the lungs and cilia in the bronchi and bronchioles to move matter out of the system.
•Components in the respiratory system that allow for protection from foreign material and include mucus production in the lungs and cilia in the bronchi and bronchioles.


Gas Exchange across Respiratory Surfaces

•Basic Principles of Gas Exchange
•Lung Volumes and Capacities
•Gas Pressure and Respiration
•Gas Exchange across the Alveoli

Basic Principles of Gas Exchange

•Gas is exchanged between the alveoli and the pulmonary capillaries via diffusion: gas molecules will move from an area of high concentration to an area of low concentration.
•The partial pressure of oxygen (PO2) is lower in the alveoli in comparison to the external environment, which allows for diffusion of oxygen into the alveoli.
•The partial pressure of carbon dioxide (PCO2) is higher in the capillaries than in the alveoli, which allows for diffusion into the alveoli where it is exhaled during expiration.
•The ventilation/perfusion ratio (V/Q) ensures that the ideal amount of blood and gas is received by the alveoli for efficient gas exchange.

Lung Volumes and Capacities

•The lung volumes that can be measured using a spirometer include tidal volume (TV), expiratory reserve volume (ERV), and inspiratory reserve volume (IRV).
•Residual volume (RV) is a lung volume representing the amount of air left in the lungs after a forced exhalation; this volume cannot be measured, only calculated.
•The lung capacities that can be calculated include vital capacity (ERV+TV+IRV), inspiratory capacity (TV+IRV), functional residual capacity (ERV+RV), and total lung capacity (RV+ERV+TV+IRV).

Gas Pressure and Respiration

•Atmospheric pressure is the sum of all the partial pressures of the gases in the atmosphere, including oxygen, carbon dioxide, nitrogen, and water vapor.
•In the atmosphere, the partial pressure of oxygen is much greater than the partial pressure of carbon dioxide.
•The partial pressure of oxygen in the atmosphere is much greater in comparison to the lungs, creating a pressure gradient; this allows oxygen to flow from the atmosphere into the lungs during inhalation.

Gas Exchange across the Alveoli

•The change in partial pressure from the alveoli (high concentration) to the capillaries (low concentration) drives the oxygen into the tissue and the carbon dioxide into the blood (high concentration) from the tissues (low concentration), which is then returned to the lungs and exhaled.
•Once in the blood of the capillaries, the O2 binds to the hemoglobin in red blood cells which carry it to the tissues where it dissociates to enter the cells of the tissues.
•The lungs never fully deflate, so air that is inhaled mixes with the residual air left from the previous respiration, resulting in a lower partial pressure of oxygen within the alveoli.


Breathing

•Types of Breathing
•The Mechanics of Human Breathing
•The Work of Breathing
•Dead Space: V/Q Mismatch

Types of Breathing

•Eupnea is normal quiet breathing that requires contraction of the diaphragm and external intercostal muscles.
•Diaphragmatic breathing requires contraction of the diaphragm and is also called deep breathing.
•Costal breathing requires contraction of the intercostal muscles and is also called shallow breathing.
•Hyperpnea is forced breathing and requires muscle contractions during both inspiration and expiration such as contraction of the diaphragm, intercostal muscles, and accessory muscles.
•Amphibians utilize gills for breathing early in life and later develop primitive lungs in their adult life; additionally, they are able to breathe through their skin.
•Birds have evolved a directional respiratory system that allows them to obtain oxygen at high altitudes: air flows in one direction while blood flows in another, allowing efficient gas exchange.

The Mechanics of Human Breathing

•The mechanics of breathing follow Boyle’s Law which states that pressure and volume have an inverse relationship.
•The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere; thus, air rushes in the airway.
•The process of exhalation occurs due to an elastic recoil of the lung tissue which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere; thus, air rushes out of the airway.
•There is no contraction of muscles during exhalation; it is considered a passive process.
•The lung is protected by layers of tissue referred to as the visceral pleura and parietal pleura; the intrapleural space contains a small amount of fluid that protects the tissue by reducing friction.

The Work of Breathing

•Both flow-resistive and elastic work are conducted during the act of respiration; flow-resistive work involves the alveoli and tissues, while elastic work involves the intercostal muscles, chest wall, and diaphragm.
•These types of work function in an inverse relationship; for example, increasing the rate of respiration results in an increase in the flow-resistive work and a decrease in the elastic work.
•Surfactant is a phospholipid and lipoprotein substance produced in the lungs that functions similarly to a detergent: it reduces the surface tension between alveoli tissue and air within the alveoli, thereby reducing the work needed for airway inflation.
•Lung resistance plays a key role in the ability to efficiently exchange gases; if there is obstruction (resistance) within the airways, the result will be decreased gas exchange.
•Lung compliance plays a key role in the ability to efficiently exchange gases; if there is too much of an increase or decrease in elasticity of the lung, the result will be disruption of gas exchange, which will cause obstructive or restrictive diseases.

Dead Space: V/Q Mismatch

•At times, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs, referred to as ventilation/perfusion (V/Q) mismatch.
•The two major types of V/Q mismatch that result in dead space include: anatomical dead space (caused by an anatomical issue) and physiological dead space (caused by a functional issue with the lung or arteries).
•Anatomical dead space can occur due to changes in gravity (i.e. posture positions: sitting, standing, lying); it will affect both ventilation (V) and perfusion (Q).
•Physiological dead space can occur due to changes in function, such as in cases of infection of the lung; it will typically affect ventilation if the infection is in the lung and will affect perfusion if the functional impairment is in the arteries.
•In a normal, healthy individual, changes in either ventilation or perfusion will result in correction of the other factor to ensure an appropriate V/Q ratio.


Transport of Gases in Human Bodily Fluids

•Transport of Oxygen in the Blood
•Transport of Carbon Dioxide in the Blood

Transport of Oxygen in the Blood

•Hemoglobin is made up of four subunits and can bind up to four oxygen molecules.
•Carbon dioxide levels, blood pH, body temperature, environmental factors, and diseases can all affect oxygen’s carrying capacity and delivery.
•A decrease in the oxygen-carrying ability of hemoglobin is observed with an increase in carbon dioxide and temperature, as well as a decrease in pH within the body.
•Sickle cell anemia and thalassemia are two hereditary diseases that decrease the blood’s oxygen-carrying capacity.

Transport of Carbon Dioxide in the Blood

•Carbon dioxide is more soluble in blood than is oxygen; about 5 to 7 percent of all carbon dioxide is dissolved in the plasma.
•Carbon dioxide has the ability to attach to hemoglobin molecules; it will be removed from the body once they become dissociated from one another.
•In the bicarbonate buffer system, the most common form of carbon dioxide transportation in the blood, carbon dioxide is finally expelled from the body through the lungs during exhalation.
•Importantly, the bicarbonate buffer system allows little change to the pH of the body system; it allows for people to travel and live at high altitudes because the system can adjust itself to regulate carbon dioxide while maintaining the correct pH in the body.


Appendix

Key terms

•aerobic living or occurring only in the presence of oxygen
•alveolus a small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood
•atmospheric pressure the pressure caused by the weight of the atmosphere above an area
•bifurcate to divide or fork into two channels or branches
•bronchus either of two airways, which are primary branches of the trachea, leading directly into the lungs
•carbaminohemoglobin a compound made up of hemoglobin and carbon dioxide; one of the forms in which carbon dioxide exists in the blood
•carbon monoxide a colorless, odourless, flammable, highly toxic gas
•carbonic anhydrase a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons
•coelom a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
•dead space air that is inhaled by the body in breathing, but does not partake in gas exchange
•deoxygenated having removed the oxygen atoms from a molecule
•diffusion The passive movement of a solute across a permeable membrane
•eupnea normal, relaxed breathing; healthy condition of inhalation and exhalation
•gill a breathing organ of fish and other aquatic animals
•heme the component of hemoglobin responsible for binding oxygen; consists of an iron ion that binds oxygen and a porphyrin ring that binds the globin molecules; one molecule binds one molecule of oxygen
•hemoglobin iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
•hemoglobin iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
•hydrostatic of or relating to fluids, especially to the pressure that they exert or transmit
•hyperpnea deep and rapid respiration that occurs normally after exercise or abnormally with fever or various disorders
•intercostal between the ribs of an animal or person
•mole in the International System of Units, the base unit of amount of substance
•oxyhaemoglobin the form of hemoglobin, loosely combined with oxygen, present in arterial and capillary blood
•parietal pleura the portion of the protective tissue that lines the inner surface of the chest wall and covers the diaphragm
•partial pressure the pressure one component of a mixture of gases would contribute to the total pressure
•partial pressure the pressure one component of a mixture of gases would contribute to the total pressure
•perfuse to force a fluid to flow over or through something, especially through an organ of the body
•pulmonary circulation the part of blood circulation which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart
•residual volume the volume of unexpended air that remains in the lungs following maximum expiration
•sickle cell anemia a hereditary blood disorder, characterized by red blood cells that assume an abnormal, rigid, sickle shape
•spiracle a pore or opening used (especially by spiders and some fish) for breathing
•spirometry the measurement of the volume of air that a person can move into and out of the lungs
•surfactant a lipoprotein in the tissues of the lung that reduces surface tension and permits more efficient gas transport
•systemic circulation the part of blood circulation which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart
•thalassemia an inherited disorder in which the person produces a high number of red blood cells, but the cells have lower levels of hemoglobin
•tidal volume the amount of air breathed in or out during normal respiration
•tidal volume the amount of air breathed in or out during normal respiration
•visceral pleura the portion of protective tissue that is attached directly to the lungs

Partial pressures
The partial pressures of oxygen and carbon dioxide change as blood moves through the body.

Common carp
This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water.

Insect respiration
Insects perform respiration via a tracheal system, in which openings called spiracles allow oxygen to pass into the body.

Oxygen dissociation curve
The oxygen dissociation curve demonstrates that as the partial pressure of oxygen increases, more oxygen binds hemoglobin.However, the affinity of hemoglobin for oxygen may shift to the left or the right depending on environmental conditions.

Lung structure
The trachea bifurcates into the right and left bronchi in the lungs.The larger right lung is made of three lobes.To accommodate the heart, the left lung is smaller, having only two lobes.

Oxygen transport and gills
As water flows over the gills, oxygen is transferred to blood via the veins.

Trachea and bronchi structure
The trachea and bronchi are made of incomplete rings of cartilage.

Direct diffusion
This flatworm’s process of respiration works by diffusion across the outer membrane.

Atmospheric pressure vs altitude
At high altitudes, there is a decrease in Patm, causing the partial pressures to decrease as well.

Sickle cell anemia
Individuals with sickle cell anemia have crescent-shaped red blood cells.Diseases such as this one cause a decreased ability in oxygen delivery throughout the body.

Gas exchange
This schematic demonstrates how gas is exchanged in humans between a capillary and an alveolus.

Hemoglobin
The protein inside red blood cells (a) that carries oxygen to cells and carbon dioxide to the lungs is hemoglobin (b).Hemoglobin is made up of four symmetrical subunits and four heme groups.Iron associated with the heme binds oxygen.It is the iron in hemoglobin that gives blood its red color.

Human lung volumes and capacities
The total lung capacity of the adult male is six liters.Tidal volume is the volume of air inhaled in a single, normal breath.Inspiratory capacity is the amount of air taken in during a deep breath, while residual volume is the amount of air left in the lungs after forceful respiration.

Avian respiratory system
(a) Birds have a flow-through respiratory system in which air flows unidirectionally from the posterior sacs into the lungs, then into the anterior air sacs.The air sacs connect to openings in hollow bones.(b) Dinosaurs, from which birds descended, have similar hollow bones and are believed to have had a similar respiratory system.

FEV1/FVC ratio
The ratio of FEV1 (the amount of air that can be forcibly exhaled in one second after taking a deep breath) to FVC (the total amount of air that can be forcibly exhaled) can be used to diagnose whether a person has restrictive or obstructive lung disease.

Boyles law
This graph of data from Boyle’s original 1662 experiment shows that pressure and volume are inversely related.No units are given as Boyle used arbitrary units in his experiments.

Carbon monoxide poisoning
When carbon monoxide (CO) in the body increases, the oxygen saturation of hemoglobin decreases since hemoglobin will bind more readily to CO than to oxygen.Therefore, CO exposure leads to death due to a decreased transportation of oxygen in the body.

Route of inhalation
Air enters the respiratory system through the nasal cavity and pharynx.It then passes through the trachea and into the bronchi, which bring air into the lungs.

Inhalation and exhalation
The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration.

Visceral pleura
A tissue layer called pleura surrounds the lung and interior of the thoracic cavity.

Alveolar structure
Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs.Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes.Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries.Mucus glands secrete mucus into the airways, keeping them moist and flexible.

Electron microscope image of cilia
The bronchi and bronchioles contain cilia that help move mucus and other particles out of the lungs.

Pulmonary edema
A physiological shunt can develop if there is infection or edema in the lung which decreases ventilation, but does not affect perfusion; thus, the ventilation/perfusion ratio is affected.Pulmonary edema with small pleural effusions on both sides (as shown) can cause changes in the V/Q ratio.

Diaphragmatic breathing
Animation of a diaphragm exhaling and inhaling, demonstrating diaphragmatic breathing.During inhalation, the diaphragm is contracted which increases the volume of the lung cavity.During exhalation, the diaphragm is relaxed which decreases the volume of the lung cavity.

Attribution

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•Wiktionary. “carbon monoxide.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/carbon+monoxide
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