Respiratory System Intelligent Design in Humans and Animals BIO 39 by Owen Borville August 7, 2025
The main purpose of the respiratory system in humans and animals is to deliver oxygen to the cells of the body's tissues and remove carbon dioxide, which is a waste product of the cell. This process of the respiratory system is done through the nasal cavity, trachea, and lungs.
Oxygen is used to perform metabolic functions in the body.
Smaller multicellular organisms use direct diffusion across the outer membrane to obtain oxygen. Oxygen moves from areas of higher concentration to areas of lower concentration on the organism. Flatworms use direct diffusion because its flat shape allows enough surface area for diffusion without the need for more advanced respiratory organs.
Skin is used as a respiratory organ in earthworms and amphibians by using capillary networks below the skin to facilitate gas exchange to and from the organism's skin. Gills in fish, amphibians, and marine organisms are thin branched and folded tissue filaments that help the respiratory process by obtaining dissolved oxygen from water into the bloodstream to be used throughout the body. Oxygen is diffused from the water to the surface area of the gills as the oxygen moves from higher concentration in the water to lower concentration in the blood of the gills of the organism.
Tracheal systems are used for respiration in animals like insects, where respiration is independent of the circulatory system and blood does not play a direct role in respiration. The tracheal system is a network of small tubes made of chitin that carry oxygen to the entire body. Insects also have openings called spiracles on their body on the thorax and abdomen that connect to the tubular network to allow oxygen to come into the body and release carbon dioxide.
In mammals, respiration occurs by inhaling or breathing air into the body. Air enters a nasal cavity and is warmed to body temperature and humidity in order to achieve equilibrium. Mucus keeps the respiratory tract from having direct contact with air and the mucus contains much water, which helps protect the body from cold, dry air and achieve equilibrium. Cilia in the naval passages and mucus also help remove harmful particulate matter in air as it passes through.
Air moves through the nasal cavity through the pharynx or throat, and larynx or voice box, on the way to the trachea. The trachea help move air into the lungs and help move exhaled air out of the body. The cylindrical trachea is made of cartilage and smooth muscle, and the trachea is lined with cells that produce mucus. The trachea is also lined with ciliated epithelia tissues. The cilia help remove foreign particles and the smooth muscle can contract or relax.
The trachea fork downward into the left and right lungs. The right lung is larger with three lobes and the left lung is smaller with two lobes. The muscular diaphragm is below the two lungs and facilitates breathing.
Inside the lungs, air moves into smaller passages called bronchi. Air enters the lung through the two main or primary bronchi or bronchus for singular. Each bronchus branches into smaller and smaller bronchi or bronchioles throughout the lung. The bronchi are also made of cartilage and smooth muscle. The bronchioles have elastic fibers instead of cartilage. In the bronchi, nerves control their contraction or relaxation. Smooth muscle is more abundant in the smaller bronchi.
The terminal bronchioles are divided into microscopic branches called respiratory bronchioles. These respiratory bronchioles are further subdivided into alveolar ducts. Surrounding these alveolar ducts are alveoli and spherical alveolar sacs. Gas exchange occurs only in the alveoli, which connect directly with capillaries of the circulatory system so that oxygen can be transferred throughout the body. Carbon dioxide also diffuses from the blood through the alveoli to be exhaled as a waste product. There are very many alveoli in each alveoli sac and very many alveoli sacs at the end of the alveolar duct. A lung can contain 300 million alveoli. Therefore, the lungs act as a sponge for gas exchange and a large surface area is available.
The respiratory system has protective mechanisms to prevent particulate matter, including dust, dirt, viral particles, and bacteria from causing damage to the lungs or cause allergies. In the nasal cavity, hairs and mucus capture small particles, viruses, dust, dirt, and bacteria, so that these are prevented from entering the lungs.
Sometimes particulate matter finds its way past the nose or enters the mouth and enters the lungs. However, the lungs produce mucus, which contains a glycoprotein, salts, and water that can trap the particulates. The walls of the bronchi and bronchioles also contain cilia, which are hair like projections that move in a unified manner to push mucus and particulates back up out of the bronchioles and toward the throat, where the harmful material can be swallowed to the stomach.
The 300 million alveoli in the lungs allow for maximum surface area to maximize gas diffusion. Gas exchange during respiration occurs mainly through diffusion. The diffusion process is caused by a concentration gradient, as gas molecules move from high pressure to low pressure location. So gas is exchanged from air in the lungs with the gas in the blood and oxygen levels are replenished in the blood.
Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture of gases is a sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total total gas mixture.
The lung capacity of each animal is different based on its size and amount of physical activity. Human lung capacity or volume is based on genetics, sex, and size. An adult lung can hold up to six liters of air.
The volume in the lungs can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume.
Tidal volume measures the amount of air that is inspired or expired during a normal breath, which is commonly about one half liter.
Expiratory reserve volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation.
Inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation.
The residual volume is the amount of air that is left after expiratory reserve volume is exhaled. There is always some residual air to help the lungs them reinflate.
Capacities are measurements of two or more volumes.
The vital capacity (VC) measures the maximum amount of air that can be inhaled or exhaled during a respiratory cycle and is the sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume.
The inspiratory capacity (IC) is the amount of air that can be inhaled after the end of the normal expiration and is the sum of the tidal volume and inspiratory reserve volume.
The functional residual capacity (FRC) is the volume of air remaining after a normal exhalation (sum of ERV and RV), which includes the expiratory reserve volume and residual volume.
The total lung capacity is a measure of the total amount of air that the lungs can hold and combines the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.
Lung volumes are measured by a technique called spirometry. The forced expiratory volume (FEV) measures how much air can be forced out of the lung over a specific period of time, such as one second. The forced vital capacity (FVC), which is the total amount of air that can be forcibly exhaled, is measured.
The ratio of (FEV)/(FVC) is used to diagnose lung diseases including asthma, emphysema, and fibrosis. A high ratio would show lung fibrosis. A low ratio would show asthma.
As air is a mixture of gases, the partial pressure of any gas in air can be calculated by multiplying the atmospheric pressure by the percent content in the gas mixture.
The atmospheric pressure is the sum of all of the partial pressures of atmospheric gasses added together.
In the gas exchange in the respiratory system from oxygen to carbon dioxide, the ratio of carbon dioxide production to oxygen consumption is the respiratory quotient (RQ).
The RQ is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung, the alveolar PO2.
Oxygen and carbon dioxide move independently of each other and they each diffuse down their own pressure gradients.
The change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.
In mammals, lungs are located in the thoracic cavity and are surrounded and protected by the rib cage, muscles, and chest wall. Below the lungs is the diaphragm, a muscle that facilitates breathing. Therefore, breathing requires the coordination of all of these parts.
Amphibians have gills that enable breathing when they are young and older amphibians grow lungs. Amphibians also breathe by diffusion across the skin.
Birds have lungs like mammals that enable breathing. In addition, birds have air sacs in their body that help with gas exchange, as birds use a large amount of energy from oxygen for flying and also at high altitudes. Air flows in and out of the air sacs, and opposite the direction of blood flow, which allows birds to obtain the amount of oxygen they need for flying at high altitudes.
According to Boyles Law, in a closed space, pressure and volume are inversely related. As one decreases, the other increases. This relationship helps explain breathing in animals and humans.
Intercostal muscles attached to the rib cage help increase the volume of the lungs by contracting. Lung volume expands because the diaphragm contracts and the intercostal muscles contract, so that the thoracic cavity expands.
The chest wall expands out and away from the lungs, which are elastic and help the lungs deflate after inflating during breathing by elastic recoil. The diaphragm also moves up and helps the lungs release air outward by relaxing and increasing the pressure toward the thoracic cavity.
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax. The space between these layers is the intrapleural space and contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed.
The respiratory rate is measured by the number of breaths per minute. This rate contributes to the alveolar ventilation, which is how much air moves in and out of the alveoli. This alveolar ventilation prevents carbon dioxide buildup in the alveoli. Alveolar ventilation is kept constant by increasing the respiratory rate while decreasing tidal volume of air per breath or decreasing the respiratory rate while increasing the the tidal volume per breath.
There are two types of work done performed by respiration. Flow-resistive work refers to the work of the alveoli and tissues in the lung. Elastic work refers to the work of the intercostal muscles, chest wall, and diaphragm. Increasing the respiration rate increases the flow-resistive work of the airways and decreases the elastic work of the muscles. Decreasing the respiration rate reverses the type of work required.
Surfactant is a complex mixture of phospholipids and lipoproteins that work to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli, preventing the alveoli from collapsing.
Diseases of the lungs reduce the rate of gas exchange in and out of the lungs. Two main causes of reduced rate of gas exchange are compliance, or how elastic the lung is, and resistance, or how much obstruction exists in the airways.
Restrictive diseases where the airways are stiff or fibrotic are respiratory distress syndrome and pulmonary fibrosis, where patients have trouble exhaling air.
Obstructive diseases like emphysema, asthma, and pulmonary edema. Causes include smoking tobacco products, where the walls of the alveoli are destroyed, and surface area for gas exchange is reduced. Asthma is inflammation of the airways caused by environmental factors. The obstruction may be cause by edema, or fluid accumulation, smooth muscle spasms in the bronchioles, increased muscle secretion, or damage to the epithelia of the airways.
Systemic circulation delivers oxygenated blood from the heart to the rest of the body, while pulmonary circulation carries deoxygenated blood from the heart to the lungs for oxygenation and then back to the heart. Pulmonary circulation pressure is low compared to systemic circulation, because of recruitment, a process of opening airways that normally remain closed when cardiac output increases (like during exercise).
As cardiac output increases, the number of capillaries and arteries filled with blood increases. Sometimes there is a mismatch between the amount of air ventilation and the amount of blood perfusion in the lungs, called the ventilation/perfusion (V/Q) mismatch.
This mismatch produces dead space, or regions of broken down or blocked lung tissue. These can impact breathing by reducing the surface area for gas diffusion. Oxygen in the blood is reduced and carbon dioxide level is increased. Dead space is created when no ventilation or perfusion takes place. Dead space can be caused by anatomical failure such as from gravity effects on the lungs or functional impairment of the lungs and arteries such as from an infection or obstruction.
The lungs can adjust for these mismatches in ventilation and perfusion as blood vessels dilate and bronchioles constrict (if ventilation is greater than perfusion), or vise versa. Blood vessels constrict and bronchioles dilate when ventilation is less than perfusion.
Hemoglobin is a protein molecule found in red blood cells (erythrocytes) that binds to and carries most oxygen (98.5 percent) to the tissues. The rest of the oxygen dissolves into the blood itself.
The hemoglobin (Hb) molecule is composed of two alpha subunits and two beta subunits. Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, and allows each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme group are more bright red in color, while those with less oxygen are darker red in color.
The second and third oxygen molecule are easier to bind to the hemoglobin molecule than the first. The fourth oxygen molecule is more difficult to bind. The binding of oxygen molecules to hemoglobin is a function of partial pressure of oxygen in the blood versus the relative hemoglobin-oxygen saturation. An oxygen dissociation curve can be plotted. As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.
The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. Partial pressure of oxygen dissolved in blood (PO2) and other environmental factors and diseases affect oxygen carrying capacity.
Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity. Carbon dioxide makes the blood more acidic and reduces hemoglobin's affinity for oxygen. Increased temperature also reduces hemoglobin affinity for oxygen. Diseases like sickle-cell anemia and thalassemia decrease the body's ability to deliver oxygen to tissues and its oxygen-carrying capacity.
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Properties of carbon dioxide affect its transport in blood. Carbon dioxide is more soluble in blood than oxygen. Carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin (carbaminohemoglobin). Carbon dioxide can also be unbound from hemoglobin when it reaches the lungs and removed from the body.
Most carbon dioxide molecules are carried in the blood as part of the bicarbonate buffer system. Carbon dioxide diffuses into red blood cells. Carbonic anhydrase (CA) in the red blood cells converts carbon dioxide into carbonic acid H2CO3, which is unstable and and dissociates into bicarbonate ions, HCO3- and H+ ions.
A chloride shift occurs when the bicarbonate ion is transported out of the red blood cell into the blood in exchange for a chloride (Cl-) ion. In the lungs, the bicarbonate ion is transferred back into the red blood cell in exchange for the chloride ion. The H+ ion disassociates from the hemoglobin and binds to the bicarbonate ion. This produces carbonic acid intermediate, which is converted back into carbon dioxide by CA.
The bicarbonate buffer system prevents the carbon dioxide from changing the pH of the system. So pH is regulated in the blood and preserves equilibrium in the system. Excess acids and bases are buffered to preserve pH and equilibrium. Increased breathing rate can also remove excess carbon dioxide, while reduced breathing rates can retain carbon dioxide to keep pH levels balanced.
Carbon monoxide has a stronger affinity for hemoglobin that oxygen, and therefore carbon monoxide prevents oxygen from binding to hemoglobin. Exposure to carbon monoxide can lead to sickness or death. Carbon monoxide gas is colorless, odorless, and difficult to detect.
The main purpose of the respiratory system in humans and animals is to deliver oxygen to the cells of the body's tissues and remove carbon dioxide, which is a waste product of the cell. This process of the respiratory system is done through the nasal cavity, trachea, and lungs.
Oxygen is used to perform metabolic functions in the body.
Smaller multicellular organisms use direct diffusion across the outer membrane to obtain oxygen. Oxygen moves from areas of higher concentration to areas of lower concentration on the organism. Flatworms use direct diffusion because its flat shape allows enough surface area for diffusion without the need for more advanced respiratory organs.
Skin is used as a respiratory organ in earthworms and amphibians by using capillary networks below the skin to facilitate gas exchange to and from the organism's skin. Gills in fish, amphibians, and marine organisms are thin branched and folded tissue filaments that help the respiratory process by obtaining dissolved oxygen from water into the bloodstream to be used throughout the body. Oxygen is diffused from the water to the surface area of the gills as the oxygen moves from higher concentration in the water to lower concentration in the blood of the gills of the organism.
Tracheal systems are used for respiration in animals like insects, where respiration is independent of the circulatory system and blood does not play a direct role in respiration. The tracheal system is a network of small tubes made of chitin that carry oxygen to the entire body. Insects also have openings called spiracles on their body on the thorax and abdomen that connect to the tubular network to allow oxygen to come into the body and release carbon dioxide.
In mammals, respiration occurs by inhaling or breathing air into the body. Air enters a nasal cavity and is warmed to body temperature and humidity in order to achieve equilibrium. Mucus keeps the respiratory tract from having direct contact with air and the mucus contains much water, which helps protect the body from cold, dry air and achieve equilibrium. Cilia in the naval passages and mucus also help remove harmful particulate matter in air as it passes through.
Air moves through the nasal cavity through the pharynx or throat, and larynx or voice box, on the way to the trachea. The trachea help move air into the lungs and help move exhaled air out of the body. The cylindrical trachea is made of cartilage and smooth muscle, and the trachea is lined with cells that produce mucus. The trachea is also lined with ciliated epithelia tissues. The cilia help remove foreign particles and the smooth muscle can contract or relax.
The trachea fork downward into the left and right lungs. The right lung is larger with three lobes and the left lung is smaller with two lobes. The muscular diaphragm is below the two lungs and facilitates breathing.
Inside the lungs, air moves into smaller passages called bronchi. Air enters the lung through the two main or primary bronchi or bronchus for singular. Each bronchus branches into smaller and smaller bronchi or bronchioles throughout the lung. The bronchi are also made of cartilage and smooth muscle. The bronchioles have elastic fibers instead of cartilage. In the bronchi, nerves control their contraction or relaxation. Smooth muscle is more abundant in the smaller bronchi.
The terminal bronchioles are divided into microscopic branches called respiratory bronchioles. These respiratory bronchioles are further subdivided into alveolar ducts. Surrounding these alveolar ducts are alveoli and spherical alveolar sacs. Gas exchange occurs only in the alveoli, which connect directly with capillaries of the circulatory system so that oxygen can be transferred throughout the body. Carbon dioxide also diffuses from the blood through the alveoli to be exhaled as a waste product. There are very many alveoli in each alveoli sac and very many alveoli sacs at the end of the alveolar duct. A lung can contain 300 million alveoli. Therefore, the lungs act as a sponge for gas exchange and a large surface area is available.
The respiratory system has protective mechanisms to prevent particulate matter, including dust, dirt, viral particles, and bacteria from causing damage to the lungs or cause allergies. In the nasal cavity, hairs and mucus capture small particles, viruses, dust, dirt, and bacteria, so that these are prevented from entering the lungs.
Sometimes particulate matter finds its way past the nose or enters the mouth and enters the lungs. However, the lungs produce mucus, which contains a glycoprotein, salts, and water that can trap the particulates. The walls of the bronchi and bronchioles also contain cilia, which are hair like projections that move in a unified manner to push mucus and particulates back up out of the bronchioles and toward the throat, where the harmful material can be swallowed to the stomach.
The 300 million alveoli in the lungs allow for maximum surface area to maximize gas diffusion. Gas exchange during respiration occurs mainly through diffusion. The diffusion process is caused by a concentration gradient, as gas molecules move from high pressure to low pressure location. So gas is exchanged from air in the lungs with the gas in the blood and oxygen levels are replenished in the blood.
Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture of gases is a sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total total gas mixture.
The lung capacity of each animal is different based on its size and amount of physical activity. Human lung capacity or volume is based on genetics, sex, and size. An adult lung can hold up to six liters of air.
The volume in the lungs can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume.
Tidal volume measures the amount of air that is inspired or expired during a normal breath, which is commonly about one half liter.
Expiratory reserve volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation.
Inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation.
The residual volume is the amount of air that is left after expiratory reserve volume is exhaled. There is always some residual air to help the lungs them reinflate.
Capacities are measurements of two or more volumes.
The vital capacity (VC) measures the maximum amount of air that can be inhaled or exhaled during a respiratory cycle and is the sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume.
The inspiratory capacity (IC) is the amount of air that can be inhaled after the end of the normal expiration and is the sum of the tidal volume and inspiratory reserve volume.
The functional residual capacity (FRC) is the volume of air remaining after a normal exhalation (sum of ERV and RV), which includes the expiratory reserve volume and residual volume.
The total lung capacity is a measure of the total amount of air that the lungs can hold and combines the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.
Lung volumes are measured by a technique called spirometry. The forced expiratory volume (FEV) measures how much air can be forced out of the lung over a specific period of time, such as one second. The forced vital capacity (FVC), which is the total amount of air that can be forcibly exhaled, is measured.
The ratio of (FEV)/(FVC) is used to diagnose lung diseases including asthma, emphysema, and fibrosis. A high ratio would show lung fibrosis. A low ratio would show asthma.
As air is a mixture of gases, the partial pressure of any gas in air can be calculated by multiplying the atmospheric pressure by the percent content in the gas mixture.
The atmospheric pressure is the sum of all of the partial pressures of atmospheric gasses added together.
In the gas exchange in the respiratory system from oxygen to carbon dioxide, the ratio of carbon dioxide production to oxygen consumption is the respiratory quotient (RQ).
The RQ is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung, the alveolar PO2.
Oxygen and carbon dioxide move independently of each other and they each diffuse down their own pressure gradients.
The change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.
In mammals, lungs are located in the thoracic cavity and are surrounded and protected by the rib cage, muscles, and chest wall. Below the lungs is the diaphragm, a muscle that facilitates breathing. Therefore, breathing requires the coordination of all of these parts.
Amphibians have gills that enable breathing when they are young and older amphibians grow lungs. Amphibians also breathe by diffusion across the skin.
Birds have lungs like mammals that enable breathing. In addition, birds have air sacs in their body that help with gas exchange, as birds use a large amount of energy from oxygen for flying and also at high altitudes. Air flows in and out of the air sacs, and opposite the direction of blood flow, which allows birds to obtain the amount of oxygen they need for flying at high altitudes.
According to Boyles Law, in a closed space, pressure and volume are inversely related. As one decreases, the other increases. This relationship helps explain breathing in animals and humans.
Intercostal muscles attached to the rib cage help increase the volume of the lungs by contracting. Lung volume expands because the diaphragm contracts and the intercostal muscles contract, so that the thoracic cavity expands.
The chest wall expands out and away from the lungs, which are elastic and help the lungs deflate after inflating during breathing by elastic recoil. The diaphragm also moves up and helps the lungs release air outward by relaxing and increasing the pressure toward the thoracic cavity.
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax. The space between these layers is the intrapleural space and contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed.
The respiratory rate is measured by the number of breaths per minute. This rate contributes to the alveolar ventilation, which is how much air moves in and out of the alveoli. This alveolar ventilation prevents carbon dioxide buildup in the alveoli. Alveolar ventilation is kept constant by increasing the respiratory rate while decreasing tidal volume of air per breath or decreasing the respiratory rate while increasing the the tidal volume per breath.
There are two types of work done performed by respiration. Flow-resistive work refers to the work of the alveoli and tissues in the lung. Elastic work refers to the work of the intercostal muscles, chest wall, and diaphragm. Increasing the respiration rate increases the flow-resistive work of the airways and decreases the elastic work of the muscles. Decreasing the respiration rate reverses the type of work required.
Surfactant is a complex mixture of phospholipids and lipoproteins that work to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli, preventing the alveoli from collapsing.
Diseases of the lungs reduce the rate of gas exchange in and out of the lungs. Two main causes of reduced rate of gas exchange are compliance, or how elastic the lung is, and resistance, or how much obstruction exists in the airways.
Restrictive diseases where the airways are stiff or fibrotic are respiratory distress syndrome and pulmonary fibrosis, where patients have trouble exhaling air.
Obstructive diseases like emphysema, asthma, and pulmonary edema. Causes include smoking tobacco products, where the walls of the alveoli are destroyed, and surface area for gas exchange is reduced. Asthma is inflammation of the airways caused by environmental factors. The obstruction may be cause by edema, or fluid accumulation, smooth muscle spasms in the bronchioles, increased muscle secretion, or damage to the epithelia of the airways.
Systemic circulation delivers oxygenated blood from the heart to the rest of the body, while pulmonary circulation carries deoxygenated blood from the heart to the lungs for oxygenation and then back to the heart. Pulmonary circulation pressure is low compared to systemic circulation, because of recruitment, a process of opening airways that normally remain closed when cardiac output increases (like during exercise).
As cardiac output increases, the number of capillaries and arteries filled with blood increases. Sometimes there is a mismatch between the amount of air ventilation and the amount of blood perfusion in the lungs, called the ventilation/perfusion (V/Q) mismatch.
This mismatch produces dead space, or regions of broken down or blocked lung tissue. These can impact breathing by reducing the surface area for gas diffusion. Oxygen in the blood is reduced and carbon dioxide level is increased. Dead space is created when no ventilation or perfusion takes place. Dead space can be caused by anatomical failure such as from gravity effects on the lungs or functional impairment of the lungs and arteries such as from an infection or obstruction.
The lungs can adjust for these mismatches in ventilation and perfusion as blood vessels dilate and bronchioles constrict (if ventilation is greater than perfusion), or vise versa. Blood vessels constrict and bronchioles dilate when ventilation is less than perfusion.
Hemoglobin is a protein molecule found in red blood cells (erythrocytes) that binds to and carries most oxygen (98.5 percent) to the tissues. The rest of the oxygen dissolves into the blood itself.
The hemoglobin (Hb) molecule is composed of two alpha subunits and two beta subunits. Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, and allows each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme group are more bright red in color, while those with less oxygen are darker red in color.
The second and third oxygen molecule are easier to bind to the hemoglobin molecule than the first. The fourth oxygen molecule is more difficult to bind. The binding of oxygen molecules to hemoglobin is a function of partial pressure of oxygen in the blood versus the relative hemoglobin-oxygen saturation. An oxygen dissociation curve can be plotted. As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.
The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. Partial pressure of oxygen dissolved in blood (PO2) and other environmental factors and diseases affect oxygen carrying capacity.
Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity. Carbon dioxide makes the blood more acidic and reduces hemoglobin's affinity for oxygen. Increased temperature also reduces hemoglobin affinity for oxygen. Diseases like sickle-cell anemia and thalassemia decrease the body's ability to deliver oxygen to tissues and its oxygen-carrying capacity.
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Properties of carbon dioxide affect its transport in blood. Carbon dioxide is more soluble in blood than oxygen. Carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin (carbaminohemoglobin). Carbon dioxide can also be unbound from hemoglobin when it reaches the lungs and removed from the body.
Most carbon dioxide molecules are carried in the blood as part of the bicarbonate buffer system. Carbon dioxide diffuses into red blood cells. Carbonic anhydrase (CA) in the red blood cells converts carbon dioxide into carbonic acid H2CO3, which is unstable and and dissociates into bicarbonate ions, HCO3- and H+ ions.
A chloride shift occurs when the bicarbonate ion is transported out of the red blood cell into the blood in exchange for a chloride (Cl-) ion. In the lungs, the bicarbonate ion is transferred back into the red blood cell in exchange for the chloride ion. The H+ ion disassociates from the hemoglobin and binds to the bicarbonate ion. This produces carbonic acid intermediate, which is converted back into carbon dioxide by CA.
The bicarbonate buffer system prevents the carbon dioxide from changing the pH of the system. So pH is regulated in the blood and preserves equilibrium in the system. Excess acids and bases are buffered to preserve pH and equilibrium. Increased breathing rate can also remove excess carbon dioxide, while reduced breathing rates can retain carbon dioxide to keep pH levels balanced.
Carbon monoxide has a stronger affinity for hemoglobin that oxygen, and therefore carbon monoxide prevents oxygen from binding to hemoglobin. Exposure to carbon monoxide can lead to sickness or death. Carbon monoxide gas is colorless, odorless, and difficult to detect.