Flashcards in BIOL #19: Respiration Deck (27):
Gas exchange, also known as respiration (not to be confused with cellular respiration), is the uptake of molecular O2 from the environment and the discharge of CO2 to the environment.
Gas exchange involves four steps: ventilation, gas exchange, circulation, and cellular respiration.
Ventilation occurs when air or water moves through a specialized gas-exchange organ, such as lungs or gills.
Gas exchange takes place as CO2 and O2 diffuse between air or water and the circulatory fluid at the respiratory surface.
Through circulation, the dissolved O2 and CO2 are transported throughout the body.
Gas exchange between circulatory fluid and cells occurs in tissues, where cellular respiration has led to low O2 levels and high CO2 levels; O2 and CO2 diffuse between circulatory fluid and cells.
The respiratory system is responsible for ventilation and gas exchange that brings O2 into the body and removes CO2.
- The respiratory system is comprised of the structures responsible for gas exchange between the individual and its environment.
- In some animals the gas-exchange surface is the skin, but in most species it is located in a specialized organ like the lungs of tetrapods, the tracheae of insects, or the gills found in mollusks, arthropods, and fish.
Air and Water as Respiratory Media
Gas exchange between the environment (water or air) and cells is based on diffusion.
Oxygen is high in the environment and low in tissues, while carbon dioxide is high in tissues and low in the environment.
Oxygen thus tends to move from the environment into tissues and carbon dioxide tends to move from tissues to the environment.
To understand how gases move by diffusion, it is important to express their presence in terms of partial pressures instead of percentages.
- Partial pressure is the pressure of a particular gas in a mixture of gases.
To calculate the partial pressure of a particular gas, multiply the fractional composition of that gas by the total pressure exerted by the entire mixture.
- Atmospheric pressure at sea level is 760 mm Hg and O2 makes up 21% of the atmospheric gases:
Thus, PO2 = 0.21*760 = 160 mm Hg
Oxygen and carbon dioxide diffuse between the environment and cells along their respective partial-pressure gradients.
- In both air and water, oxygen and carbon dioxide move from regions of high partial pressure to regions of low partial pressure.
Coordination of Circulation and Gas Exchange
The partial pressures of O2 and CO2 in the blood vary at different points in the circulatory system.
- Blood flowing into the lung (alveolar) capillaries (2) from the body has a lower Po2 and higher Pco2 than the air in the lungs, thus O2 diffuses into the blood and CO2 diffuses out of the blood.
Oxygen and carbon dioxide move from regions of high partial pressure to regions of low partial pressure.
How Do O2 and CO2 Behave in Water?
In both air and water, oxygen and carbon dioxide move from regions of high partial pressure to regions of low partial pressure.
To obtain oxygen, water breathers face a much more challenging environment than air breathers do.
Water contains much less oxygen than air does, thus, to extract a given amount of oxygen, an aquatic animal has to process 30-40 times more water than the amount of air a terrestrial animal breathes.
Water is about a thousand times denser than air and flows less easily, so water breathers (e.g. fish and lobsters) have to expend more energy to ventilate their respiratory surfaces than do air breathers.
Aquatic organisms have evolved adaptations that enable efficient gas exchange – many of which involve the organization of respiratory surfaces to allow sufficient ventilation.
Characteristics of Respiratory Surfaces
Like all living cells, the cells that carry out gas exchange have a plasma membrane that must be in contact with an aqueous solution, thus respiratory surfaces must always be moist.
The movement of O2 and CO2 across moist respiratory surfaces takes place entirely by diffusion – the rate of diffusion is proportional to the surface area across which it occurs and inversely proportional to the square of the distance through which molecules must move
- In other words, gas exchange is fast when the area for diffusion is large and the path for diffusion is short!
- Many respiratory surfaces reflect adaptations that increase the rate of gas exchange, e.g. long thin filaments, folded, and branching structures.
Gas Exchange in Animals
Relatively simple animals, such as sponges, cnidarians, and flatworms, have every cell in close contact with the external environment, allowing for efficient gas exchange to occur
In many animals, however, the bulk of the body’s cells lack immediate access to the environment, thus the respiratory surface in these animals is a thin, moist epithelium that constitutes a respiratory organ – we will discuss three such organs: gills, tracheae, and lungs.
- Some more complex organisms breath through their skin (e.g. earthworms and some amphibians) – such organisms have a dense network of capillaries just below the skin to facilitate gas exchange and must remain in damp places to keep the skin moist.
Gills In Aquatic Animals
Gills are outfoldings of the body surface that are suspended in water and used for gas exchange in aquatic animals.
Gills are efficient solutions to the problems posed by water breathing, because they present an extremely large surface area for oxygen to diffuse across an extremely thin epithelium.
Among invertebrates, the structure of gills is extremely diverse; gills can be external or internal.
Ventilation of Gills
Movement of the respiratory medium over the respiratory surface is called ventilation
- This process maintains the partial pressure gradients of O2 and CO2 across the gill that are necessary for gas exchange
Different methods of moving water over gills:
- Crayfish and lobsters use paddle-like appendages to ventilate their gills
- Mussels and clams move water over gills with cilia
- Octopuses and squid ventilate their gills by drawing in and ejecting water
- Fish using the motion of swimming plus mouth and gill movements to ventilate their gills
Fish Gill Structure
Movement of water over gills is unidirectional.
Long, thin structures called gill filaments extend from each gill arch.
Each gill filament is composed of hundreds or thousands of gill lamellae—sheet-like structures through which a capillary bed runs.
How Do Fish Gills Work?
The fish gill is a countercurrent system --the flow of blood through the capillaries is in the opposite direction to the flow of water over the gill surface, which sets up a countercurrent exchange system in each lamella.
Countercurrent flow makes fish gills extremely efficient in extracting oxygen from water, because it ensures that the difference in the partial pressure of O2 and CO2 in water versus blood is large over the entire gas-exchange surface.
Tracheal Systems In Insects
The most common respiratory structure among terrestrial animals is the tracheal system – one variation on the theme of an internal respiratory surface
The tracheal system is made up of air tubes that branch throughout the body
- The largest tubes, called trachaea, open to the outside and the finest branches (tracheoles) extend close to the surface of nearly every cell, where gas is exchanged by diffusion across the moist epithelium that lines the tips of the tracheoles
How Do Insect Tracheae Work?
Ventilation (breathing movements) may play a role in gas exchange in some insects with high energy demands (e.g. an insect in flight).
- In many winged species, tracheae alternately open and close as the wing muscles around them contract and relax. As a result, the trachaea take in air when muscles relax and they expel air when the muscle contract.
Lungs are localized respiratory organs, unlike the tracheal system, which branches throughout in insect body.
The lungs are infoldings of the body surface that are typically subdivided into numerous pockets.
Because the surface of the lung is not in direct contact with all other parts of the body, the gap in gas exchange must be bridged by the circulatory system.
Lungs have evolved in many different types of organisms, including spiders, land snails, and many vertebrates.
Mammalian Lung Structure
Air inhaled via the nose travels through the nasal cavity to the pharynx, where a tube known as the trachea carries inhaled air to narrower tubes called bronchi. The bronchi branch off into even narrower tubes called bronchioles.
The lungs, which enclose the bronchioles and part of the bronchi, are infoldings of the throat that function in gas exchange.
In frogs and other amphibians, the lung is a simple sac lined with blood vessels.
Mammalian lungs are divided into tiny sacs called alveoli (singular = alveolus), which greatly increase the surface area for gas exchange.
- Humans have approximately 150 million alveoli per lung with a surface area of 50x that of the skin
O2 in the air entering the alveoli dissolves in the moist aqueous film lining the inner surfaces and rapidly diffuses across the epithelium into a web of capillaries surrounding each alveolus
Net diffusion of CO2 occurs in the opposite direction – from the capillaries across the epithelium of the alveolus into the air space.
Lung Ventilation Varies Among Species
In the simple lungs of snails and spiders, air movement takes place by diffusion only.
Vertebrates actively ventilate their lungs by pumping air via muscular contractions.
- One mechanism for pumping air is positive pressure ventilation, in which air is pushed into the lungs. Frogs use this mechanism to breathe.
- Humans and other mammals pull air into their lungs via negative pressure ventilation
Ventilation of the Human Lung
The air inside the human lung is under negative pressure.
Humans inhale by contracting a thin muscular sheet called the diaphragm. This produces a downward motion which draws air into the lungs.
At the same time rib muscles contract which causes the rib cage to expand
As the pressure surrounding the lungs drops, air flows into the airways along a pressure gradient (from high pressure outside the body to low pressure inside the lungs).
Exhalation, in contrast, is a passive process driven by the elastic recoil of the lungs and chest wall as the diaphragm and rib muscles relax.
Upon muscle relaxation, the increased pressure in the alveoli forces air up the breathing tubes and out of the body.
The volume of air inhaled and exhaled with each breath is called tidal volume
- The average in a resting human is ~500 mL
Vital capacity refers to the volume during maximum inhalation
- The average for college-age women and men is 3.4 L and 4.8 L, respectively
Residual volume is the air that remains after a forced exhalation.
Because the lungs of mammals do not completely empty with each breath, and because inhalation occurs through the same airways as exhalation, fresh air consistently mixes with oxygen depleted air – thus, the maximum Po2 in alveoli is always considerably less than in the atmosphere, always favoring diffusion of O2 into the blood vessels of the lungs
Homeostatic Control of Ventilation
Although you can voluntarily hold your breath or breathe faster and deeper, most of the time your breathing is regulated by involuntary mechanisms.
The neurons mainly responsible for regulating breathing are in the medulla oblongata, near the base of the brain
- These neural circuits form a breathing control center that establishes the breathing rhythm
In regulating breathing, the medulla uses the pH of the surrounding tissue fluid as an indicator of blood CO2 concentration
The reason pH can be used in this way is that blood CO2 is the main determinant of the pH of cerebrospinal fluid (CSF), the fluid surrounding the brain and spinal cord (which includes the medulla oblongata)
Increased CO2 reacts with water in the blood and cerebrospinal fluid (CSF) to form carbonic acid, H2CO3, which quickly dissociates into a hydrogen ion, H+, and a bicarbonate ion, HCO3:
CO2 + H2O H2CO3 H+ + HCO3
The release of hydrogen ions (H+) lowers the blood and CSF pH, which is sensed by specialized neurons – this leads to the medullary respiratory center increasing the breathing rate, which removes the excess CO2 from the body and returns the CSF pH to a normal value
Blood is a connective tissue that consists of cells in a liquid extracellular matrix called plasma.
The remainder of the blood is made up of formed elements: platelets, red blood cells, and several types of white blood cells.
Platelets are cell fragments that minimize blood loss from ruptured blood vessels by releasing material that assists in the formation of clots.
White Blood Cells
White blood cells, which are part of the immune system, fight infections.
Red Blood Cells
Red blood cells (RBCs) transport oxygen from the lungs to body tissues, and participate in transporting carbon dioxide from tissues to lungs. In humans, red blood cells make up 99.9 % of the formed elements in blood.
Red blood cells contain an oxygen-carrying molecule called hemoglobin.
Hemoglobin consists of four polypeptide chains, each of which binds to a nonprotein group called a heme.
Each heme contains an iron ion (Fe2+) that can bind to an oxygen molecule. Each hemoglobin molecule can thus bind up to four oxygen molecules.
- The iron in hemoglobin produces the characteristic red color.
Each RBC contains ~250 million molecules of hemoglobin, thus can transport up to a billion O2 molecules.
In blood, 98.5% of the oxygen is bound to hemoglobin. The other 1.5 % is dissolved in plasma.
Hemoglobin binds O2 reversibly, loading in the lungs or gills and unloading it in other parts of the body
This process depends on the cooperation between hemoglobin subunits, referred to as cooperative binding
- When O2 binds to one subunit, the other subunits change shape slightly, increasing their affinity for O2
- When four O2 molecules are bound and one subunit unloads its O2, the other three subunits more readily unload O2, because an associated shape change lowers their affinity for O2
Blood leaving human lungs and entering the body has a Po2 greater than that of muscles and other tissues. This difference creates a diffusion gradient that unloads O2 from hemoglobin to the tissues.
- Oxygen and carbon dioxide move from regions of high partial pressure to regions of low partial pressure
Cooperative binding makes hemoglobin very sensitive to changes in the Po2 of tissues -- in response to a relatively small change in tissue Po2, there is a relatively large change in the percentage saturation of hemoglobin.
- The oxygen dissociation curve plots the percentage saturation of hemoglobin in RBCs versus the Po2 in blood within tissues.
The Affect of CO2 On Hemoglobin Binding
The production of CO2 during cellular respiration promotes the unloading of O2 by hemoglobin
CO2 decreases pH when it accumulates in tissues
- CO2 reacts with water, forming carbonic acid, which lowers pH
Low pH decreases the affinity of hemoglobin for O2, an effect called the Bohr shift.
Thus, where CO2 production is greater, hemoglobin releases more O2, which can then be used for cellular respiration.