Gas Exchange: Reviewing the Basics
Gas exchange is ultimately serving another process: energy conversion
Oxygen is necessary as the ultimate electron acceptor, driving the entire process.
Carbon dioxide is produced as waste, during the breakdown of the six-carbon glucose molecule.
Glucose
+
Oxygen
→
Carbon dioxide
+
Water
+
ATP
C6 H12 O6
+
6 O2
→
6 CO2
+
6 H2 O
+
ATP4-
Energy in one form
→
Energy in another form
This energy conversion process happens in every cell .
Location : cytoplasm and mitochondria.
Metabolic pathways :
Gas Exchange in Animals: Overview
A key challenge for most animals is to perform gas exchange.
Move the oxygen in the environment to every cell of the body.
Remove carbon dioxide from the cells and return it to the environment.
What is needed to do this:
Respiratory surface or system to help move the gases into or out of the organism.Circulatory system
How Gases Move Across Membranes
Diffusion is the movement of molecules from a high concentration to a lower concentration.
Caused by the kinetic energy of the molecules.
Two things facilitate this process for O2 and CO2 :
A moist surface
Plenty of surface area
Animals use a variety of strategies to increase the surface area of their gas exchange system.
Gills (for gas exchange in water)
Lungs and tracheal system (for gas exchange in air)
Fish Use Gills to Increase Surface Area
Oxygen gas is dissolved in water.
Gills increase the surface area so that gas exchange occurs more easily.
A countercurrent exchange system increases the efficiency of gas exchange
The direction of blood flow is opposite of the flow of the water across the gills.
This maintains a concentration gradient between the water and the blood.
Since diffusion is “driven” by having a concentration gradient, better exchange can occur.
Comparison
Concurrent flow: 50 % O2 exchange
Countercurrent flow: 80 % O2 exchange
Insects Use a Tracheal System
Insects have a system of internal tubes that branch repeatedly and eventually provide air to every cell in the body.
This is called the tracheal system.
The air enters through spiracles in the side of the insect.
The open circulatory system of insects is not used for gas exchange!
Birds Have One-way Lungs
Birds have a high metabolic rate and therefore need a constant supply of oxygen.
This is facilitated by “one-way” lungs (i.e. air is continually moving throughout the circulatory system).
Posterior and anterior air sacs allow this one-way movement of air to occur.
The Mammalian Lung
Basic anatomy:
Oral cavity
→ pharynx
→ larynx (vocal cords)
→ trachea (C-shaped cartilage)
→ split into two bronchi (singular: bronchus)
→ bronchioles
→ alveoli (singular: alveolus)
Alveoli
Increased surface area.
Surrounded by capillaries.
Ventilation
Inhalation : contraction of the diaphragm and rib muscles
Causes the thoracic cavity to expand.
This creates a negative pressure, causing the lungs to expand as well.
Air enters the lungs.
Exhalation : relaxation of both sets of muscles.
Causes the thoracic cavity to contract.
This creates a positive pressure, causing the lungs to contract.
Air leaves the lungs.
Atmospheric Pressure
Content of the atmosphere:
Pressure of the atmosphere:
Can be measured by the ability to raise mercury (Hg).
760 mmHg (101 325 Pa )
Partial pressure of oxygen: 21 % of 760 mmHg = 159.6 mmHg
Diffusion and Gases
Gas will always diffuse from a region of higher partial pressure to a region of lower partial pressure.
Therefore, the higher the partial pressure, the more oxygen you will transfer to your tissues.
Altitude and Oxygen Partial Pressure
At the top of Everest the partial pressure of oxygen is only 50 mmHg; that is why it is called the “death zone.”
Body tissues are “dying” because they are not receiving sufficient oxygen.
Help in Oxygen Transportation
Oxygen isn't very soluble in water, so there is a transportation problem.
This is solved by having red blood cells packed full of hemoglobin.
Hemoglobin is a quarternary protein with a heme group in the center of each of its four subunits.
The heme group has iron in the center.
Each iron atom can bind one oxygen molecule.
One hemoglobin molecule can carry four oxygen molecules
This is one of the reasons we need iron in our diet.
How Hemoglobin Helps
Cooperativity : a substrate that binds to one subunit of a protein causes the other subunits to become active and bind more easily to other substrates.Oxygen binds to one subunit of hemoglobin, allowing the other subunits to bind more easily to other oxygen molecules.
Control of Breathing
Breathing control centers are located in two places in the brain stem:
How it works:
Detectors are located in three places:
Medulla
Aorta
Carotid artery
The most important detector is in the medulla, and it monitors carbon dioxide levels in the blood.
This seems paradoxical, and confuses many students since they think it should measure the oxygen levels in the blood.
Since carbon dioxide is a byproduct of cellular respiration, it is just as good an indicator as is oxygen.
As carbon dioxide levels increase in the blood, the carbon dioxide combines with water to form carbonic acid.
This lowers the pH of the blood (because it is slightly more acidic than normal).
Response to a lower pH:
The breathing control center in the medulla sends a signal to the lung muscles to breathe faster and deeper.
This rids the body of carbon dioxide and increases the oxygen intake.
Aorta and carotid artery sensors
These sensors also detect a change in pH levels, and send that information to the breathing control centers at the pons and medulla (in the brain stem).
They also respond to really low oxygen levels (like when at high altitudes) to stimulate faster and deeper breathing.
This entire system is under homeostatic control.
