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Gas Exchange
6.4.1 – 6.4.5 & H.6.1 – H.6.7
Thursday, January 29, 2015
• Quiz
• Lecture:
•
6.4.1 – 6.4.5
• HW
•
TEST Moved to Tuesday, February 10th [Transportation and Gas Exchange – 100 pts.]
•
BioFlix: Part I and II
•
IP Physiology: Respiration
• Anatomy Review: Respiratory Structure
• Questions: 1-3; 9-12; 15-21; 28,30 & 32 [17 questions total]
• For some of the questions you will need to print the images and label accordingly. Print all material first, then begin watching and
answering the questions as you go along.
• This is for a grade!!!
6.4.1 Distinguish between ventilation, gas
exchange and cell respiration
• Respiration is the transport of oxygen to cells where energy production takes
place, and involves three key processes:
• Ventilation: The exchange of air between the lungs and the atmosphere; it is
achieved by the physical act of breathing
• Gas exchange: The exchange of oxygen and carbon dioxide in the alveoli and
the bloodstream; it occurs passively via diffusion
• Cell Respiration: The release of ATP from organic molecules; it is greatly
enhanced by the presence of oxygen (aerobic respiration)
IP Physiology: Anatomy of Respiration
• BioFlix –
• Human Respiratory System
• Transport of Respiratory Gases
6.4.4 Draw an label a diagram of the ventilation system,
including trachea, lungs, bronchi, bronchioles and
alveoli
6.4.2 Explain the need for a ventilation system
• Because gas exchange is a passive process, a
ventilation system is needed to maintain a
concentration gradient within the alveoli
• Oxygen is needed by cells to make ATP via aerobic
respiration, while carbon dioxide is a waste product
of this process and must be removed
• Therefore, oxygen must diffuse from the lungs into
the blood, while carbon dioxide must diffuse from the
blood into the lungs
• This requires a high concentration of oxygen - and a
low concentration of carbon dioxide - in the lungs
• A ventilation system maintains this concentration
gradient by continually cycling the air in the lungs
with the atmosphere
6.4.3 Describe the features of alveoli that adapt
them to gas exchange -TRIM
• Thin wall: Made of a single layer of
flattened cells so that diffusion distance is
small
• Rich capillary network: Alveoli are
covered by a dense network of capillaries
that help to maintain a concentration
gradient
• Increased SA:Vol ratio: High numbers of
spherically-shaped alveoli optimize surface
area for gas exchange (600 million alveoli =
80 m2)
• Moist: Some cells in the lining secrete fluid
to allow gases to dissolve and to prevent
alveoli from collapsing (through cohesion)
6.4.5 Explain the mechanism of ventilation of the lungs in terms of
volume and pressure changes caused by the internal and external
intercostal muscles, the diaphragm and abdominal muscles
• Breathing is the active movement
of respiratory muscles that
enable the passage of air to and
from the lung
• The mechanism of breathing is
described as negative pressure
breathing as it is driven by the
creation of a negative pressure
vacuum within the lungs,
according to Boyle's Law
(pressure is inversely
proportional to volume).
Inhalation (pg 6
• Diaphragm muscles contract and flatten
downwards
• External intercostal muscles contract, pulling
ribs upwards and outwards
• This increases the volume of the thoracic
cavity (and therefore lung volume)
• The pressure of air in the lungs is decreased
below atmospheric pressure
• Air flows into the lungs to equalize the
pressure
Expiration
• Diaphragm muscles relax and diaphragm curves
upwards
• Abdominal muscles contract, pushing diaphragm
upwards
• External intercostal muscles relax, allowing the
ribs to fall
• Internal intercostal muscles contract, pulling ribs
downwards
• This decreases the volume of the thoracic cavity
(and therefore lung volume)
• The pressure of air in the lungs is increased
above atmospheric pressure
• Air flows out of the lungs to equalize the pressure
H.6.1 Define partial pressure – Daltons Law (pg 2-4)
• Partial pressure is the pressure
exerted by a single type of gas when
it is found within a mixture of gases
• The partial pressure of a given gas
will depend on:
• The concentration of the gas in the
mixture (e.g. O2 levels may differ in
certain environments)
• The total pressure of the mixture (air
pressure decreases at higher
altitudes)
*the partial pressures of the gases
within the alveoli are not the same as
their atmospheric partial pressures
H.6.2 Explain the oxygen dissociation curves of
adult hemoglobin, fetal hemoglobin and myoglobin
• Transport of Respiratory Gases
• Hemoglobin is composed of four polypeptide
chains, each with an iron-containing heme
group capable of reversibly binding oxygen
• As each oxygen molecule binds, it alters the
conformation of hemoglobin, making it easier
for others to be loaded (cooperative binding)
• Conversely, as each oxygen molecule is
released, the change in hemoglobin makes it
easier for other molecules to be unloaded.
Step wise saturation of hemoglobin
Oxygen Dissociation Curves
• Oxygen dissociation curves show the
relationship between the partial pressure
of oxygen and the percentage saturation
of oxygen carrying molecules
• At low O2 levels (i.e. hypoxic tissues)
percentage saturation will be low, while at
high O2 levels (e.g. in alveoli) molecules
will be fully saturated
• Because binding potential increases with
each additional oxygen molecule,
hemoglobin displays a sigmoidal (Sshaped) dissociation curve
Adult Hemoglobin
• Dissociation curves displays a typical
sigmoidal shape (due to cooperative
binding)
• There is low saturation of oxygen when
partial pressure is low (corresponds to
environment of the tissue, when oxygen is
released)
• There is high saturation of oxygen when
partial pressure is high (corresponds to
environment of the alveoli, when oxygen is
taken up)
Fetal Hemoglobin
• The hemoglobin of the fetus has a slightly
different molecular composition to adult
hemoglobin
• Dissociation curve is to the left of the adult
hemoglobin curve (indicating a higher
affinity for oxygen)
• This is important as it means that oxygen
will move from adult hemoglobin to fetal
hemoglobin in the capillaries of the uterus
Myoglobin
• Myoglobin is an oxygen-binding molecule found in
muscles that is made of a single polypeptide with
only one heme group
• Dissociation curve is to the left of the hemoglobin
curve and does not display a sigmoidal shape
(myoglobin cannot undergo cooperative binding)
• Myoglobin's affinity for oxygen is greater than
hemoglobin and becomes saturated at low oxygen
concentrations.
• Under normal conditions (at rest) myoglobin is
saturated with oxygen and will store it until O2
levels in the body drop with intense exercise
• This allows it to provide oxygen when levels are
very low (e.g. in a respiring muscle) and so delays
anaerobic respiration and lactic acid formation
Myoglobin
Other factors that affect dissociation
• Temperature:
• Increase in temperature increases the
unloading of oxygen at the tissues.
• pH
• DPG – 2,3-Diphosphoglycerate
• created in erythrocytes during glycolysis
• increase in DPG production increases the
efficiency of O2 unloading
H.6.3 Describe how carbon dioxide is carried by the blood,
including the action of carbonic anhydrase, the chloride shift
and buffering by plasma proteins
• Carbon dioxide is transported from the tissues to the lungs in one of three
ways:
• IP Physiology: pg 16 - 19
• Bohr effect vs. Haldane
• Some is bound to hemoglobin to form HbCO2
• A very small fraction gets dissolved in the blood plasma (CO2 dissolves poorly in
water)
• The majority (~85%) diffuses into the erythrocyte and is converted into carbonic
acid
Transport as Carbonic Acid
• When CO2 enters an erythrocyte, it combines with water in a reaction catalyzed by carbonic
anhydrase to form carbonic acid (H2CO3)
• The carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate (HCO3–)
• Bicarbonate is pumped out of the cell and exchanged with chloride ions to ensure the
erythrocyte remains uncharged – this is called the chloride shift
• The bicarbonate combines with sodium ions in the blood plasma to form sodium bicarbonate
(NaHCO3), which travels to the lungs
• The hydrogen ions in the erythrocyte make the environment less alkaline, causing the
hemoglobin to release its oxygen to be used by cells
• The hemoglobin absorbs the H+ ions and acts as a buffer to restore pH, the H+ ions will be
released in the lungs to reform CO2 for expiration
H.6.4 Explain the role of the Bohr shift in the
supply of oxygen to respiring tissues
• The oxyhemoglobin dissociation curve describes the saturation of hemoglobin by
oxygen in cells under normal metabolism
• Cells with increased metabolism (e.g. hypoxic tissue) release greater amounts of carbon
dioxide into the blood
• Carbon dioxide lowers the pH of the blood (via its conversion into carbonic acid) which
causes hemoglobin to release its oxygen
• This is known as the Bohr effect – a decrease in pH shifts the oxygen dissociation curve
to the right in tissues
• Hence more oxygen is released at the same partial pressure of oxygen, ensuring
respiring tissues have enough oxygen when their need is greatest
Bohr Effect
• Cells with increased metabolism (e.g. hypoxic
tissue) release greater amounts of carbon dioxide
into the blood
• Carbon dioxide lowers the pH of the blood (via its
conversion into carbonic acid) which causes
hemoglobin to release its oxygen
• This is known as the Bohr effect – a decrease in pH
shifts the oxygen dissociation curve to the right in
tissues
• Hence more oxygen is released at the same partial
pressure of oxygen, ensuring respiring tissues have
enough oxygen when their need is greatest
H.6.5 Explain how and why ventilation rate varies
with exercise
• During exercise metabolism is increased, oxygen is becoming limited and there is a
build up of both carbon dioxide and lactic acid in the blood
• This lowers the blood pH, which is detected by chemoreceptors in the carotid artery
and the aorta
• These chemoreceptors send impulses to the breathing center in the brain stem to
increase the rate of respiration
• Impulses are sent to the diaphragm and intercostal muscles to change the rate of
muscular contraction, hence changing the rate of breathing
• This process is under involuntary control (reflex response) – as breathing rate
increases, CO2 levels in the blood will drop, restoring blood pH
• Long term effects of continual exercise include an improved vital capacity
H.6.6 Outline the possible causes of asthma and
its effects on the gas exchange system
• Asthma is a common, chronic inflammation
of the airways to the lungs (i.e. bronchi /
bronchioles)
• Inflammation leads to swelling and mucus
production, resulting in reduced airflow and
bronchospasms
• During an acute asthma attack, constriction of
the bronchi smooth muscle may cause
significant airflow obstruction, which may be
life threatening
• Common asthma symptoms include
shortness of breath, chest tightness,
wheezing and coughing
Asthma may be caused by a number of variable
and recurring environmental triggers, including:
• Allergens (e.g. pollen, molds)
• Smoke and scented products (e.g. cigarettes, perfumes)
• Food preservatives and certain medications
• Arthropods (e.g. dust mites)
• Cold air
• Exercise (increased respiratory rate)
• Stress and anxiety
H.6.7 Explain the problem of gas exchange at high
altitudes and the way the body acclimatizes
• At high altitudes, air pressure is lower and hence there is a lower
partial pressure of oxygen (less O2 in the air)
• This makes it more difficult for hemoglobin to take up and transport
oxygen from the alveoli (lower Hb % saturation), meaning tissues
receive less O2
• A person unaccustomed to these conditions will display symptoms
of low oxygen intake – fatigue, breathlessness, rapid pulse, nausea
and headaches
• Over time, the body will begin to acclimatize to the lower oxygen
levels at high altitudes:
• Red blood cell production increases to increase oxygen transport
H.6.7 Cont.,
• Red blood cells will have a higher hemoglobin content with an increased affinity for
oxygen
• Ventilation rate increases to increase gas exchange (including a larger vital
capacity)
• Muscles produce more myoglobin and have increased vascularization (denser
capillary networks) to encourage oxygen to diffuse into muscles
• Kidneys will begin to secrete alkaline urine (improved buffering of blood pH) and
there is increased lactate clearance within the body
• People living permanently at high altitudes will have a greater lung surface area
and larger chest sizes