Shortened Summary Module B
HAP-20306
Respiration
Respiration (gas exchange) happens at 2 levels: internal respiration (oxygen use in mitochondria) and
external respiration (gas exchange in lungs). Mechanisms of external respiration are largely
determined by the environment: water contains far less oxygen, so aquatic animals must work
efficiently, land animals must keep respiratory surfaces wet to allow diffusion of gases. Aquatic animals
have developed evaginations, land animals have developed invaginations (like lungs).
Small organisms can afford direct diffusion of gases, cutaneous respiration. Insects and terrestrial
arthropods have a branching system of tracheae, where the smallest tracheoles are filled with fluid.
Air enters and exits this system via spiracles (can close to prevent water loss). The system reaches all
body parts, so does not depend on circulatory system. Figure 4
Aquatic animals developed gills, branchia. These can be simple external extensions, like dermal
papulae or branchial tufts. They can be parapodia, with blood vessels. Most efficient are internal gills,
like those of fishes. They contain thin filaments and epidermal folds, lamellae. They are covered with
a movable bony flap, operculum, to protect, streamline, and to form a pumping system (breathing
occurs pulsatile). There are lots of blood vessels arranged in such a way that counter-current flow is
possible (maximum oxygen extraction). Ram ventilation is forcing water into the mouth and through
the gills by rapid swimming.
On land, gills would collapse and become dry, so internal cavities are necessary. Figure 2 The most
rudimentary lungs of vertebrates are found in lungfishes. Amphibians have varying lung shapes. They
(and lungfishes) create a positive pressure to force air in the lungs by drawing air into the mouth, close
it, and raise the buccal cavity to push air in the lungs. Reptiles have lobular lungs and can show apnea,
stop breathing for a while. They can, because they are ectothermic and don’t possess a secondary plate
for breathing while eating. Mammals have the most elaborate lungs, but gas exchange can only take
place in the alveoli and alveolar ducts, so it is very inefficient. Birds have a more efficient way of
breathing. ¼ of the incoming air flows through the parabronchi for gas exchange, the other ¾ air flows
directly to the air sacs. When the bird exhales, oxygenated air still passes the parabronchi. So there is
a continuous flow of oxygenated air.
Mammals, birds and reptiles use a negative pressure instead of a positive pressure to draw air into the
lungs (aspiratieve ademhaling).
Respiratory system; humans
External respiration in humans (and other animals) encompasses pulmonary respiration (movement
of air by bulk flow), gas exchange between air and blood, gas transportation and gas exchange between
blood and tissues.
The right lung has 3 lobes, the left lung has 2 lobes. Figure 3 The upper airways (passages in head and
neck leading to pharynx) lead to the respiratory tract. This is separated into conducting zone, also
called the dead space (glottis, larynx, trachea, bronchi, secondary bronchi, tertiary bronchi,
bronchioles, terminal bronchioles) and the respiratory zone (respiratory bronchioles and alveoli in
alveolar sacs). The function of the conducting zone is conduct, humidify, warm, and purify the air. The
,function of the respiratory zone is gas exchange. The wall of the conducting zone starts off with lots of
goblet cells (mucus secretion) and cilia in larynx and trachea. Figure 1 The smaller the ducts get, the
less cartilage rings are present, and the more smooth muscle (cartilage rings are present until
bronchioles). The wall of the alveoli contains type I alveolar cells (also pneumocytes, septal cells), a
basement membrane, type II alveolar cells (secrete pulmonary surfactant, lecithin), and alveolar
macrophages. The capillary and alveolar walls together form the respiratory membrane. Figure 5 & 6
Breathing is done by the internal intercostal and external intercostal muscles, and the diaphragm.
The interior surface of the chest wall is lined with the parietal pleura, each lung is lined with the
visceral pleura (pleural sac); the pace in between the pleura is the intrapleural space, filled with fluid.
Figure 7
BULK FLOW
Pulmonary ventilation is movement by bulk flow and driven by 4 forces: Figure 8
- Atmospheric pressure (Patm); varies at height, all other pressures are relative to this pressure.
- Intra-alveolar pressure (Palv); equal to Patm at rest, difference Patm and Palv is the actual gradient.
- Intrapleural pressure (Pip); pressure in intrapleural space, always negative value.
- Transpulmonary pressure; difference Palv – Pip, represents distending force: increase in
pressure leads to larger distending pressure and more expansion of lungs.
Boyle’s law says that if volume increases, pressure falls. Flow of air can then be described as
Flow = (Patm – Palv) / R (resistance).
COMPLIANCE & RESISTANCE
Compliance and resistance affect pulmonary ventilation. Lung compliance = (ΔV) / Δ(Palv – Pip). So,
higher compliance means a smaller change in transpulmonary pressure leads to a greater change in
volume (less muscle contraction needed). It depends on elasticity of the lungs and the surface tension
of fluid in alveoli: high surface tension decreases compliance (more work needed to spread fluid).
Pulmonary surfactant interferes with hydrogen bonding, so decreases surface tension and increases
compliance. Surfactant also prevents the alveoli from collapsing.
Resistance is analogous to the total peripheral resistance of the cardiovascular system: the radius of
the ducts has the greatest effect. As radius increases, resistance increases (branching of the respiratory
system leads to higher total radius, thus decreased overall resistance). Radius is affected by passive
forces (decrease resistance during inspiration because of outward distending and increase resistance
during expiration because of inward recoiling), mucus secretion, and smooth muscles
(bronchoconstriction and bronchodilation, just like with cardiovascular system). Sympathetic nerves
and epinephrine cause bronchodilation, parasympathetic nerves cause bronchoconstriction.
Histamine causes contraction (allergic reaction), local high CO2 concentration causes dilation.
MEASURING RESPIRATION
With a spirometer, you can measure inspired and expired air: lung volumes. Figure 9
- Tidal volume (Vt); air flow during a single breath.
- Inspiratory reserve volume (IRV); maximum inspired air (from end of normal inspiration).
- Expiratory reserve volume (ERV); maximum expired air (from end of normal expiration).
- Residual volume (RV); remaining air in lungs after maximal expiration (cannot be measured).
- Inspiratory capacity (IC); Vt + IRV.
- Vital capacity (VC); Vt + IRV + ERV.
- Total lung capacity (TLC); Vt + IRV + ERV + RV.
, - Forced expiratory volume (FEV); maximum expired air in 1 second.
The minute ventilation (Ve) is the total amount of air inspired/expired in a minute (Vt x RR) (RR =
respiration rate). Alveolar ventilation (Va) is the amount of fresh air reaching alveoli every minute: Va
= (Vt x RR) – (dead space volume x RR).
With cystic fibrosis, there is less fluid under the mucus, causing growth delay, airway infections etc.
Other diseases are obstructive, increased resistance, or restrictive, decreased compliance. A lower FVC
(forced vital capacity = vital capacity) indicates restrictive diseases, low FEV indicates obstructive
diseases. Peak expiratory flow rate (PEFR) is decreased by both diseases (maximum expiration rate).
Gas exchange and respiratory regulation
The respiratory quotient is the ratio of the amount CO2 produced and O2 consumed, which depends
on metabolism.
Diffusion of gas is influenced by some factors. First, each gas has a partial pressure: the individual gas
pressure proportion of the pressure of a gas mixture. It is influenced by the fractional concentration
(molarity) and the total pressure. Another factor is the solubility of gases: gases in liquids dissolve until
they reach an equilibrium of partial pressure. CO2 is 20
x more soluble in water than O2. When the partial
pressure in air and liquid are the same, that does not
mean that the concentrations are the same. Henry’s
law says: C = k x P, where C is molar concentration, P is
partial pressure, and k is a constant based on
temperature and the gas.
Gas diffusion is also described in Fick’s law: D = (k x A x Δp) / d;
D = amount of particles diffused in time, k = constant factor, A = surface area, Δp = difference in partial
O2 pressure, d = distance (thickness of respiratory membrane).
A gas diffuses down its partial pressure. The partial pressure of CO2 in systemic capillaries is the same
as when it left the lungs, the partial pressure in veins depends on metabolic activity and blood flow.
When the different veins return in the atrium, they therefore are mixed. Thus, blood in pulmonary
artery is called mixed venous blood.
PO2 and PCO2 in alveoli is affected by the partial pressures of inspired air (dependent on altitude), minute
alveolar ventilation, and consumption O2 and production CO2. When consumption and production
increase, alveolar ventilation increases: hyperpnea. When alveolar ventilation is insufficient:
hypoventilation, when alveolar ventilation is too much: hyperventilation. Figure 10
In an aquatic environment, O2 and CO2 can diffuse easily, CO2 is thus easy to eliminate. However, there
is a lower O2 concentration, water has a low viscosity and high density. So Fick’s law value D is 10 000x
smaller than in air.
In a terrestrial environment, O2 concentration is high, air has low density and viscosity, and energy
requirement for respiration is low. However, respiratory membranes must be kept moist, evaporation
must be avoided and CO2 is harder to eliminate, which risks low pH. But Fick’s law value D is very high.
HEMOGLOBIN
In some invertebrates, oxygen just dissolves in the body fluid. However, solubility of oxygen is very
low, so this can only happen in animals with low metabolic rate. Many invertebrates and somewhat all
