RESPIRATION
Respiration encompasses all of the mechanisms and concepts associated with GAS EXCHANGE. Animals extract OXYGEN from the surrounding water or air and exchange this with CARBON DIOXIDE produced during cellular metabolism.
At sea level, the atmospheric pressure of dry air is about 760 mm Hg (Torr). The total pressure of this gas mixture is the sum of the component partial pressures.
Dry air is about 20.95% O2, with the remainder being N2 plus small amounts of inert gases plus CO2 ( ~0.03%).
The partial pressure of O2 is thus 20.95% of the total pressure of dry air.
e.g. if total pressure = 760 torr, then
PO2 = 760 x 0.2095 = 159.2 torr
Water in equilibrium with air will have the same gas partial pressures, yet will contain much less gas per unit volume.
[see Figure] Water contains much less O2 per L than does air. Also, water is much denser and more viscous and the O2 content decreases with increasing temperature and salinity.
GAS EXCHANGE
Gas exchange involves three fundamental concepts:
BULK FLOW
DIFFUSION
CAPACITANCE
Bulk flow and diffusion describe how gases move from place to place. Capacitance describes the combination of a respiratory gas and its medium.
BULK FLOW:
This concerns mass transport of a respiratory gas in a gaseous (e.g. air) or a liquid (e.g. blood or water) medium.
V(dot) = delta P/R
Where V(dot) = flow rate (e.g. L/min), delta P = the pressure gradient for the medium, and R = resistance to flow
DIFFUSION
Diffusion describes the transport of respiratory gases across membranes, between media, and for short distances within media.
M(dot) = -D A (delta P)/t)
Where M(dot) = mass of respiratory gas diffused per unit time, D = the diffusion constant, A = the exchange surface area, delta P = the partial pressure gradient, and t = the thickness of the exchange surface.
CAPACITANCE
Capacitance describes the change in the concentration of a respiratory gas associated with a given change in the partial pressure of that gas in the respiratory medium.
Beta = delta C/delta P
Where Beta = capacitance, C = concentration (micromoles/L), and P = partial pressure (e.g. Torr).
[see Figure]
[Fig. 13-6]
[Fig. 13-2]
[Fig. 13-3]
[Fig. 13-4] The alveoli are primarily lined with flattened (Type I) epithelial cells. However, there are also thicker (Type II) cells, which produce SURFACTANT. This reduces the surface tension and facilitates inflation of the alveoli.
PULMONARY VENTILATION
There is an inverse relationship between volume and pressure (i.e. P decreases as V increases).
During INSPIRATION (an active process), the DIAPHRAGM moves down and the RIB CAGE moves up and out, thereby increasing the volume of (and decreasing P in) the thoracic cavity.
Expiration is normally passive, but can be active as well [see Figure]
[see Animation]
[Fig. 13-13 (part)] A thin layer of intrapleural fluid separates the lungs from the thoracic wall. As the thorax expands, INTRAPLEURAL PRESSURE decreases and therefore INTRAPULMONARY (alveolar) PRESSURE decreases and air is sucked into the lungs. Relaxation of the inspiratory muscles and elastic recoil of the lungs increases intrapleural pressure and leads to exhalation.
[Fig. 13-13] Intrapleural pressure remains negative (i.e. subatmospheric) even at the end of expiration. This prevents the elastic recoil from completely collapsing the lungs.
LUNG COMPLIANCE
Compliance is a measure of how easy it is to inflate the lungs. It is defined as the change in lung volume per unit change in transpulmonary pressure.
An increased elasticity of the lung tissue decreases compliance.
SURFACTANT increases compliance by decreasing the surface tension of the liquid film lining the alveoli.
[Similar to Fig. 13-19] Tidal volume can increase markedly over resting levels by utilizing the IRV and ERV. Vital capacity, however, remains unchanged. The residual volume is that amount of air that cannot be forcibly expelled.
ALVEOLAR VENTILATION
The volume of gas moved per minute = Respiratory minute volume = TV (ml/breath) x frequency (breaths/min) = ml/min
However, the really important factor is the amount of air actually ventilating the alveoli (i.e. the alveolar ventilation = V(dot)
A).V(dot)
A = frequency x (TV - dead space)The anatomic DEAD SPACE refers to that volume of air that does not participate in gas exchange.
Example:
Dead space = 150 ml
A: frequency = 10/min, TV = 600 ml
Minute volume = 10 x 600 = 6000 ml/min
V(dot)
A = 10 x (600 - 150) = 4500 ml/minB: frequency = 30/min, TV = 200 ml
Minute volume = 30 x 200 = 6000 ml/min
V(dot)
A = 30 x (200 - 150) = 1500 ml/min[Fig. 13-20]
The composition of the alveolar gas changes relatively little over time. In the example above, 300 ml of "fresh" air and 150 ml of "stale" dead space air are added during each inhalation. The remaining 150 ml of "fresh" air remains in the dead space and is exhaled unused.
[see Animation]
[see Figure]
AIR FLOW AND RESISTANCE
Just as for circulation, flow (F) is dependent on the pressure gradient and on airway resistance.
F = delta P/R
Total resistance is normally too low to significantly impede air flow.
Locally, air flow and blood flow tend to e matched to maximize gas exchange. This is accomplished by local dilation or constriction of bronchioles and/or arterioles in response to changing conditions.
[Fig. 13-21]
[Fig. 13-22] Gas Transport and Diffusion
[Fig. 13-27] Hemoglobin can combine reversibly with oxygen. Each Hb molecule can combine with 4 molecules of oxygen. Hb is saturated when all molecules are carrying their full quota of oxygen.
[Fig. 13-30] The curve shifts to the right in response to in creased temperature or acidity (= BOHR EFFECT).
2, 3 DPG reduces the affinity of Hb for oxygen.
[see Figure] Fetal Hb has a higher affinity for oxygen, which facilitates gas exchange with the maternal blood.
[Fig. 13-29] Most of the oxygen is transported in the erythrocytes bound to Hb.
CARBON DIOXIDE TRANSPORT
about 10% in physical solution
about 30% combined with Hb
about 60% as bicarbonate ions
The enzyme CARBONIC ANHYDRASE in the erythrocytes catalyzes the reversible reaction:
CO2 + H2O <–c.a.–> H2CO3 <–> HCO3- + H+
[Fig. 13-31] Carbon dioxide transport:
RESPIRATORY QUOTIENT
CO2 produced/O2 consumed
Typical values:
1.0 for carbohydrate
0.7 for lipid
0.8 for protein
[Fig. 13-40] Ventilation is dependent on control centers in the medulla oblongata and pons.
Blood gases are monitored by peripheral and central chemoreceptors. Changes in CO2 and associated H+ are the most important.
ACID - BASE BALANCE
Normal human blood pH is about 7.4
PH is largely determined by the bicarbonate to carbonic acid ratio (HCO3-/H2CO3), which is normally about 20.
Too low a ratio = ACIDOSIS
Too high a ratio = ALKALOSIS
The respiratory center helps to maintain balance by increasing (which lowers blood H2CO3) or decreasing (which elevates H2CO3) the elimination of CO2.
DIVING PHYSIOLOGY
Many air-breathing vertebrates have secondarily reinvaded the water. While underwater, most air-breathers are in an effectively ANOXIC situation. Some animals (e.g. sea otters) have large lungs, which they inflate prior to submergence. However, there are potential problems with this approach (increased BUOYANCY and, for deep divers, enhanced risk of DECOMPRESSION SICKNESS), and some diving animals actually exhale (i.e. empty their lungs) before a dive.
Diving animals have developed a variety of adaptations, which allow them to maximize their time underwater. These include (but are not limited to):
The aforementioned cardiovascular adjustments (bradycardia & peripheral vasoconstriction) represent the DIVING REFLEX.
This reflex is triggered by apnea and is augmented by facial immersion (especially in cold water). These collective responses slow the depletion of oxygen stores while continuing to supply oxygen to the most hypoxia-sensitive organs (e.g. the brain and the heart).
Though most highly developed in natural divers, such as turtles, ducks, seals, and whales, the diving reflex occurs even in non-divers (including humans) in response to apnea or forced submersion.
[see Figure]
DECOMPRESSION SICKNESS
Water pressure increases about 1 atmosphere for every 10 m depth. This high pressure forces large amounts of gas in the lungs into the blood. Given enough time, the blood will equilibrate with the prevailing gas pressures.
The problem gas is nitrogen, which makes up nearly 80% of inspired air. If the blood of a deep diver contains nitrogen at high pressure and if the animal returns to the surface (i.e. decompresses) too quickly, the nitrogen will bubble out of solution in the blood vessels and tissues, causing major problems (especially in joints and brain capillaries). This condition is known as decompression sickness (aka caisson disease and as the bends). Note: this can also occur upon too rapid ascent in a non-pressurized aircraft.
Decompression sickness can be avoided if the diver ascends slowly (or in stages).
How do animals avoid the bends?
Probably the two main things are:
Nitrogen is normally considered to be a physiologically inert gas. However, at very high pressures, nitrogen gas has effects similar to those of the anesthetic gas, nitrous oxide. This is known as NITROGEN NARCOSIS and is responsible, in human divers, for experiences such as "Rapture of the deep".
At extreme depth, the pressure itself creates neurological problems (HIGH PRESSURE NEUROLOGICAL SYNDROME). This is probably the ultimate barrier to human exploitation of the deep ocean.