Blood gases
Respiration
the total process of delivering oxygen tothe cells and carrying away thebyproduct of metabolism, carbon dioxide.
includes gas exchange in the lungs,circulation of gases through the bloodstream, and transfer of gases at thecellular level.
Ventilation
the process of moving gases through therespiratory tract.
inspiration (breathing in) occurs when themuscles of the diaphragm and chest wallcontract. The contraction of these musclesincreases the volume of the chest cavity,lowering the pressure inside. As thepressure in the airways decrease, airrushes in as the chest volume increases.
Ventilation-contd
expiration (breathing out) is apassive process. As the musclesrelax, the elastic recoil of the lungsputs pressure on the gases inside.The pressure in the chest is nowhigher than outside pressure, so airrushes out. Expiration stops whenthe recoil of the lung and the“spring” of the ribs balance eachother.
Inspiration
during quiet breathing, the decrease inpressure in the chest starts about 2.5mm Hg (compared to the outsidepressure), decreasing to around 6 mmHg towards the end of inspiration. Strongbreathing efforts can produce a pressuredecrease (vacuum) in the chest as highas 30 mm Hg.
Inspiration-contd
the muscles of the chest wall, includingthe diaphragm, contract to expand thevolume of the chest. This “vacuum” istransmitted through the fluid-filled pleuralspace, lowering the pressure in the airsacs of the lung. This draws air in. As themuscles relax, the elastic recoil of thelung pushes air back out.
Passage of air into the lungs
Mouth or nose-moisten and purify
Oropharynx
Larynx-location of vocal cords
Trachea-humidify and clean air
Bronchi
Bronchioles
Respiratory bronchioles
Alveoli-the walls contain capillaries
Respiratory unit
Respiratory unit
The blood/gas barrier
Only a very thin membrane (about0.3 micrometers thick) separates theair from the red blood cells in thecapillaries.
Partial pressure of gases
dry air is composed of 20.98% O2, 0.04% CO2,78.06% nitrogen, and 0.92% other gases suchas argon and helium.
the amount of a gas present is expressed interms of “partial pressure.” This is the amountof total gas pressure due to the substancebeing measured.
at sea level the total atmospheric pressure is760 mm Hg. Of this O2 contributes: 0.21 x 760= 160 mm Hg; the partial pressure of oxygenat sea level in dry air is 160 mm Hg.
Partial pressure contd
if atmospheric pressure is lower, thepartial pressure of a gas will beproportionately decreased.
in Salt Lake City, the atmosphericpressure is 647 mm Hg. The partialpressure of oxygen in dry air in Salt Lakeis: 0.21 x 647 = 136 mm Hg.
Partial pressure-contd
the partial pressure of carbon dioxide in dry airat sea level is 0.04 x 760 = 0.3 mm Hg.However, in the lung carbon dioxide exits theblood to raise the carbon dioxide content ofthe air.
the partial pressure of carbon dioxide in thelung air sacs is around 40 mm Hg. Becausethis carbon dioxide gas must displace oxygenand nitrogen, the partial pressure of oxygen inthe lung air sac will be lower than in outsideair.
Gas exchange
occurs by diffusion
at the alveolar membrane, each gas diffuses inthe direction where the partial pressure of thatgas is less
oxygen diffuses towards the blood and istaken up by hemoglobin
carbon dioxide diffuses towards the alveolusand mixes with the air. No “active process” isinvolved. Oxygen simply diffuses through themembrane and plasma, and is taken up by thered blood cells.
Gas exchange-contd
as blood circulates through the body, anopposite change occurs in the capillariesof the systemic circulation.
oxygen diffuses from the area of higherpressure — the blood — into the area oflower pressure-the cells.
carbon dioxide diffuses from the cellsinto the blood.
Oxygen transport
hemoglobin is a molecule composed offour subunits.
each subunit is a protein chain attachedto a porphyrin ring containing one ironatom.
as each iron atom can bind one oxygen(O2) molecule, hemoglobin can carryone, two, three, or four oxygenmolecules.
Hemoglobin
Hemoglobin
Effect of acid on O2 transport
oxygen status is affected by acid-base status.
at a given oxygen pressure, oxygen saturation in theblood is lowered by increasing either carbon dioxide orhydrogen ion concentrations.
the presence of acid “shifts the curve to the left,”meaning that less oxygen can be bound at a givenPaO2.
this mechanism assists hemoglobin in unloadingoxygen in the capillaries, where acid concentration ishigher. Raising the pH, conversely, increases theoxygen binding, allowing more total oxygen to becarried — a change that occurs in the alveolus as acidis eliminated through CO2.
Oxygen-hemoglobin diss curve
Shifting of the O/Hg curve-Left
increase in pH,lowering H+concentration
decrease inpCO2
decrease intemperature
CO2 transport
transported through the blood stream byconversion to H2CO3, which dissociates to H+and HCO3-.
H+ binds to hemoglobin, and is transported tothe lungs.
hemoglobin is acting as a buffer for the acid,but also is acting as an effective “transportationvehicle” for ferrying carbon dioxide to thelungs.
hemoglobin and bicarbonate act as buffers forthe acid produced by metabolism, effectivelytransporting this acid to the lungs forelimination.
CO2 transport-contd
as carbon dioxide is formed in the cells(from aerobic metabolism) it diffuses intothe plasma of the capillary. As it entersthe red blood cells (which containcarbonic anhydrase) it’s quicklyconverted to H2CO3, which breaks downto H+ and HCO3-
about two-thirds of the HCO3- will diffuseout into the plasma (and is replaced bychloride in the red cell). Only smallamounts of carbon dioxide remaindissolved or attach to other compounds.
CO2 transport-contd
~50 ml of CO2 gas in each 100 ml ofarterial blood, almost exclusively asHCO3-.
as the blood goes through thecapillaries, it picks up about 5 ml ofadditional CO2. With this addition ofacidic CO2, the pH drops from 7.4 to7.36.