blood gases Flashcards
Acid:
a substance that can yield a hydrogen ion (H) or hydronium ion when dissolved in water
Base:
a substance that can yield hydroxyl ions (OH-)
Dissociation constant
(ionization constant K value): describes relative strengths of acids & bases
pK:
negative log of ionization constant & pH in which protonated & unprotonated forms are present in equal concentrations
Buffer:
combination of weak acid or weak base & its salt; a system that resists changes in pH
Acid–base balance
Maintenance of homeostasis of the hydrogen-ion concentration of body fluids
Defined by the degree of acidity or alkalinity of a body fluid
Determined by the pH or negative log of the hydrogen-ion concentration [H+] in moles/L
Acidity of a solution
Determined by the concentration of hydrogen ions (cH+)
An acid is a hydrogen donor.
Description of Acids & Bases
Carbonic acid (H2CO3) can donate one H+ through dissociation (Acid)
H2CO3 —– H +HCO3
As a base, bicarbonate (HCO3−), is an H+ or proton acceptor (Base)
H + HCO3 —– H2CO3
Equal numbers of H+ and OH− produce water, which is neutral—neither acidic nor alkaline.
H + OH —- H2O
Maintenance of H+
Reference values = 7.35 – 7.45
pH is controlled by systems that regulate retention of acid and bases using the lungs & kidney
Acidosis:
a pH level below reference range (<7.34)
Alkalosis:
a pH level above reference range (>7.44)
Buffer Systems
Buffer systems are body’s first line of defense against extreme changes in H concentration.
All buffers consist of a weak acid & its salt or conjugate base.
Bicarbonate-carbonic acid system has low buffering capacity, but is still important buffer for 3 reasons:
1. H2CO3 dissociates into CO2 and H2O, allowing CO2 to be eliminated by lungs and H as water.
2. Changes in CO2 modify ventilation (respiration) rate.
3. HCO3- concentration can be altered by kidneys.
Other buffers:
phosphate system
- plays a role in plasma & RBC exhange of Na2+ in urine
Most proteins have a net (-) charge and bind to H+
plasma protein
Bicarbonate Buffer System
Most important blood buffer is the carbonic acid–bicarbonate pair
Accounts for the majority of the buffering capacity in the extracellular space
Large amount of carbon dioxide (CO2) is produced within the body as a whole.
Potential for large amounts of acid to build up is greatest
Henderson–Hasselbalch Equation
The ratio of bicarbonate to carbonic acid can be determined from this equation and is useful to the clinician who is assessing acid–base disorders.
The ratio of HCO3− to H2CO3 can be deduced.
Important ratio to use when evaluating acid–base disorders
In plasma at 37oC the pKa’ = 6.1
Temperature and solvent affect the constant
When kidneys and lungs are functioning properly a ratio of HCO3- to H2CO3- = 20:1 or pH = 7.40
cHCO3- proton acceptor (base)
Oxygen and Carbon Dioxide
CO2 is produced and released into the blood.
The lungs control the cH2CO3 in blood.
Plasma cHCO3− is primarily under the control of the kidneys.
Destruction of alveoli
Lung disease( emphysema) causes ↓in O2
Pulmonary edema
Pulmonary embolism, pulmonary hypertension or cardiac failure causes ↓ blood to the lungs
Airway blockage
Asthma and bronchitis common ailments that prevent air from reaching alveoli
Inadequate blood supply
Diffusion of CO2 and O2
O2 diffuses 20 times slower than CO2
O2 concentration ↑ 60% is toxic to lungs
Hemoglobin Buffer System
Most O2 in arterial blood is transported to tissue by hemoglobin
Role of Hemoglobin
Transports H+, O2, and CO2.
As a buffer, it is the second most important regulator of pH in blood.
Hemoglobin allows for large amounts of CO2 produced by metabolism to be carried in the blood with little or no change in pH.
Diffusion of Oxygen and Carbon Dioxide
Most important fundamental mechanism of O2 and CO2 transport.
Diffusion is the movement of an uncharged, hydrophobic solute through a lipid bilayer.
No expenditure of energy is involved.
The driving force for diffusion is the concentration gradient.
Transport Oxygen in the Blood
Hemoglobin can bind O2 only when the iron is in the ferrous (Fe2+) state.
When heme is part of hemoglobin, interactions with about 20 amino acids cradle the heme in the globin so O2 loosely and reversibly binds to Fe2+.
Most important amino acid in this reaction is histidine, which binds Fe2+.
Oxyhemoglobin (O2Hb)
O2 reversibly bound to hemoglobin
Deoxyhemoglobin (HHb):
hemoglobin not bound to O2 but capable of forming a bond when O2 is availabl
Carboxyhemoglobin (COHb):
hemoglobin bound to CO
Methemoglobin (MetHb):
hemoglobin unable to bind O2 because iron (Fe) is in an oxidized rather than reduced state
Blood Gas Units of Measurement
1 atmosphere (atm) = 760 mmHg *1 torr = 1/760 atm 760 mmHg = 760 torr 1 torr = 1 mmHg 1 mmHg = 0.133 kilopascal (kPa) 1 kPa = 7.5 mmHg 760 mmHg = 101,325 pascals** *A torr is a unit of pressure equal to 1 millimeter rise of mercury in a barometer. **Pascal is the Système International unit of pressure and is equal to n (newton)/m2 or m-1 • kg • s-2
Oxygen and Gas Exchange
7 Conditions needed for adequate tissue oxygenation Atmospheric oxygen Adequate ventilation Gas exchange between lungs and arterial blood Loading oxygen onto hemoglobin Adequate hemoglobin Adequate transport (cardiac output) Release of oxygen to the tissues BP @ sea level = 760 mm Hg Atmosphere contains O2 = 20.93% CO2 = 0.03% N = 78.1 %
Hemoglobin Buffer System
Hemoglobin (Hb) serves several roles in acid–base balance and the respiration process.
75% of CO2 is present as bicarbonate.
5% is present as dissolved gas.
20% is bound to hemoglobin as a carbamino compound.
Hemoglobin accounts for about 80% of buffering capacity.
Hemoglobin–Oxygen Dissociation
O2 must be released at tissues from its carrier, hemoglobin.
Oxygen dissociates from adult (A1) hemoglobin in characteristic fashion (S-shaped curve).
Shape of oxygen-dissociation curve & affinity of hemoglobin for O2 are affected by:
Hydrogen ion activity
pCO2 & CO levels
Body temperature
2,3-Diphosphoglycerate (2,3 DPG)
A= left shift – retain oxygen
B = normal
C= right shift – oxygen release
What is the p50?
The oxygen tension when the hemoglobin is 50% saturated with oxygen. Its used to measure hemoglobin-oxygen affinity or the ability of the arterial blood to release oxygen to the tissues.
RIGHT SHIFT:
In general, as pH ↓ or as PCO2, temp and 2,3 DPG increase, the O2 curve shifts to the right, indicating that hemoglobin has a lower affinity for oxygen and the p50 or the midpoint of the curve (see —– line) will be increased. Hemoglobin readily gives up the oxygen its carrying so that oxygen is able to diffuse into the tissues. The deoxyhemoglobin is then free to act as a buffer by picking up H+. High rates of cellular metabolism causes increased levels of PCO2 which results in the formation of more carbonic acid that dissociates into bicarbonate (HCO3-) and H+.
Right shift happens when tissues become acidic following exercise, with decreased perfusion or at high altitudes, and in anemic states. Anemia results in a decrease of oxygen carrying capacity and is characterized by a low hemoglobin. As long as the 2,3-DPG remains high, the patient doesn’t suffer effects of hypoxia
LEFT SHIFT:
In cases of hypothermia, hyper ventiliation, transfusion, abnormal hemoglobin (fetal, hemoglobinopathies) or alkalosis, O2 curve shifts to the left, indicating that the hemoglobin has a higher affinity for oxygen and the P50 will be decreased. Higher oxygen-hemoglobin retention means that tissues aren’t getting the oxygen it needs resulting in poor tissue perfusion, and results in organ ischemia.
Shows the relationship between percent hemoglobin saturation with oxygen
Can be used to determine the percent hemoglobin saturation for a given PO2 and O2 content
The curve is sigmoid or S-shaped.
The reasons for this involve the four oxygen binding sites on the hemoglobin molecule.
Presence of disease can result in significant changes in the dissociation curve.
