Acid Base Balance Flashcards
1
Q
Describe the pH scale
A
- pH measures H+ concentration
- pH is a negative scale so when the numbers go down, H+ goes up
- pH is a log scale so the difference between one pH unit is a factor of 10
2
Q
What is the normal pH range for blood?
A
- 7.35 to 7.45
- lower pH indicates a physiological acidosis (ie. it is still alkaline when under 7 but more acidic than it should be)
- above 7.45 is physiological alkalosis
3
Q
Why is blood pH so closely homeostatically regulated?
A
- we can not survive long excursions from the normal pH range
- alkalosis is much more rare a problem and is almost never fatal
- acidosis is a serious problem and can be fatal
- at pH of about 7.1 arrhythmia occurs and at 7.0 the CNS becomes so depressed that normal functioning ceases
- fetus is always more acidic than mom
- CO2 is high in fetus so that concentration gradient flows from fetus to mom
4
Q
What is an equilibrium reaction?
A
- type of reversible chemical reaction wherein both products and reactants are present at the same time
- the ratio in the products and reactants is maintained such that if one of the parts of the equilibrium is removed the reaction moves in such a way as to compensate for the loss
- all of the buffers participate in equilibrium reactions so they always act to keep the balance between reactants and products
- ultimately a buffer system can be completely consumed by attempting to balance large changes in H+
- there are intracellular and extracellular buffers
5
Q
Describe intracellular buffers
A
- inside of cells proteins and amino acids act as buffers
- in red blood cells, hemoglobin (which is constructed partially of the protein globin) is an effective buffer
- the carboxyl and amino groups can absorb or give off H+
6
Q
Describe extracellular buffers
A
- in the serum and other extracellular spaces we have the bicarbonate buffer system
- this system consists of carbon dioxide, water, carbonic acid, bicarbonate ion, and hydrogen ions
- the elements of the system are normally always present because CO2 and water are normally present
- the urine has the phosphate buffer system and bicarbonate buffer
- kidneys work to preserve bicarbonate
7
Q
What happens within the bicarbonate system when you increase CO2 levels?
A
- CO2 combines with water and carbonic acid levels go up
- some of the carbonic acid decomposes to yield H+ and bicarbonate
- in the end, the increased levels of H+ brings the pH down a very small amount and the whole system is shifted to the right to maintain a balance in the reactants and products
8
Q
What happens when you increase H+ in the bicarbonate buffer?
A
9
Q
What happens to CO2 produced in metabolically active cells in the body?
A
- metabolically active cells produce CO2
- separated from the blood stream by the basal lamina and the endothelium
- CO2 will diffuse to areas of low concentration such as the plasma in the capillaries
- CO2 combines with water to form carbonic acid which quickly converts to H+ and bicarbonate
- the initial CO2 and H2O combining is slow because there are no enzymes in the capillaries
- some CO2 goes into red blood cells and combines with hemoglobin to form carboxyhemoglobin
- some CO2 can dissolve in the fluid inside the red blood cells which is rapidly coverted to carbonic acid in the presence of carbonic anhydrase
- some remains as native CO2 in the red blood cell or the plasma
10
Q
What occurs to CO2 at the lungs?
A
- alveolar air is low in CO2
- normally we have thin lung epithelial cells, small basal lamina, and endothelial cells
- some dissolved CO2 in the plasma goes down its concentration gradient into the alveolar air (some CO2 in the RBCs can do this too)
- bicarbonate in the plasma can convert back to CO2 and water (slowly) because the CO2 that was in the plasma is not there anymore so the equilibrium shifts
- bicarbonate in RBCs is quickly converted to make up for CO2 that was lost
- carboxyhemoglobin will give us CO2 and hemoglobin which can diffuse into alveolar air
- pH changes slightly when CO2 is lost
11
Q
What are respiratory causes of acidosis?
A
- anything that interferes with respiration will increase dissolved CO2 and you will not exhale the CO2 produced
- lung damage (ie. emphysema), loss of patency of the airways (ie. a foreign body), or chest wall damage in breathing (eg. damage to the muscles of respiration)
- damage (trauma) or incapacitation (opiate poisoning) of respiratory centres in medulla oblongata
- holding your breath or running
- acute respiratory acidosis is very sympathetically stimulating
12
Q
What are non-respiratory causes of acidosis?
A
- anything except for CO2 causing increased H+ is called nonrespiratory or metabolic acidosis
- anaerobic metabolism: lactic acid produced during anaerobic glycolysis
- kidney dysfunction: normally the kidney secretes large amounts of acid and when it is not functioning properly pH will fall
- incomplete breakdown of fatty acids: uncontrolled diabetes mellitus and other forms of starvation lead to large increases in fatty acids
- consumption of ethanol in large quantities: it is converted from ethanol to acetylaldehyde and then acetic acid, or small amounts of methanol or other toxic alcohols
- normal metabolism produces sulfuric acid and other acids but these are nonvolatile
- acidic fruits which have citric and other acids (minor contribution)
- diarrhea: loss of bicarbonate rich intestinal fluid
13
Q
What are respiratory causes of alkalosis?
A
- caused by low CO2 in the blood
- you can easily develop low CO2 levels in the blood because of hyperventilation
- hyperventilation drives down the alveolar CO2 and since your blood CO2 is in equilibrium with the alveolar levels then your blood loses CO2
-
14
Q
What are nonrespiratory causes of alkalosis?
A
- vomiting: loss of H+ from the extremely acidic contents of the stomach will lead to an increase in blood pH
- ingestion of bicarbonate
- constipation: absorption of extra bicarbonate from the feces, normally feces are expelled with a small amount of water and bicarbonate, if fecal matter stays in large bowel long enough it becomes very dehydrated and more bicarbonate is reabsorbed
15
Q
What is respiratory compensation?
A
- if there is too much acid or lots of CO2 we breathe more
- if there is too little acidity or low CO2 in the blood stream we breathe less
- chemoreceptors are driven by CO2 and H+ (increases drive ventilation) and send messages to respiratory centre in pons/medulla which causes you to breathe more
16
Q
What is renal compensation?
A
- if there is too much acid, H+ leaves through urine
- if there is too much base, we retain acid and get rid of bicarbonate through the urine
- for long term compensation of acidosis, the kidneys can preserve bicarbonate
- kidneys are the only way of dealing with nonvolatile acids (ie. acids that can not leave in the respiratory gas)
- takes hours for this sytem to kick in and days to optimize
17
Q
How does the kidney compensate for pH changes?
A
- Na+/H+ antiporter in PCT
- excess CO2 in the PCT
- converted quickly to carbonic acid which will break down to give bicarb and H+
- sodium is normally in large supply in the filtrate so it can be exchanged through the antiporter with H+
- this will get rid of the H+ through the urine
- Na+ will leave through Na/K ATP pump
- bicarbonate facilitated diffusion transporter allows bicarb to move down its concentration gradient into interstitial fluid and into the plasma
18
Q
How do you figure out the cause of acidosis or alkalosis?
A
- the first thing you need to know is the pH of the blood
- next value to check is the CO2 in the blood to discover if it is higher or lower than normal
- if the CO2 levels explain the pH it will be called “respiratory”
- if the CO2 levels do not agree with the pH, then it will be non-respiratory or metabolic
- can also check bicarb levels in blood
- bicarb is a base so the pH levels should vary directly with the bicarb levels
- if bicarb is causing the problem, then it is a metabolic problem (ie. low pH with low bicarb levels and high pH with high bicarb levels)
19
Q
A patient comes in with elevated pH and low CO2, what is the problem?
A
-respiratory alkalosis
20
Q
Blood work results show low pH, low CO2, and low bicarb
A
- metabolic acidosis
- not enough bicarb
- the low CO2 is compensatory by the respiratory system (person was likely breathing deeply because of the low pH)
- can happen with large volumes of alcohol
- alcohol is converted to acetic and lactic acid which leads to increased amounts of fixed acids
- if this person had vomited then the pH would have gone back up
21
Q
A patient has the x-ray image shown below. You notice that their breathing pattern is very shallow and slow. The most likely finding upon arterial analysis of blood would be:
a) high pH and low pCO2
b) high pH high pCO2
c) low pH and low pCO2
d) low pH and high pCO2
e) pH 7.4, normal pCO2
A
d)
- shallow breathing means they are holding onto CO2
- respiratory acidosis
