S2: The Role of Ventilation in Acid-Base Balance Flashcards Preview

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Equation for pH

pH = - log [H+]


Why is pH homeostasis important?

- Protein function (e.g. enzyme/receptor binding sites) depends on a specific secondary, tertiary and quaternary structure acheived via inter and intra chain bonding
- Various factors (PH, temperature, pressure) can denature proteins by disrupting these bonds/the structure, thus impairing function
- Protein function is also dependent on pH and different enzymes have function at varying PH ranges based on the enviroment they have been adapted to function within


Why is arterial pH tightly regulated, how is it regulated and what is the safe pH range?

Arterial PH is tightly regulated to ensure effective protein function as the circulatory system innervates all organs and tissues.
This is achieved by the presence of buffering system, and by regulating the level of molecules associated with acid and base production
- 7.35-7.45 is the safe PH range in the body


How is PH homeostasis maintained when acid production increases?

Describe the buffer

The presence of buffers (weak acid (H2CO3) + conjugate base (HCO3-)) e.g. Bicarbonate buffering system, 'mops up' excess H+.
Any H+ formed by addition of strong acid, react with conjugate base to form weak acid, hence PH remains relatively stable.


What is a buffer made of?

Buffers consist of a weak acid + conjugate base are used to resist sharp changes in pH


Name 3 intracellular buffering systems

- Phosphate buffer system
- Amino acids/proteins
- Haemoglobin (in RBC)


Name 2 extracellular buffering systems

- Bicarbonate buffer system
- Plasma protein e.g. albumin


How can respiratory and metabolic distress be diagnosed and intepreted?

Analysis of ABG and pH


What is the acid dissociation constant (ka)?

Ka determines the strength of an acid.

Ka = the equilibrium constant for the dissociation of an acid in water.

It describes the ratio of ionised to unionised species, and thus the degree to which an acid is able to donate protons (H+), generating acidity.

Ka = [H+]{A-}/{HA}


Compare Ka, pKa and strength of acid

- Higher Ka, more H+ dissociation and the stronger the acid

- Higher Ka = Lower pKa

- Higher pKa the weaker the acid


What is the Henderson-Hasselbach Equation?

The Henderson-Hasselbach equation describes the relationship between pKa & pH


What is pH proportional to?

pH = [HCO3-]/PaCO2


How does the lungs and kidneys maintain blood PH homeostasis?

What might occur if there is a problem?

The lungs maintain blood PH homeostasis by regulating PaCO2
e.g. by regulating ventilation

The kidneys maintain blood PH homeostasis by regulating [HCO3-]
e.g. by regulating reabsorption/excretion in glomerular filtrate

As blood pH is proportional to the ratio of HCO3- to CO2, excessive changes may result from respiratory (CO2) or metabolic (HCO3-) dysfunction (↓pH = acidosis, ↑pH = alkalosis).


Describe acidosis

(causes, effects, compensatory mechanisms)

Acidosis is an arterial pH of <7.35

- Hypercapnia (hypoventilation)
- Increased lactic acid (sepsis)
- Increased ketone bodies (diabetes)
- Decreased kidney acid excretion (renal failure)
- Decreased HCO3- reabsorption (renal acidosis)
- Diarrhoea (loss of HCO3- from gut)

- Tachypnoea
- Muscular weakness
- Headache
- Confusion, Coma
- Cardiac arrhythmia
- Hyperkalaemia

- Hyperventilation (↓PaCO2, respiratory compensation)
- ↑ HCO3- reabsorption (renal compensation)


Describe alkalosis

(causes, effects, compensatory mechanisms)

Acidosis is an arterial pH of >7.45

- Hypocapnia (hyperventilation)
- Vomiting (loss of H+ in HCl)
- ↑ kidney acid excretion (diuretics)
- ↑ alkalotic agent consumption (antacids, NaHCO3)

- Bradypnoea
- Muscular weakness, cramps, tetany
- Headache, Nausea
- Lightheadedness, confusion, coma
- Cardiac arrhythmia
- Hypokalaemia

- Hypoventilation (↑PaCO2, respiratory compensation)
- ↓ HCO3- reabsorption (renal compensation)


Explain Acidosis-induced hyperkalaemia

Excess amount of K+ is caused by acidosis. Excess H+ in ECF, there is no longer a concentration gradient moving H+ out and Na+ in (Na/H antiport).

Decreased Na+ exchange then affects the Na+/K+ transport causing excess K+ to accumulate in ECF.

The heart and muscles rely on AP (using Na+ and K+) causing cardiac arrhythmias and muscle weakness.


Explain Alkalosis-induced cerebral vasoconstriction

CO2 (via H+) acts as a vasodilator in blood vessels (cerebral arteries are particularly sensitive)

Hyperventilation: ↓CO2 and ↓H+ (alkalosis)
-Alkalosis causes vasoconstriction of cerebral arteries
- ↓ cerebral blood flow = headache, lightheadedness, confusion, seizures