Regulation of fluids and acid base balance Flashcards

• 75-80% water
• Proportionally more ECF than adults
• Interstitial fluid is three times larger than in adults
• At 12 months decrease to 60%

• 50% of total body weight
• Due to increased adipose tissue

• Intracellular Fluid (ICF: about 23 liters, 2/3 TBW, discontinuous small collections)
• Extracellular Fluid (ECF: about 19 liters, interstitial fluid and plasma)

• High in potassium
• High in magnesium
• Low in sodium and chloride ions

• Low in potassium
• Low in magnesium
• High in sodium and chloride ions

1. consists of all the fluids which lie in the interstices of all body tissues.
2. is the link between the ICF and the intravascular compartment.
3. Oxygen, nutrients, wastes and chemical messengers all pass through the ISF.
4. low protein concentration (in comparison to plasma).
5. Lymph is considered as a part of the ISF

1. The only major fluid compartment that exists as a real fluid collection all in one location.
2. Higher protein content and its high bulk flow
3. interstitial fluid of the blood

• ECF (plasma)
• ICF (red cell water)

135-145 mEq/L

3.5-5.5 mmoles/L

0.7-0.95 mmoles/L

2.20-2.55 mmoles/L

96-106 mmoles/L

0.8-1.3 mmoles/L

Osmosis

The osmotic pressure is the hydrostatic (or hydraulic): pressure required to oppose the movement of water through a semi-permeable membrane in response to an ‘osmotic gradient’

Serum osmolality can be measured by use of

• osmometer
• calculated as the sum of the concentrations of the solutes present in the solution

1. Insensible loss: 800 mls
2. Minimal sweat loss: 100 mls
3. Faecal loss: 200 mls
4. Minimal urine volume to excrete solute load: 500 mls

Total: 1,600 mls

External: Oral intake of fluids and food (+ IV fluids)

• Food is an important source of water
• Processed foods may have a very low water content.

Internal: Metabolic water production

• The oxidation of food
• Carbohydrates are completely metabolised to CO2 and H2O
• Metabolic water is about 350 to 400 mls/day (ie 5 mls/kg)

• Loss of HCl
• volume causes hypochloremic metabolic alkalosis
• Cl decrease > limits reabsorption of HCO3 in the kidneys

• loss of K
• loss of HCO3
• loss of Na
• causes hypokalemia, acidosis

• Loss of free water
• no electrolyte loss

• Dilution
• Diuresis
• promotes ion loss

• loss of volume
• Loss of Na
• Loss of K
• Loss of HCO3

1. Enhance the absorption of water and electrolytes
2. Replace the electrolyte deficit adequately and safely
3. Contain an alkalinising agent to counter acidosis
4. Be slightly hypo-osmolar (about 250 mmol/litre) to prevent induction of osmotic diarrhoea
5. be simple to use in the hospital and at home
6. be palatable and acceptable, especially to children
7. be readily available

1. Sodium chloride 2.6 g. Na+ 75 mmol/l, Cl– 65 mmol
2. Potassium chloride 1.5 g. K+ 20 mmol/l
3. Sodium citrate 2.9 g. citrate 10 mmol/l
4. Anhydrous glucose 13.5 g glucose 75 mmol/l
5. It is dissolved in sufficient water to produce 1 litre

lower in sodium (50–60 mmol/litre)

3 to 4 hours in most cases

• up to 50 mls/min or 2,000 mls/hr in the acclimatised adult
• losses up to 25% of total body water are possible under severe stress
• from 100 to 8,000 mls/day
• solute loss can be as much as 350 mmols/day (or 90 mmols/day acclimatised) of sodium under the most extreme conditions

• Fluid loss (hot environment, physically active)
• Solute loss (Decreases with 'acclimatisation', 0.2-1%)
• Heat loss

• From the skin (trans-epithelial)
• From the respiratory tract
• Heat loss
• No solute loss

Hypothalamus

1. Osmoreceptors
2. Thirst centre
3. OVLT & SFO (respond to angiotensin II)
4. Supraoptic & paraventricular nuclei (for ADH synthesis)

• Thirst is "the physiological urge to drink water".
• Most water intake is due to social and cultural factors, not thirst
• Thirst offers a backup to these behavioural factors and to the ADH response.

• a regulatory component (due to thirst)
• non-regulatory component (all other fluid intake)

The 4 major stimuli to thirst are:

1. Hypertonicity: Cellular dehydration acts via an osmoreceptor
2. Hypovolaemia: sensed via the low pressure baroreceptors in the great veins and right atrium
3. Hypotension: The high pressure baroreceptors in carotid sinus & aorta provide the sensors for this input
4. Angiotensin II: This is produced consequent to the release of renin

• mechanoreceptors in the mouth and pharynx
• Peripheral receptors provide input to the hypothalamus and the sensation of thirst is relieved
• This occurs even before any reduction in plasma tonicity

• nonapeptide synthesised in the hypothalamus
• regulation of water balance by its effect on the kidneys
• Also known as vasopressin
• Intravenously has a half-life of only about 15 minutes
• Rapidly metabolised in the liver and kidney to inactive products

• diuretic properties
• inhibit the pituitary secretion of ADH
• Caffeine > when ingested at levels in excess of 300mg
• Alcohol diuretic effects are largly due to the alcohol volume

1. Replace intravascular volume
2. Improve tissue perfusion
3. Replace fluid deficits (dehydration)
4. Replace ongoing losses (Vomiting, Diarrhoea, burns, etc.)
5. Fluid diuresis to eliminate toxins
6. Anaesthetic and surgical support
7. Replacement of specific components (blood, plasma)
8. Nutritional support (TPN, PPN)

• 7.4
• 7.35 – 7.45

• acidemia below 7.35
• alkalemia above 7.45
• < 6.8 or > 8.0 death occurs

• Most enzymes function only with narrow pH ranges
• Affect electrolytes (Na+, K+ , Cl-)
• Affect hormones
• Affects bone synthesis and reabsorption

•Acids taken in with foods

•Acids produced by the metabolism of lipids and proteins

•Cellular metabolism produces CO2.

pH = - log [H+]

Lung > respiratory compensation > Pco2 > carbonic acid bicarbonate buffer system

Kidneys > renal compensation > H+

1. Protein buffer system > AA, H+ by haemoglobin buffer system
2. Carbonic acid bicarbonate system > organic and fixed acids
3. Phosphate > buffer pH in ICF

1. Carbonic acid is the most important factor affecting pH of ECF
2. Sulfuric acid and phosphoric acid > catabolism of AA
3. Organic acids > Metabolic byproducts lactic acid, ketones

• H+ + HCO3- <> H2CO3 <> H2O + CO2
• convert carbonic acid to CO2 (through the enzyme carbonic anhydrase)
• Then remove CO2 from the body through respiration.

Only functions when the respiratory system and control centres are working normally

carbonic anhydrase

• ⬆️ pH > the carboxyl group acts as a weak acid
• ⬇️ pH > the amino group acts as a weak base

Hemoglobin buffer system

Prevents pH changes when PCO2 is rising or falling

• Respiratory > several minutes to hours
• Renal > several hours to days

1. Depression of the CNS through ↓ in synaptic transmission.
2. Generalized weakness
3. Deranged CNS function

Severe acidosis causes > Disorientation, coma, and death

1. causes over excitability of the central and peripheral nervous systems
2. Numbness
3. Lightheadedness
4. Nervousness
5. muscle spasms or tetany
6. Convulsions
7. Loss of consciousness
8. Death

• a physiological phenomenon where haemoglobin's affinity for oxygen decreases as carbon dioxide levels or acidity (pH) in the blood increase
• causing it to release more oxygen to tissues that need it, like actively respiring muscles.

• Carbonic acid excess occurs by blood levels of CO2 are above 45 mm Hg.

• Hypercapnia

Chronic conditions

• Drugs or head trauma > Depression of the respiratory centre in the brain that controls breathing rate
• Paralysis of the respiratory or chest muscles
• Emphysema

Acute conditions

• Adult Respiratory Distress Syndrome
• Pulmonary edema
• Pneumothorax

Kidneys eliminate hydrogen ion and retain bicarbonate ion

1. Breathlessness
2. Restlessness
3. Lethargy and disorientation
4. Tremors, convulsions, coma
5. Respiratory rate rapid, then gradually depressed
6. Skin warm and flushed due to vasodilation caused by excess CO2

1. Restore ventilation
2. IV lactate solution
3. Treat underlying dysfunction or disease

• Carbonic acid deficit
• pCO2 less than 35 mm Hg (hypocapnea)
• Most common acid-base imbalance
• Primary cause is hyperventilation

Conditions that stimulate respiratory center:

• Oxygen deficiency at high altitudes
• Pulmonary disease and Congestive heart failure – caused by hypoxia
• Acute anxiety
• Fever, anemia
• Early salicylate intoxication
• Cirrhosis
• Gram-negative sepsis

• Kidneys conserve hydrogen ion

• Excrete bicarbonate ion

1. Treat the underlying cause
2. Breathe into a paper bag
3. IV Chloride-containing solution – Cl- ions replace lost bicarbonate ions

• Depletion of bicarbonate reserve
• Inability to excrete hydrogen ions at kidneys
• Production of large numbers of fixed / organic acids
• Bicarbonate loss due to chronic diarrhea

• Headache, lethargy
• Nausea, vomiting, diarrhea
• Coma
• Death

1. Increased ventilation
2. Renal excretion of hydrogen ions if possible
3. K+ exchanges with excess H+ in ECF

IV lactate solution

Bicarbonate excess greater than 26 mEq/L

Causes:

• Excess vomiting = loss of stomach acid
• Excessive use of alkaline drugs
• Certain diuretics
• Endocrine disorders
• Heavy ingestion of antacids
• Severe dehydration

• Respiration slow and shallow

• Hyperactive reflexes ; tetany

• Often related to depletion of electrolytes

• Atrial tachycardia

• Dysrhythmias

• Alkalosis most commonly occurs with renal dysfunction, so can’t count on kidneys
• • Respiratory compensation difficult – hypoventilation limited by hypoxia

1. Electrolytes to replace those lost
2. IV chloride containing solution
3. Treat underlying disorder

1. Note whether the pH is low (acidosis) or high (alkalosis)
2. Decide which value, pCO2 or HCO3 is outside the normal range and could be the cause of the problem
3. If the cause is a change in pCO2, the problem is respiratory
4. If the cause is HCO3- the problem is metabolic
5. Look at the value that doesn’t correspond to the observed pH change.
6. If it is inside the normal range, there is no compensation occurring.
7. If it is outside the normal range, the body is partially compensating for the problem.