noenates fluids proportion

At 12 months of age

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

TBW at 60 years of age

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

Fluids can be collectively discussed in physiologically relevant compartments, which are....

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

Characteristics of ICF

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

Characteristics of ECF

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

Interstitial fluid (ISF)

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

Plasma

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

The fluid compartment called the blood volume is interesting in that it is a composite compartment containing.....

• ECF (plasma)
• ICF (red cell water)

Blood levels – normal range

Sodium

135-145 mEq/L

Blood levels – normal range

Potassium

3.5-5.5 mmoles/L

Blood levels – normal range

Magnesium

0.7-0.95 mmoles/L

Blood levels – normal range

Calcium

2.20-2.55 mmoles/L

Blood levels – normal range

Chloride

96-106 mmoles/L

Blood levels – normal range

Phosphate

0.8-1.3 mmoles/L

What makes water move?

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

Components of Daily Obligatory Water Loss

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

Fluid input

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)

Pure gastric vomiting

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

Bilious vomiting

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

Panting

• Loss of free water
• no electrolyte loss

Free water gain

• Dilution
• Diuresis
• promotes ion loss

Diarrhea

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

Oral rehydration solutions should

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

The WHO oral rehydration salts formulation contains

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

Oral rehydration solutions used in the UK are

lower in sodium (50–60 mmol/litre)

Rehydration should be rapid over

3 to 4 hours in most cases

Maximum rate of sweating is

• 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

Losses due to Sweating

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

Insensible fluids losses

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

The Central Controller in Water Balance

Hypothalamus

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

What is thirst?

• 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.

Water intake can be considered to consist of two

components:

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

The 4 major stimuli to thirst are:

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

Drinking stimulates

• 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

Antidiuretic Hormone (ADH)

• 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

Properties of caffeine and alcohol? affect which hormone?

• 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

Why give fluids?

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)

Extracellular fluid pH

Blood pH

• 7.4
• 7.35 – 7.45

Acidosis pH

Alkalosis pH

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

Small changes in pH can produce major disturbances such as

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

The body produces more acids than bases

•Acids taken in with foods

•Acids produced by the metabolism of lipids and proteins

•Cellular metabolism produces CO2.

pH equation

pH = - log [H+]

pH Regulation organs

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

Kidneys > renal compensation > H+

Three major buffering systems

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

Common acids include...

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

Carbonic Acid-Bicarbonate Buffering

• 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

Enzyme converting carbonic acid to carbon dioxide

carbonic anhydrase

Explain protein buffering

• ⬆️ 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

Rates of correcting pH

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

Acidosis symptoms

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

Severe acidosis causes > Disorientation, coma, and death

Alkalosis symptoms

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

The Bohr effect is

• 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.

Respiratory Acidosis

why? in which conditions?

• 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

Compensation for Respiratory Acidosis

Kidneys eliminate hydrogen ion and retain bicarbonate ion

Signs and Symptoms of Respiratory Acidosis

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

Treatment of Respiratory Acidosis

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

Respiratory Alkalosis

Why? In which conditions?

• 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

Compensation of Respiratory Alkalosis

• Kidneys conserve hydrogen ion

• Excrete bicarbonate ion

Treatment of Respiratory Alkalosis

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

Major causes of metabolic acidosis

• 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

Symptoms of Metabolic Acidosis

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

Compensation for Metabolic Acidosis

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

Treatment of Metabolic Acidosis

IV lactate solution

Metabolic Alkalosis causes

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

Symptoms of Metabolic Alkalosis

• Respiration slow and shallow

• Hyperactive reflexes ; tetany

• Often related to depletion of electrolytes

• Atrial tachycardia

• Dysrhythmias

Compensation for Metabolic Alkalosis

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

Treatment of Metabolic Alkalosis

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

Diagnosis of Acid-Base Imbalances

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.