BODY FLUID COMPARTMENTS

In a normal adult man, 50-60% of bodyweight is water; females have pro-portionately more body fat than males and total body water is about 45-50% of bodyweight. In a healthy 70 kg male, total body water is approximately 42 L. This is contained in three major compartments:

■ Intracellular fluid (28 L, about 35% of lean bodyweight)

■ Extracellular fluid - the interstitial fluid that bathes the cells (9.4 L, about 12%)

■ Plasma (also extracellular) (4.6 L, about 4-5%).

The intracellular and interstitial fluids are separated by the cell membrane; the interstitial fluid and plasma are separated by the capillary wall.

Table 8.1 The normal daily water and sodium balance in a 75 kg man

Input

Output

Water (mL)

Water (mL)

Drink

1500

Urine

1500

Food

800

Insensible loss (skin, lungs)

800

Metabolism

200

Faeces

200

Total

2500

Total

2500

Sodium (mmol)

Sodium (mmol)

Food and drink

100

Urine

100

Sweat

Negligible

Faeces

Negligible

Osmotic pressure is the primary determinant of the distribution of water among the three major compartments. Osmolality is determined by the con-centration of osmotically active particles. Thus 1 mole of sodium chloride dissolved in 1 kg of water has an osmolality of 2 mmol/kg, as sodium chloride freely dissociates into two particles, the sodium ion Na+ and the chloride ion Cl-. One mole of urea (which does not dissociate) in 1 kg of water has an osmolality of 1 mmol/kg. Osmolarity is the osmoles of solute per litre of solution (mmol/L) and for dilute aqueous solutions is essentially equivalent to osmolality.

The intracellular fluid contains mainly potassium (K+) (most of the cell Mg2+ is bound and osmotically inactive). In the extracellular compartment sodium salts predominate in the interstitial fluid and proteins in the plasma. Body Na+ stores are the primary determinant of extracellular fluid volume but plasma volume is determined by oncotic pressure as well as protein concen-tration (see later). The composition of intracellular and extracellular fluids is shown in Table 8.2. The cell membrane separates the intracellular and extracellular fluid compartments and maintains, by active and passive trans-port mechanisms, the different electrolyte compositions within each compart-ment. A change in osmolality in one compartment will trigger water movement across the cell membrane to re-establish osmotic equilibrium.

Distribution of extracellular fluid

The capillary wall separating the intravascular (plasma) and interstitial spaces is freely permeable to Na+, K+ and glucose, and these solutes do not therefore contribute to fluid distribution between these spaces. However, plasma pro-teins, e.g. albumin, have a limited ability to traverse the capillary bed, and act to hold water in the vascular space. The distribution of extracellular water between vascular and extravascular (interstitial) space is determined by the

Table 8.2 Normal adult electrolyte concentrations of intracellular and extracellular fluids

Plasma

Interstitial fluid

Intracellular fluid

(mmol/L)

(mmol/L)

(mmol/L)

Na+

142

144

10

K+

4

4

160

Ca2+

2.5

2.5

1.5

Mg2+

1.0

0.5

13

Cl-

102

114

2

hco3-

26

30

8

PO42-

1.0

1.0

57

SO42-

0.5

0.5

10

Organic acid

3

4

3

Protein

16

0

55

equiNbrium between hydrostatic pressure (i.e. intracapNlary blood pressure), which tends to force fluid out of the capillaries, and oncotic pressure (i.e. osmotic pressure exerted by plasma proteins), which acts to retain fluid within the vessel. The net flow of fluid outwards is balanced by ‘suction' of fluid into the lymphatics, which returns it to the bloodstream (Fig. 8.1). Oedema is defined as an increase in interstitial fluid and results from:

■ Increased hydrostatic pressure, e.g. sodium and water retention in cardiac failure

■ Reduced oncotic pressure, e.g. as a result of nephrotic syndrome with hypoalbuminaemia

■ Obstruction to lymphatic flow

■ Increased permeability of the blood vessel wall, e.g. at a site of inflam-mation, the cytokines lead to an increase in vascular permeability.

Inspection and palpation are usually sufficient to identify oedema. Compres-sion of the skin of the affected area with a finger tip for 10 seconds results in ‘pitting'. Localized oedema is most likely to result from a local cause, e.g. venous obstruction. The location of generalized oedema, e.g. with cardiac failure, is often most prominent in the legs and feet in ambulatory patients and in the sacral region in those who are confined to bed.

Intravenous fluids in clinical practice

Intravenous fluids are frequently used in hospital to maintain fluid balance in patients who are unable to take fluids via the normal route and to replenish

Intravenous fluids in clinical practice

Fig. 8.1 Distribution of water between the vascular and extravascular (interstitial) spaces.This is determined by the equilibrium between hydrostatic pressure, which tends to force fluid out of the capillaries, and oncotic pressure, which acts to retain fluid within the vessel. The net flow of fluid outwards is balanced by ‘suction’ of fluid into the lymphatics, which returns it to the bloodstream. Similar principles govern the volume of the peritoneal and pleural spaces.

substantial deficits or continuing losses. Crystalloids, e.g. sodium chloride 0.9%, contain low molecular weight salts or sugars that dissolve completely in water and pass freely between intravascular and interstitial compartments (Table 8.3). Glucose 5% is essentially free water and distributes evenly across total body water (i.e. across all three major fluid compartments) after intra-venous administration; very little stays in the intravascular space. Sodium chloride 0.9% remains in the extracellular space and thus about one third of a volume infused will remain in the intravascular space. Colloids (e.g. dextran 70, gelatin) contain larger molecular weight substances and remain for a longer period in the intravascular space than crystalloid solutions. Colloids are used to expand circulating volume in haemorrhage (until blood becomes available), burns and sometimes septicaemia. Although remaining in the intravascular space longer than crystalloids, comparative studies have not shown a definite advantage of colloids over crystalloids in hypovolaemic patients. Side-effects of colloids are hypersensitivity reactions including anaphylactoid reactions and a transient increase in bleeding time. They have a greater sodium concentration than plasma.

Assessment and monitoring of fluid balance is made from the history (e.g. vomiting, diarrhoea), fluid balance charts (often inaccurate), daily weights,

Table 8.3 Crystalloids in general use
Na+
mmol/L
K+
mmol/L
Cl−
mmol/L
Osmolarity
mosmol/kg
Indication
(see footnote)
Normal
plasma
values
142
4.5
103
285–295
Sodium
chloride
0.9%
154
-
154
308
1
Sodium
chloride
0.18%/
glucose 4%
30
-
-
283
2
Glucose 5%
-
-
-
278
3
Hartmann’s
solution/
Ringer’s
lactate
solution
130
4
109
273
4
NB: In a normal adult 1.5–2.5 L (25–35 mL/kg/24 hours) of fluid containing about 70–100 mmol sodium and 40–80 mmol potassium are required to maintain balance.
Fluids are given with or without potassium chloride (depending on daily requirements and plasma levels) given as ready mixed bags in preference to adding potassium chloride concentrate to a bag.
1. Volume expansion in hypovolaemic patients. Rarely to maintain fluid balance when there are large losses of sodium. Excessive sodium and chloride (compared to plasma) may cause hypernatraemia and hyperchloraemic metabolic acidosis, respectively.
2. Maintenance of fluid balance in normovolaemic, normonatraemic patients.
3. To replace water. Only given alone when there is no significant loss of electrolytes.
Also may be alternated with sodium chloride 0.9% as an alternative to (2).
4. In fluid maintenance and replacement. Provides bicarbonate from metabolism of
lactate. Also contains calcium 2 mmol/L.

urine output and clinical observations (skin turgor, capillary refiN, jugular venous pressure (JVP), pulse, lying and standing blood pressure). Invasive cardiac monitoring is also used in high-dependency patients. Measurement of Central venous pressure before and after an intravenous fluid challenge is also used to assess volume status (p. 581). In the post-operative patient there is usually a short period of oliguria occurring as a physiological response to surgery. Urine output alone should not therefore be relied on to assess fluid balance in these patients. In addition, post-operatively, the ability of the

kidneys to dilute the urine is impaired and there is a risk of dilutional hyponatraemia.

Before intravenous fluids are prescribed a clinical assessment is made to determine:

■ If fluids are for maintenance only, or to correct a deficit, e.g. hypo-volaemia as a result of diarrhoea and vomiting.

■ The type of fluid that is necessary for maintenance and to correct any deficit.

■ The appropriate rate of fluid administration and the likely time that intra-venous fluids will be necessary.

Suggestions for appropriate use of the different types of fluid is indicated in Table 8.3.

Ebook Essentials of Kumar and Clark's Clinical Medicine, 5e

1. Ethics and communication

Ethics and communication

2. Infectious diseases

Infectious diseases

3. Gastroenterology and nutrition

Gastroenterology and nutrition

4. Liver, biliary tract and pancreatic disease

Liver, biliary tract and pancreatic disease
LIVER BIOCHEMISTRY AND LIVER FUNCTION TESTS
SYMPTOMS AND SIGNS OF LIVER DISEASE
JAUNDICE
HEPATITIS
NON - ALCOHOLIC FATTY LIVER DISEASE (NAFLD)
CIRRHOSIS
COMPLICATIONS AND EFFECTS OF CIRRHOSIS
LIVER TRANSPLANTATION
TYPES OF CHRONIC LIVER DISEASE AND CIRRHOSIS
PRIMARY SCLEROSING CHOLANGITIS
BUDD - CHIARI SYNDROME
LIVER ABSCESS
LIVER DISEASE IN PREGNANCY
LIVER TUMOURS
GALLSTONES
THE PANCREAS
CARCINOMA OF THE PANCREAS
NEUROENDOCRINE TUMOURS OF THE PANCREAS

5. Haematological disease

Haematological disease
ANAEMIA
Assessment and treatment of suspected neutropenic sepsis
HAEMOLYTIC ANAEMIA
INHERITED HAEMOLYTIC ANAEMIAS
ACQUIRED HAEMOLYTIC ANAEMIA
MYELOPROLIFERATIVE DISORDERS
THE SPLEEN
BLOOD TRANSFUSION
THE WHITE CELL
HAEMOSTASIS AND THROMBOSIS
THROMBOSIS
THERAPEUTICS

6. Malignant disease

Malignant disease
MYELOABLATIVE THERAPY AND HAEMOPOIETIC STEM CELL TRANSPLANTATION
THE LYMPHOMAS
THE PARAPROTEINAEMIAS
PALLIATIVE MEDICINE AND SYMPTOM CONTROL

7. Rheumatology

Rheumatology
COMMON INVESTIGATIONS IN MUSCULOSKELETAL DISEASE
COMMON REGIONAL MUSCULOSKELETAL PROBLEMS
BACK PAIN
OSTEOARTHRITIS
INFLAMMATORY ARTHRITIS
THE SERONEGATIVE SPONDYLOARTHROPATHIES
Clinical features, Investigations
INFECTION OF JOINTS AND BONES
AUTOIMMUNE RHEUMATIC DISEASES
SYSTEMIC INFLAMMATORY VASCULITIS
DISEASES OF BONE
THERAPEUTICS

8. Water, electrolytes and acid–base balance

WATER AND ELECTROLYTE REQUIREMENTS
BODY FLUID COMPARTMENTS
REGULATION OF BODY FLUID HOMEOSTASIS
PLASMA OSMOLALITY AND DISORDERS OF SODIUM REGULATION
DISORDERS OF POTASSIUM REGULATION
DISORDERS OF MAGNESIUM REGULATION
DISORDERS OF ACID - BASE BALANCE
THERAPEUTICS

9. Renal disease

Renal disease
INVESTIGATION OF RENAL DISEASE
GLOMERULAR DISEASES
NEPHROTIC SYNDROME
URINARY TRACT INFECTION
TUBULOINTERSTITIAL NEPHRITIS
HYPERTENSION AND THE KIDNEY
RENAL CALCULI AND NEPHROCALCINOSIS
URINARY TRACT OBSTRUCTION
ACUTE RENAL FAILURE/ACUTE KIDNEY INJURY
CHRONIC KIDNEY DISEASE
RENAL REPLACEMENT THERAPY
CYSTIC RENAL DISEASE
TUMOURS OF THE KIDNEY AND GENITOURINARY TRACT
DISEASES OF THE PROSTATE GLAND
TESTICULAR TUMOUR
URINARY INCONTINENCE

10. Cardiovascular disease

COMMON PRESENTING SYMPTOMS OF HEART DISEASE
INVESTIGATIONS IN CARDIAC DISEASE
CARDIAC ARRHYTHMIAS
HEART FAILURE
ISCHAEMIC HEART DISEASE
RHEUMATIC FEVER
VALVULAR HEART DISEASE
PULMONARY HEART DISEASE
MYOCARDIAL DISEASE
CARDIOMYOPATHY
PERICARDIAL DISEASE
SYSTEMIC HYPERTENSION
ARTERIAL AND VENOUS DISEASE
ELECTRICAL CARDIOVERSION
DRUGS FOR ARRHYTHMIAS
DRUGS FOR HEART FAILURE
DRUGS AFFECTING THE RENIN - ANGIOTENSIN SYSTEM
NITRATES, CALCIUM - CHANNEL BLOCKERS AND POTASSIUM - CHANNEL ACTIVATORS

11. Respiratory disease


Respiratory disease
TUBERCULOSISnd
DIFFUSE DISEASES OF THE LUNG PARENCHYMA
OCCUPATIONAL LUNG DISEASE
CARCINOMA OF THE LUNG
DISEASES OF THE CHEST WALL AND PLEURA
DISORDERS OF THE DIAPHRAGM

12. Intensive care medicine

Intensive care medicine

13. Drug therapy, poisoning, and alcohol misuse

Drug therapy, poisoning, and alcohol misuse

14. Endocrine disease

Endocrine disease
PITUITARY HYPERSECRETION SYNDROMES
THE THYROID AXIS
MALE REPRODUCTION AND SEX
FEMALE REPRODUCTION AND SEX
THE GLUCOCORTICOID AXIS
THE THIRST AXIS
DISORDERS OF CALCIUM METABOLISM
DISORDERS OF PHOSPHATE CONCENTRATION
ENDOCRINOLOGY OF BLOOD PRESSURE CONTROL
DISORDERS OF TEMPERATURE REGULATION
THERAPEUTICS

15. Diabetes mellitus and other disorders of metabolism

DIABETES MELLITUS
DIABETIC METABOLIC EMERGENCIES
COMPLICATIONS OF DIABETES
SPECIAL SITUATIONS
HYPOGLYCAEMIA IN THE NON - DIABETIC
DISORDERS OF LIPID METABOLISM
THE PORPHYRIAS

16. The special senses

THE EAR
THE NOSE AND NASAL CAVITY
THE THROAT
THE EYE

17. Neurology

COMMON NEUROLOGICAL SYMPTOMS
COORDINATION OF MOVEMENT
THE CRANIAL NERVES
COMMON INVESTIGATIONS IN NEUROLOGICAL DISEASE
UNCONSCIOUSNESS AND COMA
STROKE AND CEREBROVASCULAR DISEASE
EPILEPSY AND LOSS OF CONSCIOUSNESS
NERVOUS SYSTEM INFECTION AND INFLAMMATION
HYDROCEPHALUS
HEADACHE, MIGRAINE AND FACIAL PAIN
SPINAL CORD DISEASE
DEGENERATIVE NEURONAL DISEASES
DISEASES OF THE PERIPHERAL NERVES
MUSCLE DISEASES
MYOTONIAS
DELIRIUM
THERAPEUTICS

18. Dermatology

Dermatology

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