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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3050_Библиотеки_им_академика_М_И_Перельмана
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104.5°
122 Fluid and electrolyte balance
Figure 6.1 The water molecule (H
2
O). Two hydrogen atoms are bonded to one oxygen atom at an
angle of 104.5° to make one water molecule. The hydrogen end is positively (+) charged,
and the oxygen is negatively (−) charged, so the whole molecule becomes a dipole (i.e.
two poles).
Water is critical in the body because:
1 Cellular metabolism depends on water. Many chemical reactions involve water, and
other reactions that do not use water directly work through the medium of water, i.e.
they would not happen if the components were dry
2 Water is vital for movement of substances around the body, notable by the blood and
the lymphatic systems, which are both important water-based transport systems. This is
because water is an excellent solvent, and many compounds are dissolved in water in
the human body. At a cellular level, water is instrumental in the movement of dissolved
substances across membranes
3 Water is also vital for elimination purposes, not just through urine but also through
the other water-based excretory systems: faeces and sweat. Many waste substances for
excretion are dissolved in water (e.g. urea)
4 Water helps to keep cells in their correct shape through internal pressures, and water
shortage in the cells causes shrinkage with a corresponding reduction in cellular
function.
The body requires between 1 and 7 litres of water intake per day to avoid dehydration.
The exact amount is dependent on levels of activity, the body and ambient temperatures,
the atmospheric humidity and other factors, but averages about 2–2.5 litres per day for
most people.
Water occupies three body compartments (Figure 6.2):
1 Intracellular i.e. inside the cells, the largest uid compartment, with an average of
about 30 litres of water
2 Extracellular i.e. outside or between the cells, also called tissue uid, with an average
of about 12 litres of water
3 Blood, both inside the blood cells and as the main constituent of plasma, with an aver-
age of about 3 litres of water.
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Fluid and electrolyte balance 123
I
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(
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f
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B
l
o
o
d
Figure 6.2 The three body water compartments. Water occupies the intracellular (inside the cells),
extracellular (outside the cells, also called tissue uid) and blood compartments.
CW CM CM CW
B
l
o
o
d
E
C
F
Cells
E
C
F
B
l
o
o
d
Oral
intake
Excretion
Figure 6.3 W ater passes from input (oral) to output (excretion) through the three body compart-
ments: blood, extracellular uid (ECF) and cells. CW: capillary wall; CM: cell membrane.
The overall total for all 3 compartments is about 45 litres. However, water is never
static in these compartments but moves across semipermeable membranes (SPMs) from
one compartment to another and is also replaced all the time by new water. The move-
ment and replacement sequence for water is as follows: new water entering the digestive
system from drinking is absorbed into the blood, then passes into extracellular uid, then
intracellular uid, back to extracellular uid, then blood again, either directly or via the
lymphatic system and is nally excreted (Figure 6.3).
Water moves down its own concentration gradient, i.e. it ows from areas of high con-
centration to areas of low concentration, a process called osmosis (Figure 6.4).
In addition, water will also move down pressure gradients. This process allows the
formation of tissue uid, derived from the net outow of uid from plasma at the high-
est pressure (arterial) end of the capillary. Meanwhile, uid is returning to the plasma,
caused by the net inow of uid at the lowest-pressure (venous) end of the capillary
(Figure 6.5).
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To
lymph
25 mmHg
25 mmHg
ECF
Venous Blood Arterial
12 mmHg
32 mmHg
end capillary end
Net inflow = Net outflow =
13 mmHg 7 mmHg
124 Fluid and electrolyte balance
SPM
Pure
water
Figure 6.4 Osmosis. In this example, pure water can be considered as a high concentration when
compared with the water in the same volume of a dense salt solution. Water will pass
through a semipermeable membrane (SPM, water ow shown by large arrow) until the
concentration of water is equal on both sides (i.e. water is moving down its own con-
centration gradient). This would increase the volume on the salt side and decrease the
volume on the pure water side. This process, called osmosis, is in action in the body and
is one means by which water moves from one compartment to another.
Figure 6.5 T he formation and return of tissue uid. Blood pressure in the capillary forces water out
of plasma into the extracellular uid (ECF). The pressure at the arterial end (32 mmHg) is
higher than the pressure at the venous end (12 mmHg) as blood pressure falls throughout
the capillary bed. The plasma proteins pull water back into the capillary with an attrac-
tive force (measured as a pressure of 25 mmHg) across the capillary bed. Therefore, at
the arterial end, there is a net outow from the capillary (i.e. 32 − 25 = 7 mmHg), which
constantly creates new tissue uid from blood plasma, and at the venous end there is a
net inow into the capillary (i.e. 25 − 12 = 13 mmHg), which constantly returns water
back to the blood plasma. The cells interchange water with the ECF, and some extra
water escapes the ECF to become lymphatic uid.
Water gain versus water lost
The uid balance chart
Humans gain and lose water constantly throughout the day. Some of these gains and
losses are known to us, and to some extent they are under our control. However, other
gains and losses are invisible, known as insensible gains or losses, and they function with-
out any conscious control by us.
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Fluid and electrolyte balance 125
In Fluid Out
Oral ˜ 2,500 ml
Metabolism ˜ 300–500 ml
Urine ˜°1,500 ml
Respiration ˜ 400 ml
Faeces ˜ 100–200 ml
Sweat ˜ 400–900 ml
Figure 6.6 Fluid balance. To balance on both sides, input (oral and metabolism) must equal output
(urine, faeces, respiration and sweat).
Humans gain water mainly through drinking (oral intake, about 2,500 ml per day) at
regular intervals throughout the day (Figure 6.6). This may be as plain water or avoured
as in tea, coffee or soft drinks. Even alcoholic drinks contain a large proportion of water,
notably beer. Drinking water is not only driven by thirst, but we take in uids for pleasure
and comfort. Thirst is the bottom line as a driving force for drinking uids, i.e. we should
never wait for thirst to occur before taking uids by mouth.
Apart from oral intake of uids, we also gain much smaller quantities (about 300–500
ml per day) from tissue metabolism. In Chapter 1, Figure 1.4 shows the formation of
metabolic water from energy production. At the end of the electron transport chain, two
hydrogen protons (2H
+
) join with oxygen (half O
2
, i.e. O) to produce water (H
2
O). This
quantity of water is too small to sustain life, so oral intake daily becomes essential.
Humans lose much of our daily uids through urine output, i.e. about 1,500 ml per
day (Figure 6.6 and Chapter 8), but an important loss is also through the skin by evapora-
tion of sweat (400–900 ml per day; see Chapters 1 and 10). This skin loss is part of our
insensible loss since it cannot be directly measured and has to be estimated. Other forms
of insensible loss are water vapour exhaled through the lungs (about 400 ml per day) and
water lost in faeces (about 100–200 ml per day). These insensible losses must be part of
the estimation.
Fluid balance records the amount of intake versus output to see if there is net gain
or loss of uid over a 24-hour period. Everything the patient drinks and passes as urine
for 24 hours is measured (in millilitres, ml) and recorded on a uid balance chart (see
Figure 6.7).
The insensible losses and gains are estimated and recorded, and a net total is achieved.
The object is to try to maintain a neutral uid balance over 24 hours, i.e. no excess of
intake, positive balance or excess of output, negative balance. Excess water intake or out-
put over one or two periods of 24 hours is not normally a problem in health as the body
makes adjustments (usually to urine output) to correct any decits (see Chapter 8).
Producing a lower urine output will conserve water if intake is low. Similarly, a higher
urine output will correct the balance if oral intake is high, e.g. the person drinking lots of
beer who then needs to go to the toilet frequently! The physiology of these changes can
be found in Chapter 8.
If uid balance recordings show a persistent excessive loss or gain of uid over several
days or weeks, this will indicate that there is a problem which needs investigation and
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Fluid Balance Chart
Date
24 hour total total
Oral/IV/enteral intake
Time Description Oral
IV/enteral
Total
Total
Oral/IV/enteral intake
Output
Time Urine Stool Other Total
Total
Output 24 hour
126 Fluid and electrolyte balance
Figure 6.7 The 24-hour uid balance chart. The patient’s input and output are measured and
entered on the chart along with the time. Totals for the 24 hours are calculated, taking
into account the insensible losses. IV: intravenous.
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Fluid and electrolyte balance 127
attention. Special considerations must be made in relation to uid balance under certain
circumstances, as follows:
• Patients on intravenous (IV) infusions and blood transfusions must have the volumes of
administered uid measured and included in the uid balance record. Giving uids by
intravenous route is an effective way of rehydrating patients who cannot drink enough,
but it raises the risk of rapid uid overload if renal output is poor. Intravenous overload
must be avoided as it puts unnecessary strain on the heart and kidneys.
• Patients who are designated as nil by mouth (NBM) i.e. eating or drinking nothing
for longer than a few hours require careful uid monitoring to identify any risk of
dehydration. Patients who cannot eat or drink long term will need alternative uid
replacement, either by IV route or by feeding tube. This includes coma patients and
those in critical care. Short term NBM, e.g. during the perioperative period, i.e.
just before and just after surgery is not normally a problem for otherwise healthy
individuals.
• Patients who are vomiting have little or no appetite for food and drink. A single bout
of vomiting is not a major problem for uid balance, but if this persists for more than a
few hours then uid balance may need to be monitored and an alternative to oral uid
input considered. Because of their smaller size, children may be particularly prone to
rapid dehydration due to persistent vomiting.
Central venous pressure (CVP) is a measure of the pressure of the venous blood return-
ing to the heart, i.e. the blood pressure in the major veins close to the right atrium.
Normal CVP is 3–10 mmHg (or 5–12 cmH
2
O). CVP will be altered (higher or lower)
when blood volume changes occur, and severe under-hydration or overhydration will
affect blood volumes, as will signicant blood loss. CVP changes may well occur
earlier than blood pressure changes, and as such they will act as an early warning of
problems with cardiovascular pressures. Therefore, CVP measurements may be impor-
tant when rehydrating patients rapidly. CVP is covered in more detail in Chapter 3 (see
page 56).
Fluid balance in children
The rst years of life are the time for the fastest changes occurring in water physiology, but
these changes slow down gradually throughout childhood until adult levels are reached.
The foetus has the highest total body water content per kg of body weight (i.e. as much as
90% of total body weight is water). The extracellular uid (ECF) compartment (see page
122) accounts for 65% of this water. This slowly adjusts by the time of birth when water
accounts for 75% of body weight. As they get older, there are further changes (e.g. water
makes up 65% of body weight by 6 months old and 60% by the end of the rst year).
Young children also have an ability to concentrate urine lower than an adult’s, which
again changes to near adult values by the age of 2 years, and they have less losses through
sweating than adults (mainly in boys). Contrary to this, children have a higher insensible
water loss (see page 125) than adults, and again this reduces as they get older. This all
amounts to a faster water turnover in very young children than at any other age, especially
within the rst few weeks of life.
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128 Fluid and electrolyte balance
Problems of water imbalance
Oedema
Oedema is excessive uid in the tissues causing the tissues to swell. This excess of water
is mostly in the extracellular (tissue uid) compartment, with very little extra uid in
the intracellular compartment, as the cell membranes would rupture if they took on too
much water. Some extra uid can be housed in the blood compartment, but this is limited
because excess water in circulation boosts blood volume and thus raises blood pressure,
and the excess would normally be removed by the kidneys. High blood pressure (hyper-
tension) can be treated using drugs that promote water loss from the blood compartment
through the kidneys (drugs called diuretics; see
Chapters 3 and 8).
Oedema results either from the intake of water persistently exceeding the output, i.e.
a positive uid balance, over several days or weeks or due to the body’s failure to handle
water properly, i.e. heart or kidney failure. A poor diagnostic outcome in acute renal
failure has been linked to positive uid balance (Payen et al. 2008). In normal health, it is
difcult to achieve a persistent positive balance because the kidneys remove the excess
water and therefore improve the output, thus restoring the balance, i.e. the more water
drunk, the more urine passed (see Chapter 8). There are also local causes of oedema, i.e.
specic causes of oedema occurring in a particular part of the body, e.g. ascites, which
is oedema in the peritoneal space resulting in abdominal swelling. Oedema is therefore
a pathological state, i.e. a symptom of some underlying pathology. The problem of tissue
oedema can be narrowed down to ve causes:
1 Obstructed venous return of blood, as seen in heart failure. Right heart failure causes
a backlog of blood waiting to get into the right side of the heart, and this backlog can
increase venous pressure sufciently that the veins cannot accept any more blood
from the capillaries, which then engorge and leak uid into the tissues (called systemic
oedema). Left heart failure causes a backlog of blood from the lungs waiting to enter
the left ventricle, and this increases the venous blood pressure coming from the lungs,
which then causes uid to leak out of the capillaries into the lungs. Fluid then collects
in the alveoli (called pulmonary oedema). At a more local level, congestion of venous
return can occur if there are any tight restrictions to a limb preventing venous return,
e.g. an inated blood pressure cuff left on for too long
2 Excess sodium in circulation and the tissues. The kidneys remove excess sodium from
the body by excreting it in the urine (see Chapter 8). Kidney problems can cause a fail-
ure to remove sodium, which then accumulates in the blood (called hypernatraemia)
and in the tissues. One property of sodium is that it attracts water. If sodium is retained
in the body, so will be water, causing an oedematous state. Excessive sodium intake
will also contribute to oedema, especially again if the kidneys are unable to excrete it
all. The hormone aldosterone (from the adrenal cortex) conserves sodium in the body
(see Chapter 8), and excess of this hormone may also contribute to a hypernatraemia
and thus oedema
3 Inadequate plasma protein concentration. One of the vital functions of proteins in
plasma is to attract uid back to the blood from the extracellular compartment (see
plasma proteins, Chapter 4, page 67). This attracting force draws water back at a pres-
sure of 25 mmHg along the capillary length (Figure 6.5). Capillaries have semiperme-
able walls to allow this water to pass through. In the event of low plasma proteins, this
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Fluid and electrolyte balance 129
attracting force is reduced, i.e. the 25 mmHg pressure drops, and water is then added
to the tissues faster than it is withdrawn, and thus it accumulates in the tissues. Low
plasma proteins can occur in starvation – especially very low dietary protein intake –
and in liver disease since the liver produces albumen, the most abundant of the plasma
proteins (see Chapter 4, page 67).
4 Obstructed lymphatic drainage of the tissues. Normally, the lymphatic system drains
away any excess uid from the extracellular tissue space, thus preventing any uid
build-up (Figure 6.5). Anything that obstructs this drainage will cause oedema. Exam-
ples of this are new growths or inammation of the lymphatic glands (or nodes) and
any external pressures on the lymphatic vessels.
5 Increased capillary wall permeability. This occurs as a process of inammation. The
normally permeable capillary wall becomes even more permeable (i.e. ‘leaky’) as a
result of the inammatory process. This causes more water to escape into the tissues.
Proteins in the plasma are normally too big to pass through the capillary wall, but in
inammation capillaries can become leaky enough to allow proteins to escape the
plasma into the tissue uid. The protein-rich environment surrounding the capillaries
attracts even more water out of the capillaries and causes local swelling (localised
oedema) of the tissues. Swelling is one of the cardinal signs of inammation, along
with redness, pain and heat (see Chapter 10).
Pulmonary oedema is a very serious problem that is recognised by acute dyspnoea
(breathlessness), severe cyanosis (due to shunting; see Chapter 5, page 109) and cough.
Fluid collects in the air sacs (alveoli), reducing the surface area available for oxygen and
carbon dioxide exchange (see Chapter 5, page 99). The surprise is the fact that symp-
toms of pulmonary oedema occur so quickly after left heart failure has occurred. This is
because the distance between the left heart and the lungs is very short, and any backlog
of blood ow reaches the lungs rapidly. Loop diuretics (see Chapter 8) are drugs used to
treat pulmonary oedema caused by left ventricular failure. They work by promoting uid
loss from the kidneys, which in turn withdraws uid from the lungs.
Systemic uid overload can be identied by the following observations: weight gain,
raised CVP and raised blood pressure, fast pulse and breathing rates, raised urine output
if the kidneys are functioning normally or lower urine output if the heart is in failure
and oedematous swelling of tissues (
Mooney 2007). The swelling of the tissues is mostly
in the limbs (i.e. tissues farthest from the heart), especially the legs and ankles as uid
drains downwards to the lowest points due to gravity. This gravity-driven oedema is
known as dependent oedema. Dependent oedema also occurs in the back and sacral
areas in patients lying on their backs while conned to bed and may contribute towards
pressure sore formation. Systemic oedema can also be recognised by applying gentle
pressure from a nger to create a shallow dent. Normally the dent should rebound back
to normal levels when the nger is removed. If this dent remains the problem is systemic
oedema. Reduction of this oedema in the legs (the commonest site) can be assisted by
seating the patient with legs elevated, thus employing gravity to gradually drain the tis-
sue uid back into circulation. Also, it can be prevented by the application of bandages
or pressure stockings, which provide enough external pressure to stop uid leakage into
the tissues.
Cerebral oedema is swelling of the brain, due to inammation of the brain or its cov-
erings, or sometimes due to head injury, i.e. concussion, which is ‘brain shaking’ (see
Chapter 11). Because the brain is encased in the skull, the problem with swelling is the
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130 Fluid and electrolyte balance
risk of increasing intracranial pressure (raised intracranial pressure, RICP) which will put
the brain under pressure (see Chapter 11, page 239).
Dehydration
Insufcient uid in any or all of the three tissue compartments is called dehydration. This
is due to the following three conditions:
1 A persistent negative uid balance, where output exceeds input for more than a few
days. The exact time it takes to establish dehydration depends on how poor the uid
intake is or how large the uid loss is. A total lack of all uid intake will start to cause
minor dehydration within hours. Thirst is the normal body response to early dehydra
-
tion (i.e. before tissue harm is caused) in an attempt to correct the uid balance by
increasing oral intake. The causes of negative uid loss include persistent vomiting,
diarrhoea, both diabetes mellitus and diabetes insipidus (see Chapter 8), polyurea (a
large urine output) (see Chapter 8), excessive use of diuretic drugs (see Chapter 8) and
fever (see Chapter 1)
2 Extensive blood loss causing hypovolaemia (low blood volume), usually rapidly (i.e.
before the volume loss can be corrected). Blood volume losses can be from internal or
external haemorrhage or plasma loss from extensive burns (see Chapter 10)
3 Maldistribution of uid (i.e. uid is unevenly distributed around the body), with accu-
mulation in some areas while other areas suffer dehydration (Kreimeier 2000).
Severe dehydration causes weight loss, reduced pulse pressure (see Chapter 3, page 47),
rapid shallow breathing, fast weak pulse, low volumes of highly concentrated urine, dry
wrinkled skin, poor production of thick saliva from a dry mouth (see page 307), thirst and
shrunken tissues (Mooney 2007). Folds of skin formed by gentle external digital pressure
remain as folds when the nger pressure is removed, instead of returning to the original
at position rapidly as expected in full hydration. This is known as testing for skin turgor.
Tissues around the eyes shrink and the eyes appear sunken in their sockets in severe
depletion of uids. The mouth in particular becomes very dry, with little or no saliva. This
causes the mouth to become very dirty and at risk of oral infections. This is partly because
oral uid intake is an important cleansing mechanism for the mouth, and absence pre-
disposes the mouth to poor hygiene. The dehydrated dirty mouth becomes very hard to
clean properly. An oral infection leads to throat, chest, middle ear and even brain infec-
tions if left untreated. Management of dehydration is, of course, rehydration, preferably by
mouth. However, some patients require an alternative route for uid replacement, which
is normally the intravenous (IV) route. These patients include:
• those who are severely dehydrated and need urgent rehydration, and oral intake is not
adequate enough to rehydrate the patient quickly
• those unable to drink and swallow properly (e.g. unconscious, nil by mouth and post-
operative patients).
Electrolyte balance
Water balance in the body is linked to the balance of electrolytes within the three body
compartments (cells, tissue uid and blood; Figure 6.8).
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Intracellular
fluid
180
Mg
˜°˜
Organic
phosphate
HPO
4
2˝
K
˜
SO
4
˝
Protein
˝
180
Tissue
fluid
Plasma
150
Na
˜
Cl
˝
HCO
3
˝
K
˜
150 150
Na
˜
Cl
˝
HCO
3
˝
Protein
˝
K
˜
150
150
85
65
40 40
15
5 5
0
0
0
0 0
0
K
˜°
˛°ISO mmol/L
Na
˜°
=
°
145 mmol/L
As for tissue
Cations
fluid, plus
˜
Mg
2˜°
˛ 30 mmol/L
protein ˛ 15 mmol
Cl
˝°
˛
°
110 mm/L
HCO
3
˝
˛°25 mmol/L
Protein
˝°
˛ 65 mmol/L
Bicarbonate ˛
°
40 mmol/L
Anions
Org. Phos.
°
˛ 95 mmol/L
(HCO
3
˝
)
˝
Sulphate
°
˛ 20 mmol/L
(SO
4
˝
)
Fluid and electrolyte balance 131
Figure 6.8 The relative concentrations and values of the major electrolytes are shown in the three
body compartments.
Electrolytes are ionised elements in solution. They are usually minerals derived from
the diet, such as sodium, potassium and calcium. In Chapter 7, Table 7.4 lists the major
minerals in the diet. Minerals are commonly consumed in compound form, i.e. two or
more minerals elements bound together chemically, e.g. NaCl, sodium chloride. In this
form, they are usually electrically neutral (i.e. they have no charge). In the body, these
compounds are often dissolved in water (e.g. in blood and tissue uid), where they nor-
mally disassociate (i.e. break up) into their component elements (e.g. NaCl → Na
+
+
Cl
−
). In this process, each of the separate elements takes on a charge. For example, in the
dissociation of sodium chloride, sodium (Na) loses an electron and becomes a positively
charged ion (Na
+
). Positively charged ions are known as cations. Chlorine (Cl) gains an
electron and becomes a negatively charged ion (now called chloride, Cl
−
). Negatively
charged ions are known as anions. These are now electrolytes, dissolved in solution. The
electrolyte contents and amounts that are found in body uids are shown in Figure 6.8.
From these gures, it becomes obvious that the bulk of the potassium is inside the cells
(intracellular), while the bulk of the sodium is outside the cells (extracellular), e.g. tissue
uid and blood plasma.
The majority of the electrolytes in the body are minerals, such as sodium (Na
+
), potas-
sium (K
+
), chloride (Cl
−
), magnesium (Mg
2+
) and calcium (Ca
2+
). Magnesium (Mg
2+
) and
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