Fluid balance

1. Fluid Balance
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
R.D. Gardi Medical College, Ujjain, Madhya Pradesh.
Email: dr.goothy@gmail.com

2. Introduction
 Homeostasis depends on maintaining a balance between the input
and the output of all constituents in the internal fluid environment.
The kidneys control ECF volume by maintaining salt balance and
control ECF osmolarity by maintaining water balance.
The kidneys maintain this balance by adjusting the output of salt and
water in the urine as needed to compensate for variable input and
abnormal losses of these constituents.

3. Balance concept
 The quantity of any particular substance in the ECF is a readily available internal pool.
The amount of the substance in the pool may be increased either by transferring more in from the
external environment (usually by ingestion) or by metabolically producing it within the body.
Substances may be removed from the body by being excreted to the outside or by being used up
in a metabolic reaction.
If the quantity of a substance is to remain stable within the body, its input through ingestion or
metabolic production must be balanced by an equal output through excretion or metabolic
consumption.
This relationship, known as the balance concept, is extremely important in maintaining
homeostasis.

4. Balance concept
 Not all input and output pathways apply to every body-fluid
constituent.
For example, salt is not synthesized or used up by the body, so
maintaining a stable salt concentration in the body fluids depends
entirely on a balance between salt ingestion and salt excretion

5. Balance concept
 For some ECF constituents, the ECF pool is further altered by
transferring this specific constituent into or out of storage within the
body.
If the body as a whole has a surplus or deficit of a particular stored
substance, the storage site can be expanded or partially depleted to
maintain the ECF concentration of the substance within
homeostatically prescribed limits.

6. Balance concept
For example, after absorption of a meal, when more glucose is
entering the plasma than is being consumed by the cells, the extra
glucose can be temporarily stored, in the form of glycogen, in muscle
and liver cells.
This storage depot can then be tapped between meals as needed to
maintain the plasma glucose level when no new nutrients are being
added to the blood by eating

7. Balance concept
When the total body input of a particular substance equals its total body output,
a stable balance exists.
When the gains via input for a substance exceed its losses via output, a positive
balance exists.
 The result is an increase in the total amount of the substance in the body.
 In contrast, when losses for a substance exceed its gains, a negative balance
exists and the total amount of the substance in the body decreases

9. Fluid Balance
Water is by far the most abundant component of the body, averaging
60% of body weight but ranging from 40% to 80%.
The H2O content of an individual remains fairly constant because the
kidneys efficiently regulate H2O balance, but the percentage of body
H2O varies from person to person.
Why?

10. Fluid Balance
 The main reason for the wide range in body H2O among individuals is
their variable amount of adipose tissue (fat).
 Adipose tissue has a low H2O percentage compared to other tissues.
Plasma, as you might suspect, is more than 90% H2O. Even the soft
tissues such as skin, muscles, and internal organs consist of 70% to 80%
H2O. The relatively drier skeleton is only 22% H2O.
Fat, however, is the driest tissue of all, having only 10% H2O content.

11. Fluid Balance
 Accordingly, a high body H2O percentage is associated with
leanness and a low body H2O percentage with obesity because a
larger proportion of the overweight body consists of relatively dry
fat.

13. Minor ECF compartments
 Two other minor categories are included in the ECF: lymph and transcellular fluid.
Lymph is the fluid being returned from the interstitial fluid to the plasma by means of the
lymphatic system, where it is filtered through lymph nodes for immune defense purposes.
Transcellular fluid consists of a number of small, specialized fluid volumes, all of which are
secreted by specific cells into a particular body cavity to perform some specialized function.
Transcellular fluid includes cerebrospinal fluid (surrounding, cushioning, and nourishing the
brain and spinal cord); intraocular fluid (maintaining the shape of and nourishing the eye);
synovial fluid (lubricating and serving as a shock absorber for the joints); pericardial,
intrapleural, and peritoneal fluids (lubricating movements of the heart, lungs, and intestines,
respecively); and the digestive juices (digesting ingested foods).

14. Plasma and interstitial fluid
 The two components of the ECF—plasma and interstitial fluid—are
separated by the walls of the blood vessels.
However, H2O and all plasma constituents except for plasma proteins are
continuously and freely exchanged between plasma and interstitial fluid
by passive means across the thin, pore-lined capillary walls.
Accordingly, plasma and interstitial fluid are nearly identical in
composition, except that interstitial fluid lacks plasma protei

15. ECF and ICF
 The composition of the ECF differs considerably from that of the ICF.
(1) the presence of cell proteins in the ICF that cannot permeate the enveloping
membranes to leave the cells and
(2) the unequal distribution of Na and K and their attendant anions (negatively
charged ions) as a result of the action of the membrane-bound Na–K pump
present in all cells.
 Because this pump actively transports Na out of and K into cells, Na is the
primary ECF cation (positively charged ion) and K1 is the primary ICF cation

16. Fluid balance by regulation of ECF volume and osmolarity
 All exchanges of H2O and other constituents between the ICF and
the external world must occur through the ECF, so the ECF serves as
an intermediary between the cells and the external environment.
Water added to the body fluids always enters the ECF first, and fluid
always leaves the body via the ECF.

17. Fluid balance by regulation of ECF volume and osmolarity
 Two factors are regulated to maintain fluid balance in the body: ECF
volume and ECF osmolarity.
Although regulation of these two factors are interrelated, both
depending on the relative NaCl and H2O loads in the body, the
reasons why and the mechanisms by which they are controlled are
notably different

18. Control of ECF volume
 A reduction in ECF volume causes a fall in arterial blood pressure by
decreasing plasma volume.
Conversely, expanding ECF volume raises arterial blood pressure by
increasing plasma volume.

19. Control of ECF volume
 A reduction in ECF volume causes a fall in arterial blood pressure by
decreasing plasma volume.
Conversely, expanding ECF volume raises arterial blood pressure by
increasing plasma volume.

20. Short-term regulation of blood pressure
 The baroreceptor reflex alters both cardiac output and total
peripheral resistance to adjust blood pressure in the proper direction
through autonomic nervous system effects on the heart and blood
vessels.
Cardiac output and total peripheral resistance are both increased to
raise blood pressure when it falls too low.

21. Short-term regulation of blood pressure
 Fluid shifts occur temporarily and automatically between plasma and
interstitial fluid as a result of changes in the balance of hydrostatic and
osmotic forces acting across the capillary walls that arise when plasma
volume deviates from normal.
A reduction in plasma volume is partially compensated for by a shift of
fluid out of the interstitial compartment into the blood vessels, expanding
the circulating plasma volume at the expense of the interstitial
compartment

22. Short-term regulation of blood pressure
 These two measures provide temporary relief to help keep blood
pressure fairly constant, but they are not long-term solutions.
 Furthermore, these short-term compensatory measures have a
limited ability to minimize a change in blood pressure.

23. Long-term regulation of blood pressure
 Long-term regulation of blood pressure rests with the kidneys and
the thirst mechanism, which control urinary output and fluid intake,
respectively.
Of these measures, control of urinary output by the kidneys is the
most crucial for maintaining blood pressure.

24. Control of salt balance is important to regulate ECF volume
 sodium and its accompanying anion chloride account for more than 90% of the
ECF osmotic activity.
As the kidneys conserve salt (NaCl) by actively reabsorbing Na+, with Cl-
passively following, automatically conserves H2O because H2O comes along
osmotically.
This retained salt solution is isotonic.
The more salt in the ECF, the more H2O in the ECF.
The concentration of salt is not changed.

25. Salt input = salt output
 The only avenue for salt input is ingestion, which typically is well in
excess of the body’s need for replacing obligatory salt losses.
In our example of a typical daily salt balance, salt intake is 10 g per
day; yet 0.5 g of salt per day is adequate to replace the small
amounts of salt usually lost in sweat and feces.

26. Salt input = salt output
 The only avenue for salt input is ingestion, which typically is well in
excess of the body’s need for replacing obligatory salt losses.
In our example of a typical daily salt balance, salt intake is 10 g per
day; yet 0.5 g of salt per day is adequate to replace the small
amounts of salt usually lost in sweat and feces.

27. Salt input = salt output
Carnivores (meat eaters) and omnivores (eaters of meat and plants, like humans), which
naturally get enough salt in fresh meat (meat contains an abundance of salt-rich ECF),
normally do not display a physiological appetite to seek additional salt.
 In contrast, herbivores (plant eaters), which lack salt naturally in their diets, develop
salt hunger and will travel miles to a salt lick.
Humans have a hedonistic (pleasure-seeking) rather than a regulatory appetite for salt;
we consume salt because we like it rather than because we have a physiological need.

30. Control of ECF osmolarity
Regulating ECF osmolarity is important in preventing changes in cell volume.
The osmolarity of a fluid is a measure of the concentration of the individual
solute particles dissolved in it. The higher the osmolarity, the higher the
concentration of solutes or, to look at it differently, the lower the concentration of
H2O
water tends to move by osmosis down its own concentration gradient from an
area of lower solute (higher H2O) concentration to an area of higher solute (lower
H2O) concentration

31. Control of ECF osmolarity
Regulating ECF osmolarity is important in preventing changes in cell volume.
The osmolarity of a fluid is a measure of the concentration of the individual
solute particles dissolved in it. The higher the osmolarity, the higher the
concentration of solutes or, to look at it differently, the lower the concentration of
H2O
water tends to move by osmosis down its own concentration gradient from an
area of lower solute (higher H2O) concentration to an area of higher solute (lower
H2O) concentration

32. Control of ECF osmolarity
Na+ and accompanying Cl-, being by far the most abundant solutes in the ECF in terms of
numbers of particles, account for most ECF osmotic activity.
 In contrast, K+ and its accompanying intracellular anions are responsible for ICF osmotic
activity.
Even though small amounts of Na+ and K+ passively diffuse across the plasma membrane all the
time, these ions behave as if they were nonpenetrating because of Na+–K+ pump activity.
 Any Na+ that passively diffuses down its electrochemical gradient into the cell is promptly
pumped back outside, so the result is the same as if Na+ were barred from the cells.

33. Hypertonicity of ECF
 The excessive concentration of ECF solutes, is usually associated with
dehydration, or a negative free H2O balance.
Insufficient H2O intake, such as might occur during desert travel or might
accompany difficulty in swallowing
Excessive H2O loss, such as might occur during heavy sweating,
vomiting, or diarrhea
Diabetes insipidus, a disease characterized by a deficiency of vasopressin

34. Diabetes insupidus
 Vasopressin (antidiuretic hormone) increases the permeability of the distal and collecting tubules to H2O
and thus enhances water
 conservation by reducing the urinary output of water
 Without adequate vasopressin in diabetes insipidus, the kidneys cannot conserve H2O because they cannot
reabsorb H2O from the late parts of the nephron. Such patients typically produce up to 20 liters of very
dilute urine daily, compared to the normal average of 1.5 liters per day.
 Unless H2O intake keeps pace with this tremendous loss of H2O in the urine, the person quickly
dehydrates.
 Such patients complain that they spend an extraordinary amount of time day and night going to the
bathroom and getting drinks. Fortunately, they can be treated with desmopressin administered by nasal
spray.

35. Hypertonicity of ECF
 when ECF becomes hypertonic, the cells will shrink as water moves out
of the cells.
Of particular concern is that considerable shrinking of brain neurons
disturbs brain function, which can be manifested as mental confusion and
irrationality in moderate cases and delirium, convulsions, or coma in more
severe hypertonic conditions
Circulatory problems may range from a slight lowering of blood pressure
to circulatory shock and death

36. Hypotonicity of ECF
 Hypotonicity of the ECF is associated with overhydration—that is,
excess free H2O
Patients with renal failure who cannot excrete dilute urine become
hypotonic when they consume relatively more H2O than solutes.
Hypotonicity occurs transiently in healthy people if H2O is rapidly
ingested to such an excess that the kidneys cannot respond quickly
enough to eliminate the extra H2O

37. Hypertonicity of ECF- water intoxication
 when ECF becomes hypotonic, the cells will swell as water moves into
the cells.
swelling of brain cells also leads to brain dysfunction. Symptoms include
confusion, irritability, lethargy, headache, dizziness, vomiting, drowsiness,
and in severe cases, convulsions, coma, and death.
Nonneural symptoms of overhydration include weakness caused by
swelling of muscle cells and circulatory disturbances including
hypertension and edema.

38. Source of water input
 Drinking liquids (appx one liter per day)
Eating solids (meat- 75% water , fruits and vegetables 60-96%)
Metabolically produced water
The average H2O intake from these three sources totals 2600 mL per day.
Another source of H2O often employed therapeutically is the intravenous
infusion of fluid.

39. Source of water input
 Drinking liquids (appx one liter per day)
Eating solids (meat- 75% water , fruits and vegetables 60-96%)
Metabolically produced water
The average H2O intake from these three sources totals 2600 mL per day.
Another source of H2O often employed therapeutically is the intravenous
infusion of fluid.

40. Source of water output
 Nearly a liter of H2O daily without being aware of it.
This insensible loss (loss of which the person has no sensory awareness)
occurs from the lungs and non sweating skin.
During respiration, inspired air becomes saturated with H2O within
the airways. This H2O is lost when the moistened air subsequently expires
Normally, you are not aware of this H2O loss, but you can recognize it on
cold days when H2O vapor condenses so that you can “see your breath.

41. Source of water output
 Sensible loss (loss of which the person is aware) of H2O from the skin
occurs through sweating, which represents another avenue of H2O output.
Another passageway for H2O loss from the body is through the feces.
Normally, only about 100 mL of H2O are lost this way each day.
 By far the most important output mechanism is urine excretion, with 1500
mL (1.5 liters) of urine being produced daily on average
The total H2O output is 2600 mL/day

42. Factors regulated to maintain water balance
 On the intake side, thirst influences the amount of fluid ingested;
on the output side, the kidneys can adjust how much urine is formed.
Controlling H2O output in the urine is the most important mechanism
in controlling H2O balance

44. Thirst
 Thirst is the subjective sensation that drives you to ingest H2O.
The thirst center is located in the hypothalamus close to the
vasopressin-secreting cells
Vasopressin secretion and thirst are both stimulated by a free H2O
deficit and suppressed by a free H2O excess.

45. Hypothalamic osmoreceptors
 The predominant excitatory input for both vasopressin secretion and thirst
comes from hypothalamic osmoreceptors located near the vasopressin-
secreting cells and thirst center.
As ECF osmolarity increases (too little H2O) and the need for H2O conservation
increases, vasopressin secretion, and thirst are both stimulated.
As a result, reabsorption of H2O in the distal and collecting tubules is increased
so that urinary output is reduced and H2O is conserved, while H2O intake is
simultaneously encouraged.

46. Left atrial volume receptors.
 left atrial volume receptors.
 Located in the left atrium, these volume receptors respond to pressure-
induced stretch caused by blood flowing through, which reflects the ECF
volume—that is, they monitor the “fullness” of the vascular system
In response to a major reduction in ECF volume (.7% loss of volume), left
atrial volume receptors reflexly stimulate both vasopressin secretion and
thirst.

47. Angiotensin II
 Conserves sodium.
 Increase in ADH release
Increase in aldosterone release
Stimulates thirst sensation