(1) The human body is 50-75% water which is regulated to maintain a constant volume. Water intake and output must be equal to maintain homeostasis. (2) The kidneys, lungs, skin and digestive system are involved in water regulation through urine production, evaporation, perspiration and feces. Disruptions can cause dehydration or water overload. (3) Electrolytes like sodium, potassium, calcium and magnesium are also tightly regulated by hormones and organ systems to maintain normal blood levels and cellular function. Imbalances can impact nerve and muscle function.

1. Fluid and electrolyte balance
Body fluids:
The adult human body is at least 50% water by weight. In infants, water makes
up as much as 75% of body weight.
Because the water content of adipose tissueis relatively low, people with more
adipose tissuehave a smaller proportion of water in their bodies.

2. Regulationof water content:
Before regulation, let’s discuss aboutwater input and output,
The body’s water content is regulated so that its total volumeremains constant.
Thus, the volume of water taken into the body is equal to the volume lost each
day.
The total volume of water entering the body each day is 1500–3000 mL.

3. i.water input:
Although fluid consumption is heavily influenced by habit and by socialsettings,
water ingestion does depend, at least in part, on thirst regulatory mechanisms.
The sensation of thirst results primarily from an increase in the osmolality of
the extracellular fluids and from a reduction in plasma volume, which lowers
blood pressure.

4. The thirstsensation is temporarily reduced after a person drinks a smallamount
of liquid. At least two factors are responsiblefor this temporary interruption of
the thirstsensation. First, when the oral mucosa becomes wet after it has been
dry, sensory neurons conduct action potentials to the thirst center of the
hypothalamus and temporarily decrease the sensation of thirst.
Second, consumed fluid increases the digestive tract volume, and stretch of the
digestive tract wall initiates sensory action potentials in stretch receptors. The
sensory neurons conduct action potentials to the thirst center of the
hypothalamus, where they temporarily suppress the sensation of thirst
Longer-termsuppression of thirstresults when extracellular fluid osmolality
and blood pressurearewithin their normal ranges.
ii.water output:
1.Kidneys:
The kidneys are the primary organs thatregulate the composition and volume
of body fluids by controlling the volume and concentration of water excreted
in the formof urine.
2.Evaporation:Respiratory passages-
The volume of water lost through the respiratory passages depends on the
temperature and humidity of the air, body temperature, and the volume of air
expired.
Skin-
Insensible perspiration-
Water lostthrough thediffusionand evaporation of water from theskin is called
insensible perspiration and it regulates heat loss.
Sensible perspiration-
Sweat, or sensible perspiration, is secreted by the sweat glands in contrast to
insensible perspiration, it contains solutes.
During exercise, elevated environmentaltemperature, or fever, the volume of
sweatincreases substantially and plays an important role in heat loss.

5. NOTE: During severedehydration, the change can be great enough to cause
blood viscosity to increase substantially, which increases the heart’s workload
enough that heart failure can result.
3.Feces-
Relatively little water is lost by way of feces from the digestive tract. Exceptions
areseverevomiting and diarrhea,whichcan resultin a largevolumeof fluid loss.
1.Regulation of Extracellular Fluid osmolality:
Increased blood osmolality affects hypothalamic neurons, and decreased blood
pressure (BP) affects baroreceptors in the aortic arch, carotid sinuses, and atrium.
as a result of these stimuli, the rate of antidiuretic hormone (ADH) secretion from
the posterior pituitary increases, which increases water reabsorption by the kidneys.

7. (1) Blood osmolality is in the normal range. (2) blood osmolality
increases outside the normal range, which causes homeostasis to be
disturbed. (3) The control center responds to the change in blood
osmolality. (4) The control center causes ADH to be secreted, which
increases water reabsorption at the distal convoluted tubule and the
collecting duct. (5) These changes cause blood osmolality to decrease.
(6) blood osmolality returns to the normal range and homeostasis is
restored.

8. 2.Regulationof Extracellular fluidvolume:

9. (1) Blood volume is in the normal range.
(2) Blood volume increases outside the normal range, which causes
homeostasis to be disturbed.
(3) The control centers respond to the change in blood volume.
(4) The control centers cause ADH and aldosterone secretion to
decrease, which reduces water reabsorption. The control centers also
cause dilation of renal arteries, which increases urine production. The
heart secretes ANH, which also increases urine production.
(5) These changes cause blood volume to decrease.
(6) Blood volume returns to the normal range and homeostasis is
restored
3.Regulationof specific electrolytes in the extracellular fluid:
 Electrolytes are molecules or atoms with an electrical charge.
 The major extracellular ions are Na+, Cl-, K+, Ca2+, Mg2+, and phosphate
ions (Po43-). Electrolytes are in the food and water we ingest.
 Organs—such as the kidneys and, to a lesser degree, the liver, skin, and
lungs—remove them from the body.
 The concentrations of electrolytes in the extracellular fluid are regulated,
so that they do not change unless the individual is growing, gaining
weight, or losing weight.
 The regulation of each electrolyte involves the coordinated participation
of several organ systems.
i.Regulationof sodium-
Sodium ions are the dominantextracellular cations. Because of their abundance
in the extracellular fluid, they exert substantial osmotic pressure
Although less than 0.5 g is required to maintain homeostasis, the average
individual ingests approximately 10–15 g of sodium chloride daily.
Therefore, regulation of the body’s Na+ content depends primarily on the
excretion of excess quantities of Na+.
On the other hand, when Na+ intake is very low, the mechanisms for conserving
Na+ in the body take effect.

11. A reduced plasma Na+ concentration leads to hyponatremia; an elevated
plasma Na+ concentration results in hypernatremia
ii.Regulationof Chloride-
Theelectrical attraction of anionsand cations makesit difficultto separatethese
charged particles.
Consequently, the regulatory mechanisms that influence the concentration of
cations in the extracellular fluid also influence the concentration of anions.
The mechanisms that regulate Na+, K+, and Ca2+ levels in the body are
important in influencing Cl- levels.
Because Na+ predominates, the mechanisms that regulate extracellular Na+
concentration are the most important in regulating the extracellular Cl-
concentration.

12. iii.Regulation of potassium ion-
Because the concentration gradient of potassium ions across the plasma
membrane has a major influence on the resting membrane potential of
electrically excitable cells, K+ concentrations are tightly regulated.
They are actively reabsorbed in the proximal convoluted tubules and actively
secreted in the distal convoluted tubules and collecting ducts.

13. (1) Blood K+ is in the normal range. (2) Blood K+ increases outside the
normal range, which causes homeostasis to be disturbed. (3) The
control center responds to the change in blood K+. (4) The control
center causes aldosterone to be secreted, which increases K+ secretion
at the distal convoluted tubule and the collecting duct. (5) These
changes cause blood K+ to decrease. (6) Blood K+ returns to the
normal range and homeostasis is restored.
An abnormally low level of K+ in the extracellular fluid is called hypokalemia; an
abnormally high level of K+ in the extracellular fluid is called hyperkalemia.

14. iv.Regulation of calcium:
Almost 99% of total body calcium is contained in bone
The kidneys, digestive tract, and bones are important in maintaining
extracellular Ca2+ levels
They are actively reabsorbed in the proximal convoluted tubules, loop of henle
and Distal convoluted tubule
a) Increase of Calcium levels:
Parathyroid hormone (PTH), secreted by the parathyroid glands, increases
extracellular Ca2+ levels and reduces extracellular phosphate levels.
The rate of parathyroid hormone secretion is regulated by extracellular Ca2+
levels. Parathyroidcell receptorsactas extracellular Ca2+level sensors.Elevated
Ca2+levels inhibit parathyroidhormonesecretion, and reduced levels stimulate
it.
Parathyroid hormone causes increased osteoclast activity, which results in the
degradation of bone and the release of Ca2+ and phosphate ions into body
fluids. Parathyroid hormone increases the rate of Ca2+ reabsorption from
nephrons in the kidneys and increases the concentration of phosphate ions in
the urine. It also increases the rate at which vitamin D is converted to 1,25-
dihydroxycholecalciferol, or active vitamin D. Active vitamin D acts to increase
Ca2+ absorption across the intestinal mucosa.
Increased in Calcium levels leads to decreasesecretion of parathyroid hormone
by feedback mechanism.
b) Decrease of calcium levels:
Calcitonin,whichissecreted by theparafollicularcells ofthe thyroidgland, helps
reduce extracellular Ca2+ levels.
The major effect of calcitonin is in bone, where it inhibits osteoclasts. Thus,
calcitonin prevents bone degeneration, which keeps blood Ca2+ levels from
rising.
Hypocalcemia is a below-normal level of Ca2+ in the extracellular fluid, and
hypercalcemia is an above-normal level of Ca2+ in the extracellular fluid

15. Changesin the extracellular concentration of Ca2+markedly affectthe electrical
properties of excitable tissues. Hypocalcemia increases theplasma membrane’s
permeability to Na+. As a result, nerveand muscletissuesundergo spontaneous
action potential generation. Hypercalcemia decreases the plasma membrane’s
permeability to Na+, preventing normal depolarization of nerve and muscle
cells. High extracellular Ca2+ levels cause the deposition of calcium carbonate
salts in soft tissues, resulting in irritation and inflammation of those tissues

16. V.Regulation of Magnesium:
Most of the magnesium in the body is stored in the bones or intracellular fluid.
Less than 1% of the total are ions in the extracellular fluid.
Magnesium ions are cofactors for intracellular enzymes, such as the sodium-
potassium pump involved in actively transporting Na+ out of and K+ into cells

17. (1) Blood Mg2+ is in the normal range. (2) Blood Mg2+ increases
outside the normal range, which causes homeostasis to be disturbed.
(3) The control center responds to the change in blood Mg2+. (4) The
control center prevents Mg2+ reabsorption in the kidney. (5) These
changes cause blood Mg2+ to decrease. (6) Blood Mg2+ returns to the
normal range and homeostasis is restored
Hypomagnesemiais a below-normalblood level of magnesium, and
hypermagnesemia is an above-normalblood level of magnesium

18. vi.Regulation of phosphates:
About 85% of the phosphate in the body is in the form of calcium phosphate
salts in bone (hydroxyapatite) and teeth. Most of the remaining phosphate is
inside cells.
Many of the phosphate ions are covalently bound to other organic molecules.
Phosphate ions are bound to lipids (to form phospholipids), proteins, and
carbohydrates, and they are important components of DNA, RNA, and
ATP. Phosphates also play important roles in regulating enzyme activity, and
phosphate ions dissolved in the intracellular fluid act as buffers

19. (1) Blood PO43- is in the normal range. (2) Blood PO43- increases
outside the normal range, which causes homeostasis to be disturbed. (3)
The control center responds to the change in blood PO43-. (4) The
control center prevents PO43- reabsorption in the kidney. (5) These
changes cause blood PO43- to decrease. (6) Blood PO43- returns to the
normal range and homeostasis is restored.
A below-normalblood level of phosphateis called hypophosphatemia, and an
above-normalblood level of phosphateis called hyperphosphatemia