Two major sources

Ingested in form of

Liquid & water in food

About = 2200 ml/day

Synthesized in the body

Oxidation of carbohydrates

About = 300 ml/day

Total intake = 2500 ml/day

Water intake very variable

From person to person

In the same individual

Depends on

Climate, habits, level of physical activity

Insensible fluid loss

Evaporation

In respiratory tract = 400 ml/day

Through the skin = 400 ml/day

Total insensible loss = 800 ml/day

Insensible loss via skin

Independent of sweating

Minimized by cornified layer of the skin

Burns: rate of evaporation increases

Fluid loss in sweat

Highly variable depend on

Environmental temp

Level of physical activity

Normally = 100 ml/day

Exercise, hot weather

About 1 to 2 liters/hr

Water loss in faeces

Small amount = 100 ml/day

But amount lost increases tremendously in severe diarrhoea

Water loss by kidneys

Variable

As low as 0.5l/day in dehydration

As high as 20 l/day in a person with excessive fluid intake

Normally about =1500 ml/day

In the human body there are several major fluid compartments each of which is subject to homeostatic regulation.

Fluid content of the human body ranges from 40% to 60% of bodyweight.

Intracellular fluid compartment ICF

Extracellular fluid compartment ECF

Intestitial fluid

Plasma

Transcellular

Intracellular Fluid (ICF) water inside the cell comprises 2/3 of the body’s water.

Its main function is to facilitate intracellular chemical reactions that maintain life

Is the largest compartment.

ICF is about 40% of the body weight.

The ICF is primarily a solution of potassium and organic anions, proteins etc.

The cell membranes and cellular metabolism control the constituents of this ICF.

Extracellular Fluid (ECF) is the remaining 1/3 of the body’s water, any fluid not contained inside a cell therefore comprises the extracellular compartment.

ECF is about 20% of the body weight, it consists mainly of plasma and lymph fluid (circulating compartment) and interstitial fluid (the spaces between the cells).

Communicates with interstitial fluid through capillary pores

Hence ECF constantly mixing

Plasma and interstitial fluid have same composition except for proteins

Blood contains both ECF and ICF

ICF is the fluid within the RBC

Average blood volume

Is 8% of body wt

= 5.6 (70*0.08) liters

On the average

60% of blood vol = plasma ( 3 liters)

40% of blood vol = RBC (2 liters)

Values vary considerably in different people depending on

Sex, wt, and other factors

The ECF is further subdivided into three sub-compartments.

Interstitial Fluid (ISF) surrounds the cells, but does not circulate.

It comprises about 3/4 of the ECF.

Plasma circulates as the extracellular component of blood.

It makes up about 1/4 of the ECF.

Transcellular fluid is a set of fluids that are outside of the normal compartments.

These 1-2 litres of fluid make up the Cerebrospinal Fluids (CSF), Digestive Juices, Mucus, specialized joint fluids aqueous; humour and vitreous humour the fluids in the eyeball.

The ECF provide a relatively constant environment for cells and transport substances to and from the cells.

Under normal conditions homeostasis of the total volume of the water in the body is maintained or restored primarily by adjusting output urine volume and secondarily by fluid intake

Regulation of fluid intake; When dehydration begins to develop, salivary secretion decreases, producing the sensation of thirst; individual increased fluid intake to offset increased output tends to restore fluid balance

Regulation of urine volume; two factors determine urine volume

Glomerular filtration rate, except under abnormal conditions remain fairly constant

Rate of tubular reabsorption of water fluctuates considerably; normally adjusts urine volume to fluid intake influenced by hormonal mechanisms

Some of the factors that alter fluid loss under abnormal conditions are increased rate of respiration and volume of sweat secreted, certain abnormal conditions such as vomiting, diarrhoea, or intestinal drainage can produce fluid and electrolyte imbalance

The volume of extracellular fluid can increase or decrease, even if the osmolality of the extracellular fluid is maintained within a narrow range of values

Neural mechanism,

Reninangiotensin-aldosterone mechanism,

Atrial natriuretic hormone (ANH) mechanism

Antidiuretic hormone (ADH) mechanism.

Neural mechanism changes the frequency of action potentials carried by sympathetic neurons to the afferent arterioles of the kidney in response to change in blood pressure

When baroreceptors detect an increase in arterial and venous pressure, the frequency of action potential carried by sympathetic neuron to the afferent arteriole decreases.

Consequently the afferent arteriole dilates

This increases glomerular capillary pressure, resulting in an increase in the glomerular filtration rate, an increase in filtrate volume and an increase in urine volume

When the baroreceptors detect a decrease in arterial and venous pressure the action potential carried by sympathetic neuron causes constriction of afferent arteriole and the opposite occurs

The Renin angiotensin-aldosterone mechanism responds to small changes in the blood volume

Increased blood pressure results from increased blood volume

Juxtaglomerular cells detect increase in blood pressure in the afferent arteriole and decrease the rate of renin secretion

The decrease in renin secretion results in a decreased conversion of angiotensinogen to angiotensin II

Reduced angiotensin II causes a decrease in the rate of aldosterone secretion from the adrenal cortex

Decreased aldosterone level reduces the rate of sodium (Na+) reabsorption from the distal renal tubules and collecting ducts

Consequently more Na+ remains in the filtrate and fewer are reabsorbed

The effect is to increase the osmolality of the filtrate, which reduces the ability of the kidney to reabsorb water

The water remains with the excess Na in the filtrate

Thus the volume of urine produced increases and extracellular fluid volume decreases

The opposite occur in case of decreased in blood volume

The ANH mechanism is most important in responding to increases in extracellular fluid volume

An increase in pressure in the atria of the heart, which usually results from an increase in blood volume, stimulates the secretion of ANH, which decreases Na+ reabsorption in the distal and collecting ducts

This increase the rate of Na+ and water loss in the urine

Thus increased ANH secretion decreases extracellular fluid volume

ANH does not appear to respond strongly to decreases in blood volume however a decrease in pressure in the atria of the heart inhibit the secretion of ANH

The ADH mechanism plays an important role in regulating extracellular fluid volume in response to large changes in the blood pressure

An increase in blood pressure results in a decrease in ADH secretion

As a result, the reabsorption of water from the lumen of the distal tubule and collecting ducts decreases, resulting in a large volume of dilute urine

This response helps to decrease extracellular fluid volume and blood pressure

A decrease in blood pressure results in an increase in ADH secretion and the opposite occurs.

Water enters the body via the digestive tract water is also added to the total fluid volume from each cell as it catabolises food and the resulting water enters the bloodstream

Water leaves the body via four exits

As urine through the kidney

As water in expired air through the lungs

As sweat through the skin

As faeces from the intestine

The control mechanism for water exchange between plasma and interstitial fluid consists of four pressures

Blood hydrostatic and colloid osmotic pressures on one side of the capillary membrane

Interstitial fluid hydrostatic and colloid osmotic pressure on the other side; two of the pressures for a vector in one direction and the other two in the opposite direction

Blood hydrostatic pressure (BHP) forces fluid out of capillaries into interstitial fluid (IF)

Blood colloid Osmotic pressure (BOP) draws fluid from IF into capillaries

Interstitial fluid hydrostatic pressure (IFHP) forces fluid out of IF into capillaries

Interstitial fluid colloid osmotic pressure (IFOP) draws fluid from capillaries to IF

The rate and direction of fluid exchange between capillaries and interstitial fluid are determined by the hydrostatic and colloid osmotic pressures of the two fluids

Some principles about transfer of water between blood and interstitial fluid

No net transfer of water occurs as long as (BHP + IFOP) = (IFHP + BOP)

A net transfer of fluid occurs when (BHP + IFOP0 ≠ (IFHP + BOP)

Fluid shift out of blood into interstitial fluid whenever (BHP + IFOP) > (IFHP + BOP)

Fluid shift out of interstitial fluid into blood whenever (BHP + IFOP) < (IFHP + BOP)

The balance of hydrostatic and osmotic forces causing movement out and into the capillaries is known as Starling forces

Hydrostatic pressure is generated by the pumping force of the heart

It pushes water out of the capillaries

The water potential is created due to the inability of large solutes to pass through the capillary walls

This build up of solutes induces osmosis

The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach equilibrium

The osmotic pressure drives water back into the vessels

Because the blood in the capillaries is constantly flowing, equilibrium is never reached

The balance between the two forces is different at different points in the capillaries

At the arterial end of the vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid

At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary

This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid

To prevent a build-up of tissue fluid surrounding the cells in the tissue, the lymphatic system plays a part in the transport of tissue fluid

Tissue fluid can pass into the surrounding lymph vessels, and eventually ends up rejoining the blood

Sometimes the removal of tissue fluid does not function correctly, and there is a build-up

This causes swelling, and can often be seen around the feet and ankles. The position of swelling is due to the effects of gravity.

An electrolyte balance exists when the quantities of electrolytes (molecules that release ions in water) the body gains equal those lost.

The electrolytes of greatest importance to cellular functions are sodium, potassium, calcium, magnesium, chloride, sulphate, phosphate, bicarbonate and hydrogen ions.

Sodium ions are predominant extracellular cations.

Because of their abundance in the extracellular fluid, they exert substantial osmotic pressure.

The kidneys are the major route by which Na+ is excreted.

The predominant anion in extracellular fluid is chloride ions.

The extracellular concentration of Potassium ions K+ must be maintained within narrow range.

The concentration gradient of K across the plasma membrane has a major influence on the resting membrane potential, and cells that are electrically excitable are highly sensitive to slight changes in that concentration gradient.

Therefore Potassium is important in maintaining the nerve impulse conduction and cardiac muscles fibre contractions.

Abnormal potassium (K+) levels may cause these cells to function abnormally.

Kidneys are the primary site for potassium regulation.

The extracellular concentration of calcium ions like that of K+ is regulated within a narrow range they are important for electrical properties of excitable tissue.

Parathyroid hormone increases extracellular Ca+ level and reduces extracellular phosphate levels.

Most of the phosphate ions are found inside the cells forming covalent bond with organic molecules.

Most of the magnesium in the body is stored in the bones or in the intracellular fluid as it is used as cofactor for intracellular enzymes.

Sulphur is an important element in protein synthesis; mostly are found inside the cell.

These electrolytes primarily obtained from foods, but they may also be found in drinking water and other beverages.

Some electrolytes are by-products of metabolic reactions.

The body loses some electrolytes by perspiring or through kidney during urine production.

The kidney alters renal electrolyte losses to maintain the proper composition of body fluids.

Plasma and interstitial fluid are almost identical in chemical make-up, with intracellular fluid showing striking differences.

The table below shows the approximate values of electrolytes in different body fluid compartments.

Fluid content of the human body ranges from 40% to 60% of bodyweight.

Intracellular Fluid (ICF) water inside the cell comprises 2/3 of the body’s water.

Extracellular Fluid (ECF) is the remaining 1/3 of the body’s water.

Mechanisms that regulate extracellular fluid volume are neural mechanism, Reninangiotensin-aldosterone mechanism, atrial natriuretic hormone (ANH) mechanism and Antidiuretic hormone (ADH) mechanism.

Electrolytes are primarily obtained from foods, but they may also be found in drinking water and other beverages.