B ODY FLUIDS

F LUID COMPARTMENTS The 70 kg 'standard male' contains 42 liters of water - 60% of his body weight. The hypothetical adult female contains 55% of her body weight as water: this lower percent being due to a higher fat content. These figures are standard values which are quoted frequently and are average value

V ARIATION DUE TO A GE Neonates contain more water then adults: 75-80% water with proportionately more extracellular fluid (ECF) then adults. At birth, the amount of interstitial fluid is proportionally three times larger than in an adult. Total body water as a percentage of total body weight decreases progressively with increasing age. By the age of 60 years, total body water (TBW) has decreased to only 50% of total body weight in males mostly due to an increase in adipose tissue.

V ARIATION BETWEEN T ISSUES Most tissues are water-rich and contain % water. The three major exceptions to this are: Plasma: 93% water (& 7% ‘plasma solids’) Fat: 10-15% water Bone: 20% water Variation between Individuals Obese adults have a lower TBW ratio. For any particular tissue of the body the variation is much less but any variation that occurs is still mostly due to differences in amount of adipose tissue.

C OMPARTMENTS The water in the body is contained within the numerous organs and tissues of the body. These collections are referred to as compartments. The major division is into Intracellular Fluid (ICF: about 23 liters ) and Extracellular Fluid ( ECF: about 19 liters ) based on which side of the cell membrane the fluid lies.

I NTRACELLULAR F LUID The distinction between ICF and ECF is clear and is easy to understand: they are separated by the cell membranes Intracellular fluids are high in potassium and magnesium and low in sodium and chloride ions

E XTRACELLULAR F LUID The ECF is divided into several smaller compartments : plasma, Interstitial fluid, fluid of bone and dense connective tissue and transcellular fluid. The ECF compositional similarity is in some ways, the opposite of that for the ICF (ie low in potassium & magnesium and high in sodium and chloride).

W ATER M OVEMENT ACROSS M EMBRANES The lipid cell membrane separates the intracellular fluid from the extracellular fluid Substances which are water soluble do not cross lipid membranes easily unless specific transport mechanisms are present. Water molecules cross cell membranes by diffusion

O SMOSIS is the diffusion of a solvent (frequently water) through a semi-permeable membrane, from a solution of low solute concentration (high water potential) to a solution with high solute concentration (low water potential), up a solute concentration gradient.diffusionsemi-permeable membrane It is a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane separating two solutions of different concentrations.

Net movement of solvent is from the less- concentrated ( hypotonic ) to the more- concentrated ( hypertonic ) solution hypotonic hypertonic The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent.osmotic pressure pressure

W HAT IS EDEMA ? Edema is observable swelling from fluid accumulation in body tissues. The swelling is the result of the accumulation of excess fluid under the skin in the spaces within the tissues The body's organs have interstitial spaces where fluid can accumulate. An accumulation of fluid in the interstitial air spaces (alveoli) in the lungs occurs in a disorder called pulmonary edema

C AUSES OF EDEMA (1) Depression of metabolic systems : due to the depression of blood flow causing low oxygen and low nutrients which affects the cell membrane ionic pumps ( Na/k pump) leading to high Na in cell (2) Inflammation : has a direct effect on membrane permeability allowing Na to diffuse to the interior of the cell

H OMEOSTASIS Steady state internal balance between ICF and ECF It is very important to maintain a constant composition of fluids. Kidneys play a major role in regulation of body fluids

T HE KIDNEY NEPHRON Kidney nephrons are the functional units of the kidneys. There are typically over 10,000 kidney nephrons in each of the two kidneys in the body. Has 2 major functions: a. Excrete end products of metabolism. b. Control the concentration of body fluids

D ESCRIPTIONS OF THE P ARTS OF A K IDNEY N EPHRON In the renal corpuscle blood is forced through the glomerular capillaries at higher pressure than the pressure at which the blood generally travels around the body (and also into the kidney itself). Helped by the increased pressure in the glomerular capillaries, a filtration process occurs in which some blood fluid is forced out of the glomerulus and into the capsular space of the Bowman's Capsule.

The renal tubule is the part of the kidney nephron into which the glomerular filtrate passes after it has reached the Bowman's capsule The water and solutes that have passed through the proximal convoluted tubule (PCT) enter the Loop of Henle, which consists of two portions - first the descending limb of Henle, then the ascending limb of Henle. In order to pass through the Loop of Henle, the water (and substances dissolved in it) pass from the renal cortex into the renal medulla, then back to the renal cortex

When this fluid returns to the renal cortex (via the ascending limb of Henle) it passes into the distal convoluted tubule (DCT), which is shown on the left-hand side of the diagram above. The distal convoluted tubules of many individual kidney nephrons converge onto a single collecting duct

O SMOTIC PRESSURE The amount of pressure that requires to stop osmosis is called “ Osmotic pressure ” It is determined by the number of particles per unit volume of fluid. The amount of osmotic pressure exerted by a solute is proportional to the number of molecules or ions Osmole is the unit used to express the concentration in terms of numbers of particles.

G LOMERULUS FILTRATION RATE ( GFR) Is the quantity of filtrate formed each minute in all nephrons of both kidneys. In a normal person, its approximately 125 ml/min The GFR is constant hour after hour. Any alteration will cause metabolic defects, for example: (a) GFR : fluid will pass slowly and all of it will be reabsorbed waste product not eliminated (b) GFR : fluid will pass rapidly will affect reabsorption