Physiology lecture 4: Water, fluids, spaces and compartments
Key concepts
- compartmentalisation of body fluids is an important general principle in physiology - compartmentalisation is achieved by barriers between compartments; these barriers determine which substances move between compartments - solutes and water that enter or leave the body do so via the ECF - the ICF and ECF are in osmotic balance. Water moves between these compartments only when osmotic pressure gradients exists - equilibration of ICF and ECF osmolarity occurs primarily by shifts in water and not shifts in solute - loss of isotonic solution does not alter intracellular fluid (ICF) volume, loss of hypertonic solution (resulting in hypoosmotic dehydration) will tend to expand the ICF, loss of hypotonic solution (resulting in hyperosmotic dehydration) will tend to contract the ICF
How does the kidney regulate water excretion?
- water diffuses across the plasma membrane of most cells via water channels known as aquaporins - the type and number of aquaporins differ in different cell membranes - the type and number of aquaporins in a cell membrane can be altered in response to various signals i.e. anti-diuretic hormone (ADH) - collecting tubules of kidney nephrons express numerous aquaporins that can be increased or decreased in number depending on the total body water content - dehydration: high expression of aquaporins in tubules (water reabsorbed to blood, low water loss in urine) ( small volume of highly concentrated urine) - overhydration: low expression of aquaporins in tubules (excess water excreted in urine) (high volume of low concentration urine)
In response to these impulses, which of the following cause smooth muscle on urinary bladder to contract?
The parasympathetic neurons from the spinal cord to the urinary bladder
Fluid volumes in body compartments
Total Body Water (TBW) - 0.6 of body weight (i.e. 70 kg) -42 litres Intracellular fluid (ICF) - 0.4 of body weight - 28 litres Extracellular fluid - 0.2 of body weight - 14 litres -> 3/4 of ECF is interstitial (10.5 L) and 1/4 of ECF is intravascular (3.5 L) Information on weight allows estimation of volume
If urination is not convenient, the brain sends impulses down the spinal cord to inhibit the micturition reflex. True or false?
True
The micturition reflex is coordinated by neurons in the spinal cord. True or false?
True
Measuring volumes of compartments
A) Syringe contains 10 mls of a 9 mg/ml tracer solution B) Allow tracer to disperse equally and then measure it, now measures 2.5 mg/ml (you want the tracer not to be toxic, want it to distribute evenly, do not want it metabolised or excreted majorly in the time it is being tested) - plasma compartment tracers: Evan's blue or radio-iodinated serum albumin (RISA) (stays in the intravascular compartment) - extracellular compartment tracers: mannitol or inulin - total body volume tracers: tritium oxide (THO) - volumes of interstitial (ISF) and intracellular (ICF) measured indirectly, i.e. ISF = ECF minus plasma, ICF = whole body volume minus ECF
Dehydration states
Fluid loss can be due to a variety of reasons: - skin: burns, large wounds - gastrointestinal tract diseases: diarrhoea, vomiting - kidney diseases: polyuria, overuse of diuretics - haemorrhage, oedema etc Dehydration states: Isosmotic dehydration (concentration unchanged) Hyperosmotic dehydration (concentration increased) Hypoosmotic dehydration (concentration decreased)
Fluid compartments in the body 2
Intracellular fluid compartment (ICF) and extracellular fluid compartment (ECF) ECF broken down into interstitial fluid (ISF) and intravascular fluid
Ineffective vs effective osmoles
An ineffective osmole is a substance that, although osmotically active, can easily distribute across body compartments and so does not cause fluid shifts i.e. urea - isotonic solution: causes no change in cell volume - hypertonic solution: cell shrivels-ICF decreases - hypotonic solution: cells swell-ICF increases
Measuring blood volume
Blood volume (BV) = volume of blood (both red blood cells and plasma) in the circulatory system Haematocrit (Hct): fraction of the blood that is red blood cells: usually 40% for women and 45% for men BV = Plasma volume/1-Hct Plasma water is he initial body access point for ingested nutrients and exit point for the body's waste products. Access to all cells of the body except the cells of the blood is via the interstitial space
Isosmotic dehydration
Causes: blood loss (haemorrhage), diarrhoea, vomiting, burns - gastrointestinal fluids (i.e. faeces and vomit) are isotonic Outcome: Isosmotic dehydration - volume of ECF decreases - osmolarity of ECF does not change as fluid lost was isotonic - no change in the volume or osmolarity of the ICF compartment Treatment: infusion of isotonic saline Water does not move between body compartments if the osmolarity stays the same
Hydrostatic and oncotic pressure
Hydrostatic pressure (red arrow): - pressure exerted by a fluid at equilibrium due to the force of gravity, pumping of the heart - fluid "leaks out" of the capillaries Oncotic pressure (blue arrow): - type of osmotic pressure - large plasma proteins cannot cross capillary walls and tend to draw water into the capillaries - albumin (main protein component of blood) constitute around 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid (albumin cannot leave the vascular compartment, tens to draw fluid into the vascular compartment - protein malnutrition appear bloated: fluid remains in the interstitial compartment instead of getting drawn into the vascular compartment as no proteins to bring fluid in
Composition of fluid compartments
In clinical medicine, the osmotic pressure of a body fluid is generally expressed as its osmolarity i.e. moles total solute/solvent mass (Kg) - isosmotic solution: osmolarity around 290 mM - hyperosmotic solution: osmolarity > 290 mM - hypoosmotic solution: osmolarity <290 mM Water distribution in body compartments depends primarily on the concentration of electrolytes in the body compartments - because of the Gibs-Donnan effect, more positively charged ions are attracted into the vascular compartment and less negatively charged particles - body is actively achieving this uneven distribution of Na+ and Cl- inside and outside the cell - in a healthy person, you do not see large shifts in water movement because the osmolarity of all compartments is the same
Hyposmotic dehydration
Loss of solute in excess of water Causes: renal loss of NaCl due to adrenal insufficiency (Addison's disease) Outcome: Hypoosmotic dehydration - loss of NaCl from the plasma; plasma osmolarity decreases - fluid moves from the ECF to the ICF - volume of ICF increases - osmolarities of ECF and ICF equalise at a lower level Cells are very sensitive to this, especially the cells of the brain
Hyperosmotic dehydration
Loss of water in excess of solute Causes: decrease in water intake, diabetes insipidus, diabetes mellitus, alcoholism, fever - sweat is hypotonic (loss of water in excess of salt) Outcome: Hyperosmotic dehydration - volume of ECF decreases - osmolarity of ECF increases (fluid we are loosing is more dilute) - water will leave the ICF and move into the ECF - osmolarities of ECF and ICF equalise at a higher level - volume of both compartments decreases
Movement between the intracellular fluid and the interstitial fluid
Most substances that are biologically important cannot penetrate lipid membranes - to cross a cell plasma membrane, substances move through specific integral membrane proteins which are divided into channels or transporters - hydrophobic molecules such as O2, CO2, N2 and steroids pass straight through (lipid soluble) - small uncharged polar molecules such as H20, urea and glycerol cannot move freely from the intracellular fluid to the extracellular fluid, but some do get through - large unchanged polar molecules such as glucose and sucrose struggle even more to pass through - ions such as H+, Na+, HCO3-, K+, Ca2+, Cl- and Mg2+ cannot pass through
Disturbances of body fluid balance
Oedema: abnormal accumulation of fluid beneath the skin, or in one or more cavities of the body A. Severe lymphedema (blockage of lymphatic vessels B. Severe protein malnutrition C. Systemic (pitting) oedema in the lower parts of the body particularly in the ankles (venous pressure is elevated in the legs during standing) Dehydration scenarios: - vomiting and diarrhoea - severe haemorrhage Over-hydration scenarios: - psychogenic polydipsia (urge to consume large volumes of water) - infusion therapy The only regulated route for the excretion of flid from the body is via the kidneys
Oncotic pressure
Oncotic pressure is a type of osmotic pressure, generated by impermeable proteins in solution (Gibbs-Donnan effect) - because some particles cannot diffuse across the membrane (e.g. proteins) their electrostatic presence will result in the asymmetric distribution of permeable ions. However, each side remains electrically neutral within itself. - because negatively charged proteins cannot move, they attract positive particles towards them (Na2+) and negative particles (Cl-) will come with them as well
Water intake and output
Solutes and water enter or leave the body via the ECF - inputs should match outputs to be at a balance - if exercising a lot, might release more water The body maintains steady-state water balance by ensuring that the amount of water added each day is exactly balanced by that lost or excreted. The only regulated route for excretion of water from the body is the kidneys; urine volume can vary from 0.5 to 18L/day.
When urination is desired, decreased action potentials along what causes relaxation of the external urinary sphincter?
Somatic motor neurons
Movement between body fluid compartments
The differing compositions of the compartments reflect the activities of the barriers (cell membranes) separating them - a lipid bilayer is arranged so that the hydrophilic phosphate heads point "out" to the water on either side of the bilayer and the hydrophobic tails point "in" to the core of the bilayer Cell membrane = lipid bilayer Capillary wall = fenestrated - main drivers of solute movement between the intracellular and interstitial fluid compartments are OSMOTIC PRESSURE and ELECTROCHEMICAL GRADIENTS - main drivers of solute movement between the interstitial and the plasma compartments are HYDROSTATIC PRESSURE and ONCOTIC PRESSURE
Osmotic pressure (pi)
The pressure that must be applied to a solution to prevent the net flow of water into it - throughout the body, dissolved components generate osmotic pressure - water "follows" the osmoles
Electrochemical gradient
The separation of electrical charge across a plasma membrane (the membrane potential) provides the electrical force that drive positive ions into a cell and negative ions out of a cell - virtually all eukaryotic cells maintain a nonzero transmembrane potential
Tonicity - Fluctuations in cell volume
The tonicity of a solution is related to its effect on the volume of a cell. Hypothetical situation: non penetrating (effective) solutes, only water moves (contained within a compartment and draw water (only water is moving) Normal cell volume: Intracellular fluid = 290 mOsm - in a hypertonic solution, fluid will leave the cell and the cell will shrivel - in a hypotonic solution, water enters the cell
When the urinary bladder initially becomes stretched, there is an increase in the frequency of action potentials travelling from ...
The urinary bladder to the sacral region
A woman runs the Dublin city marathon on a very hot day and loses 3 litres of sweat (hypotonic). During the marathon, she drinks 3 litres of water. For each parameter listed below, indicate whether it is increased, decreased or unchanged in the new steady state, and explain why. a) Plasma osmolarity b) Extracellular fluid volume c) Haematocrit d) Total body water
a) decrease, loosing hypotonic solution, both water and salt but proportionally more water. She lost water and salt but consumed water again, net loss of NaCl with no net loss of H2O thus plasma osmolarity decreases. b) decrease in ECF osmolarity will cause a fluid shift from ECF to ICF, leading to a decrease in ECF volume c) haematocrit will increase because 1. the decrease in ECF volume concentrates the red blood cells and 2. the shift of water into cells causes the red blood cells to swell d) no net gain or loss of H2O thus there is no change n total body water
Fluid compartments in the body
2 primary fluid compartments in the body: 1. Internal fluid compartment (ICF) 2. Extracellular fluid compartment (ECF) - interstitial fluid compartment (found in the spaces around cells) - intravascular fluid compartment - transcellular fluid compartment (smallest compartment, 2.5% - 1L)(cerebrospinal fluid, synovial fluid, serous fluids (i.e. peritoneal fluid, pleural fluid), intraocular fluid) - different compartments gave different fluid composition - the volume of a given compartment is determined by the amount of solutes within that compartment
Fluid in the body
60% of male body weight is water - "standard male" is 70 kg: 42 litres of H20 55% of female body weight is water - "standard female" is 55 kg: 30 litres of H20 - women have more fat cells, higher percentage of body fat
