Review of Critical Care Medicine

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Physiology

Posted by Dr KAMAL DEEP on February 14, 2011

1.Body Composition

image

Note :- Proteins are not in interstitial fluid (because there are more proteins in plasma than in interstitial fluid, there is a Donnan effect on ion movement across the capillary wall also.)

60% Water (40%ICF;20%ECF(15%Interstitial plus 5% Plasma Volume) 18% Protein Fat 15% Minerals 7%

TBW 60%—measured by D20,HEAVY WATER.It is ECF PLUS ICF.

ICF (40%)—Measured by TBW minus ECF.It cannot be measured directly just like interstitial fluid.

ECF (20%) –Measured by Inulin (Mannitol and sucrose have also been used to measure this space though not as much accurate as inulin)

ECF=Interstitial Fluid(15%) Plus Plasma Volume(5%).

Interstitial Fluid is measured by ECF minus Plasma Volume.It cannot be measured directly just like intracellular fluid.

Plasma Volume(3.5 liters ;5% of body weight) can be measured using dyes Evans Blue(T-1824).It can also be measured by serum albumin labeled with radioactive iodine.

In short TBW(D20,Titanium oxide and aminopyrine),ECF(INULIN,MANNITOL,SUCROSE),PLASMA(T-1824,ALBUMIN) can be measured directly by bracketed methods while Intracellular fluid(TBW-ECF)and interstitial fluid(ECF –PLASMA) can be derived from them indirectly by subtraction methods.

TBW= 40-45 LITRES;60% of body weight; (Intracellular Fluid=28.8Litres Plus Interstitial Fluid =10.5 Liters Plus Blood plasma=3.5 Liters) where ECF(interstitial plus blood plasma) is 14 L or another name is sucrose space in a 70 KG MAN.

Total Blood volume = 3500 Multiply 100/100-38 where 38 is hematocrit.Hematocrit is percentage of blood volume that is made up of red blood cells..So Total blood volume equals 5645 ml.In other words TTAL  is about 8% of body weight in which 5% part is plasma volume and 3 % is red cell volume.

The water content of lean body tissue is constant at 71-72 ml/100g of tissue.

Q:-Most accurate measurement of extracellular fluid
volume (ECF) can be done by using – (AIIMS 03)
a) Sucrose b) Mannitol
c) Inulin d) Aminopyrine

Ans is C i.e. INULIN

Q:- In an adult man 70kgs, the extracellular
fluid volume will be about – (Karn PG MEE 2006)
a) 42L b) 25L
c) 15L d) 121L  Ans is not A as given in guides…Thanks to 
drirfan_ahmad@hotmail.com

Q:-Which of the following methods is not used for measurement of body fluid volumes –
a) Aminopyrine for total body water-true (AIIMS May 05)
b)Inulin for extracellular fluid-true
c)Evans blue for plasma volume-true
d)albumin for blood volume-false it is used for plasma volume   Ans is D

Note:- Red cell volume can be determined by subtracting the plasma volume from the total blood volume..Total blood volume was determined by plasma volume and hematocrit.A commonly used tag is 15Cr,a radioactive isotope of chromium that is attached to the cells by incubating them in a suitable chromium solutions.Isotopes of iron and phosphorus and antigenic tagging have also been employed.

Heavy water (D20) or Deuterium Oxide is most frequently used for measuring the TBW.Titanium oxide and aminopyrine have also been used for this purpose.

Solvent Drag:- When solvent is moving along one direction,it tends to drag along some molecules of solute.

Q:- The percentage of the circulating blood volume in the venous system and splanchnic vessels is
normally between – (JIPMER 70, AMU 88)
a) 20-30% b) 40-50%
c) 60-70% d) None of these

Ans is c

Q:-Splanchnic vessels and venules contain what percentage of blood volume – (AMC 86,87)
a) 10-20 %
b) 20-30 %
c) 40-50 % d) 60-70 %

Ans is b given…..

Two different answers

Because different questions.

Veins are also called “capacitance vessels” because most of the blood volume (60%) is contained within veins.the splanchnic circulation contains about 25—30% of the total blood volume.

Q:-Osmolarity is – (JIPMER 98)
a)Osmolarity per kg of solvent
b)Osmolarity per litre of solvent
c)Moles per kg of solvent
d)Moles per litre of solvent

The osmolal concentration of a substance in a fluid is measured by the degree to which it depresses the freezing point, with 1 mol of an ideal solution depressing the freezing point 1.86 °C. The number of milliosmoles per liter in a solution equals the freezing point depression divided by 0.00186

Osmolarity is the number of osmoles per litre of the solution.

Osmolality is the number of osmoles per kg of the solvent.

Therefore, osmolarity is affected by the volume of the various solutes in the solution and the temperature, while the osmolality is not.

Osmotically active substances in the body are dissolved in water, and the density of water is 1, so osmolal concentrations can be expressed as osmoles per liter (Osm/L) of water. In this book, osmolal (rather than osmolar) concentrations are considered, and osmolality is expressed in milliosmoles per liter (of water).

Note that although a homogeneous solution contains osmotically active particles and can be said to have an osmotic pressure, it can exert an osmotic pressure only when it is in contact with another solution across a membrane permeable to the solvent but not to the solute.

Q:-A solution contains 1 gram-mole of magnesium sulfate per liter. Assuming full ionization of this compound , calculate the osmotic pressure of the solution (1 mosmole/liter concentration is equivalent to 19.3 mm.IIg osmotic pressure) – (AI 86)
a) 19.3 mm Hg b) 3.86 mm Hg
c) 19.300 mm Hg d) 38.600 mm Hg
e) 57.900 mm Hg

Ans is D  19.3 MULTIPLY BY 2

Magnesium sulfate (or magnesium sulphate) is a chemical compound containing magnesium, sulfur and oxygen, with the formula MgSO4 SUPPLYING 2 Osm

If a solute is a nonionizing compound such as glucose, the osmotic pressure is a function of the number of glucose molecules present. If the solute ionizes and forms an ideal solution, each ion is an osmotically active particle. For example, NaCl would dissociate into Na+ and Cl ions, so that each mole in solution would supply 2 Osm. One mole of Na2SO4 would dissociate into Na+, Na+, and SO42– supplying 3 Osm..

Q:-When solvent is moving in one direction, the
solvent tends to drag along some molecules of solute. This is called – (PGI 81„AMU 86)
a) Filtration b) Osmosis
c) Dorman effect d) Solvent drag

D is ans

Q:-Osmolality of plasma in a normal adult (in m
Osm/L) is – (Delhi 87)
a) 250 -270
b) 270 – 290
c) 300 -310 d)310-330

The freezing point of normal human plasma averages –0.54 °C, which corresponds to an osmolal concentration in plasma of 290 mOsm/L

It is important to note the relative contributions of the various plasma components to the total osmolal concentration of plasma. All but about 20 of the 290 mOsm in each liter of normal plasma are contributed by Na+ and its accompanying anions, principally Cl and HCO3. Other cations and anions make a relatively small contribution. Although the concentration of the plasma proteins is large when expressed in grams per liter, they normally contribute less than 2 mOsm/L because of their very high molecular weights. The major nonelectrolytes of plasma are glucose and urea, which in the steady state are in equilibrium with cells. Their contributions to osmolality are normally about 5 mOsm/L each but can become quite large in hyperglycemia or uremia.

Gibbs-donnan Equation

Donnan Effect

When an ion on one side of a membrane cannot diffuse through the membrane, the distribution of other ions to which the membrane is permeable is affected in a predictable way. For example, the negative charge of a nondiffusible anion hinders diffusion of the diffusible cations and favors diffusion of the diffusible anions. Consider the following situation,

image

in which the membrane (m) between compartments X and Y is impermeable to charged proteins (Prot–) but freely permeable to K+ and Cl–. Assume that the concentrations of the anions and of the cations on the two sides are initially equal. Cl– diffuses down its concentration gradient from Y to X, and some K+ moves with the negatively charged Cl– because of its opposite charge. Therefore

image

Furthermore,

image

that is, more osmotically active particles are on side X than on side Y.

Donnan and Gibbs showed that in the presence of a nondiffusible ion, the diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal:

image

Cross-multiplying,

image

This is the Gibbs–Donnan equation. It holds for any pair of cations and anions of the same valence.

The Donnan effect on the distribution of ions has three effects in the body introduced here and discussed below. First, because of charged proteins (Prot–) in cells, there are more osmotically active particles in cells than in interstitial fluid, and because animal cells have flexible walls, osmosis would make them swell and eventually rupture if it were not for Na, K ATPase pumping ions back out of cells. Thus, normal cell volume and pressure depend on Na, K ATPase. Second, because at equilibrium the distribution of permeant ions across the membrane (m in the example used here) is asymmetric, an electrical difference exists across the membrane whose magnitude can be determined by the Nernst equation. In the example used here, side X will be negative relative to side Y. The charges line up along the membrane, with the concentration gradient for Cl– exactly balanced by the oppositely directed electrical gradient, and the same holds true for K+. Third, because there are more proteins in plasma than in interstitial fluid, there is a Donnan effect on ion movement across the capillary wall.

Nernst equation deals with – (JIPMER 92)
a)Oxygen uptake
b)Chloride shift
c)Cellular ATP levels
d)Plasma bicarbonate level

The forces acting across the cell membrane on each ion can be analyzed mathematically. Chloride ions (Cl) are present in higher concentration in the ECF than in the cell interior, and they tend to diffuse along this concentration gradient into the cell. The interior of the cell is negative relative to the exterior, and chloride ions are pushed out of the cell along this electrical gradient. An equilibrium is reached between Cl influx and Cl efflux. The membrane potential at which this equilibrium exists is the equilibrium potential. Its magnitude can be calculated from the Nernst equation, as follows:

image

where

ECl = equilibrium potential for Cl
R = gas constant
T = absolute temperature
F = the faraday (number of coulombs per mole of charge)
ZCl = valence of Cl (–1)
[Clo] = Cl concentration outside the cell
[Cli] = Cl concentration inside the cell

The equilibrium potential for Cl (ECl), calculated from the standard values listed in Table 1–1, is –70 mV, a value identical to the measured resting membrane potential of –70 mV. Therefore, no forces other than those represented by the chemical and electrical gradients need be invoked to explain the distribution of Cl across the membrane

Table 1–1 Concentration of Some Ions Inside and Outside Mammalian Spinal Motor Neurons.

 

Ion Concentration (mmol/L of H2O) Equilibrium Potential (mV)
Inside Cell Outside Cell
Na+ 15.0 150.0 +60
K+ 150.0 5.5 –90
Cl 9.0 125.0 –70

The magnitude of membrane potential at any given time depends,of course,upon the distribution of Na+,K+ and CL-

Table 4–1 Nerve Fiber Types in Mammalian Nerve.a

 

Fiber Type Function Fiber Diameter (micronm) Conduction Velocity (m/s) Spike Duration (ms) Absolute Refractory Period (ms)
A
Alpha Proprioception; somatic motor 12–20 70–120
Beta Touch, pressure 5–12 30–70 0.4–0.5 0.4–1
Gamma Motor to muscle spindles 3–6 15–30
Delta Pain, cold, touch 2–5 12–30
B Preganglionic autonomic 3–15 1.2 1.2
C
Dorsal root Pain, temperature, some mechano-reception 0.4–1.2 0.5–2 2 2
Sympathetic Postganglionic sympathetic 0.3–1.3 0.7–2.3 2 2

aA and B fibers are myelinated; C fibers are unmyelinated.

Table 4–3 Relative Susceptibility of Mammalian A, B, and C Nerve Fibers to Conduction Block Produced by Various Agents.

 

Susceptibility to: Most Susceptible Intermediate Least Susceptible
Hypoxia B A C
Pressure A B C
Local anesthetics C B A

image

A) Arrangement of thin (actin) and thick (myosin) filaments in skeletal muscle (compare to Figure 5–2). B) Sliding of actin on myosin during contraction so that Z lines move closer together. C) Detail of relation of myosin to actin in an individual sarcomere, the functional unit of the muscle. D) Diagrammatic representation of the arrangement of actin, tropomyosin, and troponin of the thin filaments in relation to a myosin thick filament. The globular heads of myosin interact with the thin filaments to create the contraction. Note that myosin thick filaments reverse polarity at the M line in the middle of the sarcomere, allowing for contraction.

The width of the A bands is constant

image

image

 

During phases 0 to 2 and about half of phase 3 (until the membrane potential reaches approximately –50 mV during repolarization), cardiac muscle cannot be excited again; that is, it is in its absolute refractory period. It remains relatively refractory until phase 4. Therefore, tetanus of the type seen in skeletal muscle cannot occur. Of course, tetanization of cardiac muscle for any length of time would have lethal consequences, and in this sense, the fact that cardiac muscle cannot be tetanized is a safety feature.

Cardiac muscle is a collection of individual cells (cardiomyocytes) that are linked as a syncytium by gap junctional communication.

  • Smooth muscle cells are largely under control of the autonomic nervous system.
  • There are two broad categories of smooth muscle cells: unitary and multiunit. Unitary smooth muscle contraction is synchronized by gap junctional communication to coordinate contraction among many cells. Multiunit smooth muscle contraction is coordinated by motor units, functionally similar to skeletal muscle.
  • Smooth muscle cells contract through an actomyosin system, but do not have well-organized striations. Unlike skeletal and cardiac muscle, Ca2+ regulation of contraction is primarily through phosphorylation–dephosphorylation reactions
  • the excitation–contraction coupling in unitary smooth muscle can occur with as much as a 500-ms delay. Thus, it is a very slow process compared with that in skeletal and cardiac muscle, in which the time from initial depolarization to initiation of contraction is less than 10 ms
  • Unitary smooth muscle is characterized by the instability of its membrane potential and by the fact that it shows continuous, irregular contractions that are independent of its nerve supply. This maintained state of partial contraction is called tonus, or tone. The membrane potential has no true “resting” value, being relatively low when the tissue is active and higher when it is inhibited, but in periods of relative quiescence values for resting potential are on the order of –20 to –65 mV. Smooth muscle cells can display divergent electrical activity

image

image

EEG records showing the alpha and beta rhythms. When attention is focused on something, the 8–13 Hz alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm.EEG records showing the alpha and beta rhythms. When attention is focused on something, the 8–13 Hz alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm.

1.A person falling asleep first enters stage 1, the EEG begins to show a low-voltage, mixed frequency pattern. A theta rhythm (4–7 Hz) can be seen at this early stage of slow-wave sleep. Throughout NREM sleep, there is some activity of skeletal muscle but no eye movements occur.

2.Stage 2 is marked by the appearance of sinusoidal waves called sleep spindles (12–14 Hz) and occasional high voltage biphasic waves called K complexes.

3.In stage 3, a high-amplitude delta rhythm (0.5–4 Hz) dominates the EEG waves.

4.Maximum slowing with large waves is seen in stage 4. Thus, the characteristic of deep sleep is a pattern of rhythmic slow waves, indicating marked synchronization; it is sometimes referred to as slow-wave sleep. Whereas theta and delta rhythms are normal during sleep, their appearance during wakefulness is a sign of brain dysfunction

4 Responses to “Physiology”

  1. I A said

    Hey Kamaldeep.

    In the question from Karn PGMEE it’s asking for ECF not TBW. ECF should be 14-15 L right? TBW would be 60% of 70 kg = 42 L.

    Let me know. Good job with the blog. Maybe we can share notes? I have made tons of them.

  2. Ya you are right.
    You can share notes..Send via mail to drkamaldeeps@yahoo.com.
    I will upload it here if they are original.

  3. Thank you shering your question answer . my blog also contain only physiology MCQs and SEQs

  4. I just including the worthwhile details people supply in your articles or blog posts. Let me take a note of your web site plus test just as before below often. We’re slightly specific I’ll be educated numerous innovative goods proper below! Enjoy for one more!

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