A young woman requires dialysis for acute renal failure. Examine the

diagram, Describe the principles involved in the system for moving

solvent and solute across the semi-permeable membrane.

Blood Urea – 40 mmol/L

Osmolarity 320 mosm/L

Pressure 100 mmHg

------------SEMI-PERMEABLE MEMBRANE---------------

Dialysate Urea – 0

Osmolarity 346 mosm/L

Pressure 10 mm Hg

Answer.

SOLUTE MOVEMENT

1. Diffusion of Urea down a Concentration gradient.

2. Hydrostatic Pressure forcing fluid through membrane by a difference of

hydrostatic pressure - Filtration.

3. Solvent drag – solute molecules move with solvent. – convective

forces

SOLVENT MOVEMENT

1. Osmosis –from blood into dialysate to equilibrate osmotic pressure

Outline the physiological effects of acute hypoglycaemia

Answer

Define “acute hypoglycaemia”.

Time and criteria or level of blood sugar for hypoglycaemia

Mechanisms for hunger and nausea – role of hypothalamus

State tissues that are dependent totally on glucose as a substrate

Alternative substrates in tissues not dependent solely on glucose

Neurohumoral effects

Metabolic reserves

What are the physiological effects of the evaporative loss of 2 litres of water in a 60kg man.

1. Reduction in total body water750ml from ECF1250ml from ICF

2. ↓ blood vol → decreased VR and atrial filling → ↓ CO viaFrank-Starling mechanism

3. Reduced CO → ↓ BP

4. Baroreceptor stimulated ↑ heart ratevasoconstrictionvenoconstriction

both of which tend to restore BP

5. ↓ atrial wall stretch → ↑ ADH→ ↓ANP

6. Increased osmolarity → ↑ ADH

7. ↓ thirst → lends to drinking

8. ↑ ADH → effects on kidney

Explain the mechanism for the maintenance of oxygen supply to organs during isovolaemic haemodilution.

Haemodilution ↓ oxygen carrying capacity

Oxygen flux equation= cardiac x arterial O2

output content

≈ cardiac output x [ sat x Hb x 1.34]

Normally blood loses 25% O2 content so that remaining 75% forms reserve supply

1. ORGAN BLOOD FLOW ↑ by

a) increased cardiac outputby ↓ blood viscosity → ↑ venous → ↑ stroke volumeby ↓ afterload return

b) ↑ cardiac output via ↑ sympathetic drive increasing heart rate & stroke volume

c) ↑ regional blood flow by ↓ viscosity

d) vasodilation of organ circulation

2. O2 EXTRACTION increased

3. Haematocrit ↓ = heart & brain will receive preferential blood supplyDecreased viscosity-therefore better flow and distribution to tissues.

Briefly describe the functions of proteins in cell membranes.

Many types of proteins within cell membrane

Located at (i) outer surface

(ii) through thickness or integral proteins

(iii) inner surface

Hydrophobic part of protein orientated towards cell interior

Hydrophilic part orientated towards exterior of cell

May be Lipoproteins (proteins that contain lipids) or Glycoproteins (contain

carbohydrates)

Role:

1. Structural proteins

2. Active transport mechanisms

3. Ion channels for passive ion transport

4. Receptors for transmitters or hormones

5. G proteins

6. Surface proteins – as receptors eg Glycoprotein receptors

As antigens eg Rh factor

Factors that control intra-ocular pressure

1. Intraocular Blood Volume

Venous pressure eg coughing and straining, posture

Carbon dioxide

Posture improve drainage

2. Volume of Aqueous humour

Produced by ciliary body (thickened anterior part of choroid) by

DIFFUSION

Role of Carbonic anhydrase – intraocular pressure is reduce by CA

inhibitors e.g acetazolamide

Drainage of aqueous humour via canal of Schwlem

Describe the effects of 100ml of 8.4% sodium bicarbonate administered intravenously in a 70kg man during CPR at an asystolic arrest

Composition of Sodium bicarbonate

Na 1 mmo;l per mlHCO3 1 mmol per mlTonicity 2 mosmol per ml

Effects:1. Alkali load → K+ flux; H+

2. Na loadhypertonicity in intravascular volume

3. CO2 load clue to dissociation of HCO3-

4. Effects on O2 dissociation curve

5. CO2 diffusion intracellularly leading to intracellular acidosis

6. Effects on respiratory control

An elderly lady with bowel obstruction has nasogastric losses > 1000ml/day for 5 days.

Explain the mechanism of the development of metabolic alkalosis and justify your choice of replacement fluid.

NG fluid compositionNa50 K 10H 100 Mg 2 (mmol/L)

Metabolic alkalosisα loss of H+ ionsmaintained and persist because:

1) Hypovolaemic Na+ reabsorbed in preference to H+ at distal tubule

2) HypochloraemiaHCO3 reabsorbed with Na+ reabsorptionMore K+ and H+ loss at distal tubule

Choice of fluid0.9% NaCl with 30mmol K+

Define “basal metabolic rate”. How does the metabolic rate of a child with abdominal sepsis in the OR differ from that of a sleeping adult.

Definition BMR = rate of energy utilisation in awake subject 12 hours after meal in a comfortable or thermoneutral environment

Factors affecting BMR of childa) Age - neonate BMR higher

approx. 2 x adultb) Additonal factors

pain ↑ )sepsis ↑ ) ↑ metabolicpyrexia ↑ ) rateanxiety ↑ )if rigors present ↑ )

OR temperature ↓ ) ↓ metabolic? starvation ) rateGA

Sleeping Adultα Age Order ↓

sleep ↓

Outline the control of body water by the kidneys.

1. 180L fluid filtered through the glomeruli/day

2. Approx. 90% water filtered is reabsorbed

3. Proximal tubule70% passive reabsorption via osmotic forces following (secondary to) active Na reabsorption.

4. Loop of Henle• passive osmotic reabsorption of water in descending limb because:

a) descending limb is permeable to H2Ob) ascending limb is not permeable

• countercurrent multiplier system – produces hyperosmotic medullary interstitium which enhance H2O reabsorption. Occurs at juxtamedullary nephrons.

5. Distal tubule - passive H2 reabsorptionCollecting ducts - site of ADH “free H2O” reabsorption

Regulated by ECF osmolarity ≈ Ant Pit → ADH

6. Osmotic diuresis

Briefly explain how alveolar hypoventilation reduces the systemic arterial partial pressure of oxygen (PaO2)

1. Alveolar hypoventilation< 3600ml/min

2. AlveolarConc of gas + inspired gas conc ± output / uptake of gas

alveolar ventilation

3. With CO2 ; Insp CO2 = OPACO2 = α K (output) ∴↓ VA → ↑ PAO2

4. Apply Alveolar Air EquationPAO2 = P1O2 - PACO2

RPAα P1 in a non-linear manner (i.e. rectangular hyperbola)

5. O2 cascade Paα PA

6. PAO2 = PIO2 - V. O2 VA

Briefly describe the mechanisms that control the distribution of body water between the plasma, interstitial and cellular compartments.

1. Distribution of body H2OICF / ECF

factors: age, sex, obese

2. Forces that determine distributionmembrane permeabilitymembrane pumpsosmotic forcesGibb-Donnan effectcolloid osmotic forces

3. total body osmolarityα Na Other osmotic forces Sugar

blood urea

With regard to an invasive arterial blood pressure monitoring, briefly describe the role of

i. frequency responseii. resonant frequencyiii. optimal damping

of the measuring system (ie cannula, tubing and transducer) in achieving an accurate waveform.

Frequency response: range of frequencies to reproduce the Fourier components of the waveform ie without distortion. X10 hormones

180 beats/m = 3H2 x 10

Resonant frequency: frequency at which the signal is magnified due to oscillation and hence distort the signal

Optimal damping df = resist (friction)2√ms

≈ damping. coef 0.7 – produces the least attenuation of a wide range of frequencies and minimum change in frequency response Also calle “optimal damping”.

Transducer f.α √S/M mass = fo = R √ Σ

2 TTpl

Connecting tube:fo α short length, large diameter

stiffair; blood clots

Cannula:site, size, kinks, etc

Briefly describe the acid base changes which arise in a hypoxic person, including the compensatory processes which may restore the status to normal.

1. Definition: Clinical hypoxiaDecrease PaO2 from capillary (90mmHg) to mitochondria(≈ 1mmHg)

2. Biochemical Effects:Mitochondria: stop oxidative phosphorylation↑ Glycolytic pathway

Glucose → Pyruvate → ↑ Lactic Acid + 2 ATP

2 ATP → ADP + AMP - retard Ca ATP pump

3. Cellular Buffering HPO4; Protein

4. Interstitial buffers

5. Vascular Buffers: PaO2 : BuffersHbO2 dissociation shiftsK+

6. Acute Compensatory sResp: Hyperventilation ↑ V/QSympathetic N. Syst ↑Cardiac Output

7. Long Term Compensatory sRenalEPO

Draw and label a left atrial pressure trace. Briefly describe the factors that affect LAP.

Diagram A, C and V wavesX – Y descentValues on axes

Factors:Blood volumeSympathetic tonePostureLV contractibility (systolic)LV compliance (diastolic)LV impedance / afterloadIntrathoracic pressurePericardial pressureRV output