Physiology Flashcards
CSF flow
Lateral ventricle —> Interventricular foramen of Munro —> 3rd ventricle —> Aqueduct of Sylvius —> 4th ventricle —> Medial and lateral Foramina of Magendie and Luschka —> Subarachnoid space
Air embolism
Gold standard for detection is TOE
Blood flow during exercise
Blood supply to skeletal muscle increases from 1-4ml/100g/min at rest to 50-100ml/100g/min at exercise
Main mechanism is local autoregulation - hypoxia, hypercarbia, NO, K ions, adenosine and lactate
Overrides the sympathetic-driven vasoconstriction
Anterior pituitary hormones
Are glycoproteins which share a common ALPHA subunit and have unique BETA subunits
Shunt in pregnancy
15% in lateral position
14% in supine position
Starling forces in the lung
Interstitial oncotic pressure = 17mmHg
Capillary hydrostatic pressure = 13mmHg (arteriolar end), 6mmHg (venous
end)
Interstitial hydrostatic pressure = 0 to slightly negative
Capillary oncotic pressure = 25mmHg
Intracellular ions
Potassium 150mmol/L Phosphate 100mmol/L Magnesium 20mmol/L Sodium 15mmol/L Chloride 10mmol/L Calcium 100nnmol/L
ASA grades
I = healthy, non-smoking, minimal/no alcohol consumption II = mild, well-controlled systemic disease without symptoms. Includes: pregnancy, BMI 30-40, smoker, social alcohol consumption, well-controlled lung disease, HTN or DM III = moderate to severe systemic disease with substantive functional limitations. Includes BMI >40, COPD, poorly controlled HTN/DM, alcohol dependence, pacemaker, reduced EF, hepatitis IV = severe systemic disease that is a constant threat to life e.g. CVA/TIA/MI in past 3mths, stents, IHD, severely reduced EF, sepsis etc V = moribund patient not expected to survive without operation VI = brain-dead organ donor
IV fluids
3% NaCl - 513mmol/L Na 5% NaCl - 856mmol/L Na 0.9% NaCl - 154mmol/L Na Hartmanns- 131mmol/L Na 0.45% NaCl + 5% Glucose - 77mol/L
TBW in neonates
Approx 75% of total body weight
Higher circulating volume per unit mass and lower percentage body fat than older children
Solubility of CO2 in blood
Solubility factor = 0.03mmol/L/mmHg or 0.225mmol/L/kPa at 37’C
20x more soluble than O2
0.5ml/kPa in 100mls of blood = 3mls per 100mls in venous blood (PCO2 6.1kPa) and 2.5mls per 100mls arterial blood (PCO2 5.3kPa)
Henry’s Law: number of particles in solution is proportional to the partial pressure at the liquid surface
Content is higher in deoxygenated blood than oxygenated blood at a given PCO2 because reduced Hb is a better buffer of H ions as it is less acidic (Haldane Effect)
Changes in pregnancy
Total blood volume increases
Plasma volume increases proportionately more than red cell volume so Hb decreases
Hct and packed cell volume decrease
Increased production of T3 and T4 and TBG so plasma conc of free thyroid hormones remains constant
Changes with hypothermia
Decreased insulin production so hyperglycaemia can occur Decreased P50 of Hb Increased myocardial irritability Decreased coagulation Decreased metabolic rate
Calcium
Normal plasma conc = 2.5mmol/L
At higher pH, plasma proteins become ionised and bind more Ca so free ionised Ca conc decreases
Extracellular, unbound Ca influences PTH secretion
Iron
Is bound to transferrin in its ferric form (Fe3+)
2 molecules carried per transferrin molecule
Daily loss in a man is 0.5-1mg mainly in the faeces
Iron deficiency —> hypochromic microcytic anaemia
Pseudocholinesterase
Found in: Plasma Placenta Kidney Brain Pancreas
Synthesised in the liver
Rate dissociation of water
Is 10^-14 = [H+] x [OH-]
So [H+] is 10^-7 mol/L
Blood supply to different organs
Brain: 50ml/100g/min
Carotid bodies: 2000ml/100g/min
Meissner’s corpuscles
Rapidly adapting cutaneous mechanoreceptors which respond to light touch
Uric acid
Is filtered by the glomerulus
Reabsorbed and secreted by PCT
Surgical 3rd nerve palsy
Causes pupillary dilatation due to compression of parasympathetic supply to pupil
CSF composition
Protein conc 0.5% of plasma protein conc
Lower pH due to reduced protein buffering
Higher chloride conc to maintain electro neutrality
Closing capacity
Does not change with position
But FRC increases on standing so CC less likely to encroach on FRC
ODC
Shifted to the right in anaemia to facilitate offloading of O2
Cannot he used to determine oxygen carrying capacity
Kety-Schmidt technique
Measures CBF using difference between arterial and venous conc of a tracer eg N2O
Aldosterone
Causes water and Na retention in equal amounts so does not change urine osmolality
Changes during exercise
CO increase 5-10x
Skeletal muscle blood flow increase 20-30x due to precapillary sphincter relaxation by local auto regulation
Partial pressure gradient for O2 into mitochondria increases x2
O2 offloading from Hb increase 2-3x
SVR
= k (MAP - CVP) / CO
Cardiac AP
Takes 0.2s to travel through atria
Conduction is slowest at the AVN
Travels between atria via Bachmann’s bundle
Circulatory changes at birth
1st breath generates negative pressure of 50cmH2O
PVR falls by >80% so increased PA pressure and increased blood flow to LA
DA closes due to increased O2 levels and low PG within 24hrs
PFO fuses within 48hrs due to reversal in atrial pressures
SVR and MAP increase
Blood flow in IVC falls (due to loss of placenta blood supply)
Hypoxia, hypercarbia, acidosis and hypothermia will increase pulmonary vascular resistance and cause a right to left shunt or persistent foetal circulation
Atrial stretch receptors
Type A discharge during systole
Type B discharge during diastole
Increase in atrial pressure
Can increase HR via Bainbridge reflex
can decrease HR via baroreceptor reflex
Increase/decrease depends on initial HR (will decrease if high, increase if low)
Atrial contraction
Right atrium contracts before left atrium
Ventricular contraction
LV contracts before RV
Afterload
= the tension developed in the ventricular wall during systole
Anrep effect increases SV when afterload is high by increasing LVEDV
Likely to be low in heart failure due to low intraventricular pressure
Higher in a hypertrophied ventricle because of increased radius (La Place’s Law: P = 2T/r)
Electrolyte abnormalities and cardiac function
Hypokalaemia makes the membrane potential more negative so it is less excitable but has more automaticity and increases QT interval
Hyperkalaemia makes the membrane potential less negative so it is closer to TP
Hypercalcaemia makes the TP less negative, decreases conduction velocity and shortens refractory period
Hypermagnesaemia increases PR interval
2,3-DPG
Binds to beta chains of Hb
Formed in RBCs from a byproduct of glycolysis
Thyroid hormones, GH and androgens increase levels
Aortic valve opening
Occurs when ventricular pressure > aortic pressure, 80mmHg
Left ventricle
Is 3x thicker than RV
Has papillary muscles on the MV but not aortic valve
Veins and venules
Capacitance vessels
Hold 2/3 of the circulating volume
Venules have a diameter of 0.01-0.2mm
Veins have diameter of 0.2-5mm
Easily distensible walls so very compliant between 0 and 10mmHg
blood entering venules has pressure of 12-20mmHg
Blood entering veins has pressure of 10mmHg
Bohr Equation
VD/VT = PaCO2 - PECO2/PaCO2
Lung metabolises
Bradykinin Noradrenaline Serotonin PGE2 and F2a Leukotrienes ATI —> ATII
Blood flow to different organs
Coronary blood flow = 5% CO, 250ml/min
Renal blood flow = 25% CO, 1.2L/min, 500ml/min/100g
Cerebral blood flow = 15% CO, 700ml/min, 50ml/min/100g (majority to grey matter)
Hepatic blood flow = 25% CO (75% from HPV, 25% from hepatic artery)
Muscle spindles
Sense muscle length so respond to stretching/contraction of muscles
When they are stretched, they fire via type Ia or II fibres to efferent gamma-motor neurones
These can be altered by descending pathways in the spinal tract
Involved in polysynaptic withdrawal reflex
Swallowing
Involuntary process
Triggered by bolus of food moving into back of the mouth and sensed by swallowing receptors
The most sensitive are the tonsillar pillars
Soft palate is pulled up
Larynx is pulled up and anterior by the neck muscles
Epiglottis covers the opening of the larynx
Palatopharyngeal folds are pulled medially to prevent larger food pieces from passing
Upper oesophageal sphincter relaxes
Whole process takes 1-2seconds
Cholecystokinin
Peptide hormone
Produced in the duodenal mucosa by enteroendocrine cells
Production triggered by fats and proteins
Slows GI transit time to increase digestion of fats
Causes release of digestive enzymes from pancreas and bile from gallbladder
Reduces gastric acid secretion
Increases satiety
Drugs excreted unchanged in the urine
Aminoglycosides Cephalosporins Ephedrine Digoxin Lithium Milrinone/mannitol Neostigmine Oxytetracycline Pencilling
Lipid metabolism
90% ingested lipids are triglycerides
10-30% are broken down in the stomach, the rest in duodenum and upper jejunum
Bile salts are solubilising agents for fats and aid absorption
Cerebral metabolic rate
Increases by 8% for every 1’C increase in temperature
Lactate
Produced from pyruvate in anaerobic metabolism
So 2 molecules produced from 1 glucose
Lactate level rises sharply at 50-80% of VO2 max - in untrained people, lactate level rises sooner
80% is converted back to glucose in the liver via the Cori cycle
Filtered by the kidneys and reabsorbed to a Tmax of 75mg/min
Glucagon
Stimulated by: cortisol, infection, theophylline, PDE inhibitors
Inhibited by: alpha stimulation, insulin, glucose, ketones, phenytoin and somatostatin
Ranitidine
Increases pH and decreases volume of gastric secretions
IV dose is 50mg
Not an enzyme inhibitor (unlike cimetidine)
Barbiturates
Alkalosis increases duration of action because tautomerisation from keto to enol form occurs at higher pH and makes the drug more lipid soluble
Excreted more readily when the urine is alkalinised
Thiopental
2mins post IV dose - in vessel-rich tissues 4mins - in muscle 7mins - in fatty tissue Slowly recirculates (half life 8.4hrs) Can cause demyelination in porphyria
Heavy bupivicaine
0.5% bupivicaine + 8% dextrose
