Capillary vessel. A) permeability which can vary between tissues, within tissues at different times and along the capillary

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1 I. Capillary bed structure Single layer of endothelium supports diffusion MedSoc Teaching CRH Session 2 Capillary circualtion Chanel Tobinska Arteriole Capillary vessel Venules BLOOD Blood flow velocity depends on contractile state of arterioles Pre capillary sphincter (small area os smooth muscle to control diameter of the arterioles and the entry ) Post capillary sphincter (small area os smooth muscle to control diameter of venues and the exit ) II. Solute/substances movement across capillaries A) permeability which can vary between tissues, within tissues at different times and along the capillary B) DIFFUSION FILTRATION pinocytosis DIFFUSION exchange of substances e.g. CO2 and O2 occurs primarily by diffusion down the concentration gradient, primarily depends on the capillary permeability of substance and surface area. Expressed by the Fick s Law: J = -PS (Co-Ci) J - quantity moved per unit time P - capillary permeability S - capillary surface area C - concentration outside(o) and inside (i) FILTRATION/Reabsorption Def: movement of a volume of protein free fluid out of capillary ( filtration ) and back (reabsorption) Depends on 4 variables: 1. Capillary hydrostatic pressure Pc (pressure difference between the beginning and end of the capillary) 2. Interstitial fluid colloid osmotic pressure πi 3. Capillary(plasma)colloid osmotic pressure πp 4. Interstitial fluid hydrostatic pressure Pi

2 MedSoc Teaching CRH Session 2 Capillary circualtion 1. Capillary hydrostatic pressure Pc - major determinant Which depends on pre capillary resistance provided by the smooth muscles of the pre-capillary sphincter which can depend on the arterial blood pressure. If arteriole constrict/increase resistance (can be due to increased BP - look previous MedSoc Teaching session) then: - blood volume builds up before the sphincter/constriction > pressure before constriction/ upstream/in arterioles goes up - less blood flows into the capillary - decreased capillary hydrostatic pressure Pc ( less blood in the capillary bed) - decreased filtration (less blood that can be filtered) - Increased reabsorption (we are saving the fluid) - decreased pressure in venules/downstream (less blood that goes out) Which depends on post capillary resistance provided by the smooth muscles of the post-capillary sphincter and venous pressure If post-capillary sphincter constrict or if there is an increase in venous pressure due to e.g. congestion or varicose veins then: - blood volume builds up before the sphincter/in the capillary > pressure before constriction/in - capillary vessel goes = capillary hydrostatic pressure Pc - filtration (more blood that can be filtered ) - Decreased reabsorption (we are filtering/giving away the fluid) - because of the constriction/ congestion less blood flows into the venules > decreased pressure in the venules/downstream Clinical : Oedema - due to hypervolaemia ( more blood enters capillary bed more can be filtered) - due to inflammation ( vasodilation + increased permeability)

3 2. Interstitial fluid colloid osmotic pressure πi - minor determinant Which depends on the presence of protein in the interstitium hence the capillary permeability to protein. Normally low. Low πi = reabsorption High πi = filtration HIGHER FAVOURS FILTRATION 3. Capillary colloid osmotic pressure πc - major determinant Which depends on synthesis/breakdown of protein in liver, capillary permeability to protein, abnormal protein loss though e.g. kidney High πc = reabsorption Low πc = filtration FLUID FLUID GOES WHERE THE PROTEINS ARE 4. Interstitial fluid hydrostatic pressure Pi - minor determinant Which depends on the interstitial fluid volume, compliance of organ, effective drainage via lymphatics( permeable to proteins). Normally stable. Interstitium HIgh Pi= Interstitium Low Pi= Clinical: Oedema - due to lymphatic obstruction ( reduced drainage hence more fluid in the interstitium) - hypoproteinaemia ( less proteins in blood )

4 III SUMMARY Fluid movement ( Q ) = Kf [(Pc+ πi) - (πc+pi)] Kf - (filtration coefficient) is a constant that depends on permeability and surface area available. Pc drops across capillary. Highest in arterioles,middle in capillaries, lowest in venules. Filtration/reabsorption depends on the balance between 1-4 variables. Especially the major determinants 1 & 3 : If Pc>πc there is net filtration If Pc<πc there is net reabsorption General rules: 1. Fluid goes where there is lots of proteins and not much fluid 2. Oedema occurs when there is abnormally high filtration

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8 Heart Failure Tutorial Essential points 1. The heart is a pump which moves blood around the body 2. Failure of the pump has many causes 3. The body does two things which make heart failure worse 4. Heart failure symptoms come from the part of the circuit before the problem The heart is a pump which moves blood around the body The heart is also like a bicycle Speed of the bike = Heart failure is failure of the pump to meet the needs of the organs and tissues Failure of the pump can have many causes Muscle Electrics Valves Blood supply The body does two things which make heart failure worse

9 ! a) The RAA system b) Remodelling of the cardiac chambers The R-A-A system. Renin-Angiotensin-Aldosterone System Hang your kidneys up with your laces Hang H-ang Hepatic = angiotensinogen Kidneys - Renal Renin L-ace lungs = ACE aldosterone vasoconstriction Other neurohormonal adaptation systems: -Activation of sympathetic nervous system (. Heart rate) - ADH (antidiuretic hormone) ADDS H20 (increases -load) - ANP Atrial Natriuretic Peptide o From the Atrium o Natriuresis losing Na (sodium) in the urine o Causes sodium and water loss in the urine so acts the other systems Remodelling of the cardiac chambers

10 ! Heart failure symptoms come from the part of the circuit before the problem Left heart failure symptoms: Right Heart Left Heart Either type causes: Right heart failure symptoms: Rest of body capillary bed Cor pulmonale: literally means heart lungs heart failure due to chronic lung disease Diagnosing Heart failure 1) Symptoms and Examination 2) Echocardiogram (fancy ultrasound of the heart) 3) BNP (brain natriuretic peptide) increased levels 4) Chest xray batwing and enlarged heart