Vascular Physiology 3 Flashcards
capillary structure
*the capillary is composed of a single cell-layer with an adjoining basement membrane
*fluid/electrolytes/small hydrophilic compounds are able to pass through small water-filled CHANNEL
*lipids and cholesterol are able to pass through the endothelial CELL itself
4 things which affect the diffusion rate across the capillary
- concentration difference (ΔX)
- surface area for exchange (A)
- diffusion distance (ΔL)
- capillary wall permeability
note: surface area and concentration difference are proportional, while diffusion distance is inverse
capillary variation in water permeability
*brain capillaries are an example of capillary beds with lower water permeability
*capillary beds with higher water permeability include: kidney, bone marrow, liver
capillary variation in protein permeability
*continuous capillaries: do NOT let protein into the tissues
*discontinuous & fenestrated capillaries: very “leaky” to proteins
continuous capillaries
*prevent translocation of proteins from the capillary into the surrounding tissues
*ex: brain
fenestrated capillaries
*small holes in the capillary allow for the passage of electrolytes, fluids, and small proteins
*ex: kidneys, intestines
sinusoidal capillaries
*larger holes/gaps in the capillary allow for the passage of electrolytes, fluids, proteins, and RBCs
*ex: bone marrow, liver, spleen
pressure gradient across the capillary
*pressure varies from one end of the capillary to the other
*pressure on the arteriole side of the capillary (30 mmHg) is higher than pressure on the venule side of the capillary (10 mmHg)
*this pressure difference drives blood flow from the arteriole to the venule
capillary hydrostatic pressure (Pc)
*Pc is MUCH MORE influenced by changes in Pv (pressure in the venule) than by changes in Pa (pressure in the arterioles)
fluid exchange at the capillary
- arteriolar side of the capillary = filtration (fluid exiting the capillary, going into the interstitium)
- venule side of the capillary = reabsorption (fluid re-entering the capillary)
capillary oncotic pressure (Πc)
*because the capillary barrier is readily permeable to ions, the osmotic pressure within the capillary is principally determined by PLASMA PROTEINS that are relatively impermeable (ex. albumin)
*several different types of disease manifest as a reduced capillary oncotic pressure: advance liver disease (reduced protein synthesis), nephrotic syndrome (kidney spills a lot of protein into the urine)
tissue oncotic pressure (Πi)
*the oncotic pressure of the interstitial fluid depends on the interstitial protein concentration and the reflection coefficient of the capillary wall
*in a “typical” tissue, tissue oncotic pressure is about 5 mmHg (much lower than capillary plasma oncotic pressure)
capillary filtration and absorption
*the relationship among the factors which cause filtration and absorption is referred to as Starling’s law: (Pc - Pi) - 1(Πc - Πi)
*net filtration: (Pc-Pi) > (Πc-Πi)
*net absorption: (Pc-Pi) < (Πc-Πi)
*no net fluid movement: (Pc-Pi) = (Πc-Πi)
post-capillary sphincter
*if pressure in capillary falls (too much or inappropriately), post-capillary sphincter tightens to increase capillary pressure
pitting edema
*caused by:
1. INCREASED capillary hydrostatic pressure leading to excess fluid filtration (fluid moving out of capillary into interstitium)
2. DECREASED capillary oncotic pressure leading to fluid going to the tissues
*when you press, it leaves a thumbprint
non-pitting edema
*caused by lymphatic obstruction or translocation of proteins in the tissue, which draws out fluid (myxedema)
*when you press, it does not leave a thumbprint
compartment syndrome
*when Pi (pressure in the interstitial space/tissues) > venous pressure, it may limit/stop flow leading to critical ischemia of affected distribution
*if blood can’t exit, new blood can’t enter
*often caused by a knife wound, gunshot wound, injury, bone fracture
veins and hydrostatic pressure when lying down
*when lying down: pressure in the veins is higher than pressure in the right atrium, and blood flows from higher pressure to lower pressure
*if someone is light-headed from low blood pressure, have them lie down
venous return: skeletal muscle pump
*veins in the legs have VALVES to prevent venous blood from going backwards
*when skeletal muscle contracts, it helps to propel blood back up to the heart
venous return: respiratory cycle
*inspiration: negative thoracic pressure when we breathe in PROMOTES VENOUS RETURN (right ventricle gets bigger); however, due to low pressure, blood stays in lungs rather than go to the left ventricle
*expiration: positive thoracic pressure when we exhale reduces venous return and goes in opposite direction of inspiration
venous return curve
*venous return is greater when the pressure in the right atrium is lower
*as the right atrial pressure increases, less venous blood is returning to the heart
*many factors can influence venous return
factors that can increase venous return
*IV fluids, venoconstriction (increases venous pressure) → mean circulatory filling pressure increases
*exercise, decreased systemic vascular resistance → mean circulatory filling pressure does not change
factors that decrease venous return
*volume loss (ex. hemorrhage), venodilation (decreases venous pressure) → mean circulatory filling pressure decreases
*increased systemic vascular resistance → mean circulatory filling pressure does not change
aerobic exercise - overview
*any sustained exercise which improves heart muscle and lung function, thereby optimizing out body’s use of oxygen
*includes jogging, rowing, swimming, cycling
*typically lasts longer than 20 minutes
