 This video will cover the following objective from cardiovascular physiology part two blood vessels Describe the factors that influence net filtration pressure in the capillaries to find filtration and reabsorption Capillaries are the smallest blood vessels which are specialized for the exchange of substances between the blood and the surrounding tissues Fluid is able to flow from the blood through the capillary wall through the intercellular clefts of continuous capillaries and through the fenestration pores of fenestrated capillaries as well as through the intercellular gaps of sinusoids blood moves in response to the blood pressure which is the force of blood pushing out on the Walls of the blood vessel the heart generates blood pressure by pumping blood into the arteries So the elastic arteries have the highest blood pressure and then blood flows From an area of high pressure to lower pressure as it's flowing from elastic arteries into muscular arteries arterioles capillaries, venules and veins This blood pressure is also known as hydrostatic pressure You can see that the hydrostatic pressure in the capillaries is ranging from around 20 to 40 millimeters mercury so the capillary hydrostatic pressure ranges from 20 to 40 millimeters of mercury as the hydrostatic pressure is the force that drives blood to move through the capillary The value of the capillary hydrostatic pressure will decline as blood flows from the arterial end to the venules at the venus end in This illustration we can see the capillary hydrostatic pressure at the arterial end of a capillary bed is around 35 millimeters of mercury now there's Another force that is drawing liquid back into the blood known as the blood colloidal osmotic pressure So this is the force of osmosis which draws liquid into the blood and Is a force that's opposite to the capillary hydrostatic pressure that? forces liquid out of the blood the difference between capillary hydrostatic pressure and blood colloidal osmotic pressure is Known as the net filtration pressure if the net filtration pressure is positive Liquid will move out of the blood in a process known as filtration if the net filtration pressure is negative Liquid will move from the extracellular fluid Into the blood in a process known as reabsorption We can calculate the net filtration pressure at different points along the capillary by taking the difference between the capillary hydrostatic pressure and the blood colloidal osmotic pressure With a capillary hydrostatic pressure of 35 and a blood colloidal osmotic pressure of 25 We get the net filtration pressure of 10 driving filtration at the arterial end of the capillary bed at some point in the mid region of the capillary the Capillary hydrostatic pressure has declined to Equal the colloidal osmotic pressure therefore the net filtration pressure is Zero and there will be no net movement of fluid. There will not be any filtration or reabsorption But further towards the venules in the venous end of the capillary the capillary hydrostatic pressure has fallen below The blood colloidal osmotic pressure with a capillary hydrostatic pressure of 18 If we subtract the colloidal osmotic pressure of 25 from 18 We can calculate the net filtration pressure equals negative seven millimeters of mercury and so the negative net filtration pressure drives reabsorption Forcing liquid back into the blood at the venous end of a capillary bed