Answer
Blood flow velocity is the rate at which the blood flows through a blood vessel.
In the systemic circulation the blood leaves the left ventricle and passes first through the aorta ; it then travels through the large elastic arteries, the muscular arteries, the arterioles, and thence to the capillaries and a capillary beds. After the blood has passed trough the exchange vessels of the capillary bed it is collected by venules , transported to large and larger and eventually veins and finally delivered to the right atrium by the venae cavae.
Blood velocity at the artra is about 30-35cm/s. This is where the diameter of the blood vessel is greatest . As the blood flows through narrower and narrower arteries, the velocity drops progressively to 20cm/s then to 10cm/s. Blood flow velocity in the capillary bed s is about 1-2cm/s. From the capillary bed the blood then flows into the collecting venules at much the same speed.From the small veins to lager veins the velocity increases from to about 7-8 cm/s, and finally attains a velocity of 8-18 cm/s as the venae cavae empty into the right atrium.
There are several factors responsible for this variation in the velocity of blood flow. the most important of these factors are diameter of blood vessel. peripheral resistance, vasomotor tone, length of blood vessel,, activity of sphincters;
Diameter ( radius of blood vessel) The aorta has a larger diameter than the arteries arterioles or capillaries but there are many capillaries and the combined cross sectional ara of the smaller vessels is greater than the cross sectional area of the aorta ( or the vena cavae) Velocity is inversely related to cross sectional area so as combined cross sectional area increases the velocity of flow decreases.
Peripheral resistance is also an important factor that impacts velocity. Resistance is generated by friction against vessel walls In a wide vessel with laminar flow the middle of the stream flows faster than the side because of resistance due to friction.. The smaller the radius of a tube the greater the resistance and the greater the decrease in velocity.
The greater resistance of the exchange capillary tubes slows the blood flow and this facilitates exchange of solutes, , water and cells between capillaries and interstitial fluid and venules and tissues.
Vascular sphincters is the net important factor that can slow down velocity of blood stream.. The precapillary sphincters of metarterioles can control the amount and velocity of blood flow into the capillary bed or the thoroughfare channel.
Vasomotor tone-- whether modulated by vascular reflex arc or moderated by sympathetic inputs from the cardiovascular center of the medulla can affect velocity of blood flow by increasing blood pressure and controlling arteriolar constriction
Work Step by Step
Blood flow speed or velocity varies with blood vessel diameter, blood vessel length, peripheral resistance of blood vessels. Vasomotor tone, blood pressure, cardiac output and heart rate can also affect blood flow velocity. However, the dramatic systematic changes that take place as blood flows from aorta to capillary beds, and from venules to venae cavae are mediated primarily by vessel combined cross sectional areas(csa), vessel resistance to fluid flow ( due to friction) and changes in vasomotor tone mediated by sympathetic neural activity.
The csa of the aorta of aorta is 2,5cm$^{y}$2; csa of all capillaries 4500 cm2x$^{y}$ . So blood flow is fast in aorta (20-40cm/s) and slow in capillaries (0.3-2cm/s).
Resistance varies inversely to the fourth power of of the radius of a vessel ; therefore if the radius doubles, resistance drops to S/1/4/S-- that is one over the fourth power of the value of the original radius.
If =2 then $\Delta$ R 1/16th/S
Resistance and vessel cross sectional areas are the most potent factors that control velocity , though vasomotor tone, and arteriolar constriction are significant and more readily changeable.
