In this article we will discuss about the meaning and role of peripheral resistance in cardiovascular system.
Meaning of Peripheral Resistance:
i. It is the resistance offered by vessel wall for flow of blood.
ii. The unit used to measure resistance (pressure divided by flow) is dynes/cm. In other words, resistance in cardiovascular system is generally expressed as R units (Reynold’s number), which is obtained by dividing the pressure in mm Hg by flow in ml/sec. For example, if the mean arterial pressure is 90 mm Hg and the left ventricular output is 90 ml/sec, the total peripheral resistance is
90 mm Hg/90 m1/sec = 1 R unit
In general, the resistance offered by the vessel wall is influenced by:
R = 8 ƞ, l/πr4
Wherein
8 is the integer of velocity of flow
ƞ is viscosity of blood
l is length of blood vessel
r is the radius of blood vessel (4th power of radius)
The length and viscosity of the blood vessel do not vary much easily. Hence varying the radius of the blood vessel can bring about a lot of moment-to- moment alterations of peripheral resistance. This can be easily altered by sympathetic nerve fibers and chemical factors.
Role of Arterioles in Maintenance of Peripheral Resistance:
i. Peripheral resistance is maximum at the level of arterioles.
ii. Hence arterioles are known as seat of peripheral resistance.
iii. It is maximum at arterioles because in the walls of the arterioles, there are plenty of smooth muscle fibers.
iv. The contractility of the smooth muscle is constantly under the influence of vasoconstrictor impulses coming from the lateral horn cells of spinal cord reaching the arterioles along the sympathetic fibers.
v. Apart from the neural influence that can affect the contractility of the smooth muscle, there are local and hormonal factors, which can alter the contractile state of vascular smooth muscle.
vi. Some have constrictor influence and some other substances have dilator effect.
vii. Therefore, peripheral resistance is controlled by neural and chemical mechanisms.
Neural Mechanism:
i. The role of vasomotor center present in the brainstem is very important.
ii. The vasomotor center exerts its influence through the sympathetic nerves on the smooth muscle of the arterioles (Fig. 3.13).
iii. Sympathetic tone or arteriolar tone refers to constant excitatory influence by sympathetic nerves on smooth muscles of arterioles. About 1-3 impulses per second reach the arterioles. These impulses come from the lateral horn cells of spinal cord. The frequency of impulses can be increased up to 10/second.
iv. Intervention to the vasomotor-sympathetic pathway affects peripheral resistance and concomitant changes in the blood pressure.
v. Sympathetic constrictor influence is not only to the smooth muscles present in the walls of the arterioles, but also on the smooth muscles present at the beginning of the capillaries (pre-capillary sphincters), post-capillary sphincters and on the walls of the veins.
Hormonal Mechanism (Table 3.1):
i. Norepinephrine exerts a powerful vasoconstrictor effect.
ii. However, adrenaline in certain areas has vasoconstrictor effect and in certain other areas has vasodilator effect. In the skeletal region, the vasodilator effect is brought about by the action through beta receptors whereas in most of the other parts of body, the vasoconstrictor effect will be mediated through the alpha receptor activity. Vasoconstrictor effect of adrenaline is used clinically in the treatment of epistaxis.
iii. The role of these hormones secreted from adrenal medulla is of much importance in physiological conditions, like muscular exercise, etc. But they have vital role to play in certain pathological conditions, like cardiovascular shocks.
iv. Apart from adrenaline, and noradrenaline, the other blood-borne factors which have role in altering peripheral resistance are serotonin (5HT), angiotensin II, vasopressin, histamine, etc. In addition to blood-borne factors, there are certain other local factors like adenosine, bradykinin, etc., which also have the ability to alter peripheral resistance.
Extrinsic Control of Peripheral Blood Flow:
The relationship between the mean pressures, mean velocity and cross-sectional area is designed in different parts of the circulatory system to suit the demands of the parts of the body (Figs 3.14, 3.15 and Table 3.2).
Flow which is nothing but the blood flow is equal to effective perfusion pressure in that part of body divided by the resistance. The effective perfusion pressure is the mean trans-luminal pressure at the arterial end minus the mean pressure at the venous end.
The units of resistance (pressure divided by flow) are dynes/cm. In other words, resistance in cardiovascular system is generally expressed as R units (Reynold’s number), which are obtained by dividing the pressure in mm Hg by flow in ml/sec. For example, if the mean arterial pressure is 90 mm Hg and the left ventricular output is 90 ml/sec, the total peripheral resistance is
90 mm Hg/90 ml/sec = 1 R unit
In general, the resistance offered by the vessel wall is influenced by
R = 8 ƞ, l/πr4
Wherein
8 is the integer of velocity of flow
ƞ is viscosity of blood
l is length of blood vessel
r is the radius of blood vessel (4th power of radius)
The length and viscosity of the blood vessel do not vary much easily. Hence varying the radius of the blood vessel can bring about a lot of moment-to- moment alterations of peripheral resistance. This can be easily altered by sympathetic nerve fibers and chemical factors.
Poiseuille-Hagen Formula:
The formula explains relationship between the blood flow to that of pressure gradient that is available and the total peripheral resistance in the body. According to the formula,
F = (P1 – P2) πr4/8 ƞl
Wherein
F = flow
(P1-P2) is the pressure difference between the two ends of the blood vessel
r4 = fourth power of radius
8 is the integer of velocity of flow
ƞ is viscosity of blood
l is length of blood vessel
The flow is directly proportional to the pressure difference between the two ends of blood vessel that is in considerations. P2 is mostly the pressure that gets reflected in the down steam of blood vessel from the left ventricle.
Hence a change in P1 is not all that easy and not practical one either to regulate the blood flow in a particular part of body. P2 is the pressure at the venous end of the capillaries. If P2 is increased, the hydrostatic pressures in the capillaries are increased and this can give rise to accumulation of fluid in the interstitial spaces and edema.
The flow is inversely proportional to the fourth power of radius. Supposing the radius of blood vessel changes from one to two, the rate of change of flow that is observed will be about 16 times.
In the body, when flow of the blood has to be adjusted in a particular tissue or organ, it is the radius of the blood vessel, which gets altered in the concerned part of body to suit the required volume of blood flow:
i. The activity of the pre-capillary sphincter is altered in state of cardiovascular shocks, injury to any part of the body. These events will try to maintain the cardiovascular dynamics and reduce the volume of blood lost from the injured area respectively.
ii. There is venomotor tone, but compared to arteriolar tone this is much less.
iii. Capacitance vessels would not show much of alterations in the lumen diameter and since they are larger and very little smooth muscle is present in their walls; they do not contribute for regulation of peripheral resistance. Blood gets mobilized from these capacitance vessels to peripheral circulation in conditions, like hypovolemia thereby try to restore the normal cardiovascular dynamics.
iv. The influence of parasympathetic nerves to vascular smooth muscle is not much. There is some amount of parasympathetic innervation to the vascular smooth muscles present in head, viscera and genitalia.
Sympathectomy and Blood Flow:
In certain cases, the sympathetic innervation to the blood vessels may have to be abolished permanently. In such a case, following sympathectomy, in the initial periods there will be increase of blood flow due to loss of arteriolar tone. However, in course of time, the flow of blood gets decreased markedly. This is because of denervation hypersensitivity or denervation supersensitivity.
As long as the post-ganglionic sympathetic innervations to the vascular smooth muscles are intact, the responsiveness of the vascular smooth muscle is restricted to circulating chemical substances. However, after sometime following denervations, the vascular smooth muscle becomes hyper-responsive to circulating chemical substances. This results in increased vascular tone and causes the blood flow to decrease.