The Vascular system: Arterioles and Control of Arteriolar Smooth Muscle

Arterioles function

1. in individual organs, determine relative blood flow to the organ at any given mean arterial pressure

2. All together are the major factor in determining mean arterial pressure (MAP)

   1. causes drop in MAP as distance from heart increases

Circular smooth muscle

• state of contraction can be regulated

Arterioles and resistance to blood flow

• Vasodilation

• Vasoconstriction

• High resistance vessels due to small size

  • altering arteriolar diameter alters resistance and flow

• Vasodilation → relaxation of arteriole smooth muscle

  • increased blood flow to organs

• Vasoconstriction → contraction of arteriolar smooth muscle

  • decreases blood flow to organs

Arterioles and resistance to blood flow

• intrinsic or basal tone

• Intrinsic or basal tone

• Arteriole smooth muscle is partially contracted in the absence of external factors

• Other factors can increase or decrease the state of contraction to cause vasoconstriction or vasodilation

Arterioles and resistance to blood flow

• Extrinsic or intrinsic factors alter basal tone

• Extrinsic → factors external to the organ or tissue; whole body needs (MAP); nerves and hormones can affect constriction of arterioles muscles

• Intrinsic → local controls; organs and tissues alter their own arteriolar resistances independent of nerves or hormones, not whole body only certain organs and tissues

External controls of ANS

• Sympathetic neurons

• Parasympathetic neurons

• Noncholinergic, nonadrenergic neurons

• Sympathetic neurons

  • arterioles innervated by sympathetic postganglionic neurons

    • NE → vasoconstriction (a-adrenergic receptors)

  • Can be used to cause vasodilation

    • Sympathetic tone can be increased (vasoconstriction) or decreased (vasodilation)(in addition to vessel’s intrinsic/ basal tone)

    • To vasodilate: decrease rate of sympathetic activity to below the basal level

  • Regulating MAP

• Parasympathetic neurons

  • little/no parasympathetic innervation of arterioles

• Noncholinergic, nonadrenergic neurons

  • NO (nitric oxide) → vasodilation

Extrinsic controles: Hormones

• epinephrine from adrenal medulla

• Arteriolar smooth muscle may contain both adrenergic receptor subtypes (α/β)

Local Controls: Active hyperemia

• Arteriolar smooth muscle is sensitive to local chemical changes (e.g. O2, CO2, H+)

• Local chemical changes are the result of changes in metabolic activity

• Increased metabolic activity

  • results in vasodilation of arterioles and increased blood flow

• no nerves or hormones involved

Local Controls: Flow Autoregulation

• changes in arterial blood pressure alter blood flow to an organ

  • changes the concentration of local chemicals (e.g. O2, CO2, H+)

• Arterioles change their resistance to maintain constant blood flow in the presence of a pressure change

• Constant metabolic activity

• No nerves or hormones involved

Local Controls: Flow Autoregulation

• Flow autoregulation may also be mediated by the myogenic response

  • direct response of arteriolar smooth muscle to stretch