Question

Describe two types of blood flow regulation in detail at he level of the arteriole.

Answer 1

REGULATION OF ARTERIAL BLOOD PRESSURE
Arterial blood pressure varies even under physiological conditions. However, immediately it is brought back to normal level because of the presence of well organized regulatory mechanisms in the body. Body has four such regulatory mechanisms to maintain the blood pressure within normal limits
A. Nervous mechanism or short­term regulatory mechanism
B. Renal mechanism or long­term regulatory mechanism
C. Hormonal mechanism
D. Local mechanism.
NERVOUS MECHANISM FOR REGULATION OF BLOOD PRESSURE – SHORT-TERM REGULATION
Nervous regulation is rapid among all the mechanisms involved in the regulation of arterial blood pressure. When the pressure is altered, nervous system brings the pressure back to normal within few minutes. Although nervous mechanism is quick in action, it operates only for a short period and then it adapts to the new pressure. Hence, it is called short­term regulation. The nervous mechanism regulating the arterial blood pressure operates through the vasomotor system.   Vasomotor System
Vasomotor system includes three components:
1. Vasomotor center
2. Vasoconstrictor fibers
3. Vasodilator fibers.

MECHANISM OF ACTION OF VASOMOTOR CENTER IN THE REGULATION OF BLOOD PRESSURE
Vasomotor center regulates the arterial blood pressure by causing vasoconstriction or vasodilatation. However, its actions depend upon the impulses it receives from other structures such as baroreceptors, chemoreceptors, higher centers and respiratory centers. Among these structures, baroreceptors and chemoreceptors play a major role in the short­term regulation of blood pressure.
1. Baroreceptor Mechanism
Baroreceptors are the receptors, which give response to change in blood pressure. Baroreceptors are also called pressoreceptors.
Situation
Baroreceptors are situated in the carotid sinus and wall of the aorta .
Functions
Role of baroreceptors when blood pressure increases
When arterial blood pressure rises rapidly, baro­ receptors are activated and send stimulatory impulses to nucleus of tractus solitarius through glassopharyngeal and vagus nerves. Now, the nucleus of tractus solitarius acts on both vasoconstrictor area and vasodilator areas of vasomotor center. It inhibits the vasoconstrictor area and excites the vasodilator area.
Inhibition of vasoconstrictor area reduces vasomotor tone. Reduction in vasomotor tone causes vasodilatation, resulting in decreased peripheral resistance. Simultaneous excitation of vasodilator center increases vagal tone .This decreases the rate and force of contraction of heart, leading to reduction in cardiac output. These two factors, i.e. decreased peripheral resistance and reduced cardiac output bring the arterial blood pressure back to normal level .
Role of baroreceptors when blood pressure decreases
The fall in arterial blood pressure or the occlusion of common carotid arteries decreases the pressure in carotid sinus. This causes inactivation of baroreceptors. Now, there is no inhibition of vasoconstrictor center or excitation of vasodilator center. Therefore, the blood pressure rises.
Information regarding blood pressure within the range of 50 to 200 mm Hg (mean arterial pressure) reaches the vasomotor center through the carotid baro­ receptors. Information about the blood pressure range of 100 to 200 mm Hg goes through aortic baroreceptors.
Both carotid and aortic baroreceptors are stimulated by the rising pressure than the steady pressure and their response depends upon the rate of increase in the blood pressure.
Since the baroreceptor mechanism acts against the rise in arterial blood pressure, it is called pressure buffer mechanism or system and the nerves from baroreceptors are called the buffer nerves.
2. Chemoreceptor Mechanism
Chemoreceptors are the receptors giving response to change in chemical constituents of blood. Peripheral chemoreceptors influence the vasomotor center.   
Situation
Peripheral chemoreceptors are situated in the carotid body and aortic body.
Function
Peripheral chemoreceptors are sensitive to lack of oxygen, excess of carbon dioxide and hydrogen ion concentration in blood. Whenever blood pressure decreases, blood flow to chemoreceptors decreases, resulting in decreased oxygen content and excess of carbon dioxide and hydrogen ion. These factors excite the chemoreceptors, which send impulses to stimulate vasoconstrictor center. Blood pressure rises and blood flow increases. Chemoreceptors play a major role in maintaining respiration rather than blood pressure .
Sinoaortic mechanism
Mechanism of action
chemoreceptors in carotid and aortic region constitute sinoaortic mechanism. Nerves supplying the baroreceptors and chemoreceptors are called buffer nerves because these nerves regulate the heart rate , blood pressure and respiration .

LOCAL MECHANISM FOR REGULATION OF BLOOD PRESSURE
In addition to nervous, renal and hormonal mechanisms, some local substances also regulate the blood pressure. The local substances regulate the blood pressure by vasoconstriction or vasodilatation.
LOCAL VASOCONSTRICTORS
Local vasoconstrictor substances are derived from vascular endothelium. These substances are called endothelium-derived constricting factors (EDCF). Common EDCF are endothelins (ET), which are peptides with 21 amino acids. Three types of endothelins ET1, ET2 and ET3 are identified so far.
Endothelins are produced by stretching of blood vessels. These peptides act by activating phospholipase, which in turn activates prostacyclin and thromboxane A2. These two substances cause constriction of blood vessels and increase the blood pressure.
LOCAL VASODILATORS
Local vasodilators are of two types: 1. Vasodilators of metabolic origin 2. Vasodilators of endothelial origin.
Vasodilators of Metabolic Origin
Vasodilators of metabolic origin are carbon dioxide, lactate, hydrogen ions and adenosine .
Vasodilators of Endothelial Origin
Nitric oxide (NO) is an endothelium­derived relaxing factor (EDRF). It is synthesized from arginine. Nitric oxide synthesis is stimulated by acetylcholine, bradykinin, VIP, substance P and platelet breakdown products. As nitric oxide is a vasodilator, deficiency of this leads to constant vasoconstriction and hypertension.
Other functions of nitric oxide are penile erection with vasodilatation and engorgement of corpora cavernosa, activation of macrophages in brain, destruction of cancer cells and relaxation of smooth muscles of gastrointestinal tract.
Types of nitric oxide
i. NO3 (nitrate)
ii. NO+ (nitrosonium cation)
iii. NO– (nitroxyl anion).