Question

1. Name the adrenergic receptors. Where are they found? What happens when each is stimulated and what is the net effect?

2. Compare and contrast blood flow through one 4mm diameter blood vessel vs two 2mm blood vessels. Include volume, resistance, and rationale for your
answer.

Answer 1

Adrenergic receptors which are mostly acted upon by norepinephrine and epinephrine are mainly divided into two types, Alpha receptors and Beta receptors.

Alpha receptors are further divided into,

Alpha-1 receptors,  they are found on vascular smooth muscle. They determine both arteriolar resistance and venous capacitance, when these receptors are stimulated they cause vasoconstriction, because of which blood pressure increases.

Alpha-2 receptors, they  are found both in the brain and in the periphery, In the brain stem, they modulate sympathetic outflow, when stimulated they decrease blood pressure.

Beta receptors are furter classified as

Beta-1,they are found the heart, the kidney, and the fat cells.When these are stimulated they increase the blood pressure, increase the heart rate and myocardial contraction, and increased breakdown of fat.

Beta-2 receptors, they are located in the bronchioles of the lungs and the arteries of the skeletal muscles, when stimulated they cause bronchodilation, widening of the airways of the lungs, they also increase the levels of glucos in blood stream.

Beta-3 receptors,they are mainly found in the gallbladder, urinary bladder and adipose tissue. when stimulated they cause relaxation of urinary bladder, and can increase the breakdown of fat.

Resistance to blood flow can be determined by Poiseuille's Equation, which states that Vessel resistance (R) is directly proportional to the length (L) of the vessel and the viscosity (η) of the blood, and inversely proportional to the radius to the fourth power, R = nL/ r4.  Changes in diameter and radius are directly proportional to each other

From this equation it is very evident that, vessel resistance is very sensitive to changes in radius, the change in radius alters resistance to the fourth power of the change in radius, what it means is that if there is two fold increase in diameter than the resistance will decrease by 16 fold, as the resistance decreases the rate of flow increases, So the resistance offered by 4 mm diameter blood vessel will be very less as compared to 2 mm vessels diameter and the flow of blood will be very in the larger diameter blood vessel.