Arterioles

Angiotensin II also acts on the smooth muscle in the walls of the arterioles causing these small diameter vessels to constrict, thereby restricting the outflow of blood from the arterial tree, causing the arterial blood pressure to rise.

Angiotensin, on the other hand, causes vasoconstriction on the systemic arterioles, and acts as a dipsogen for ostriches.

Aortic pressure is highest at the aorta and becomes less pulsatile and lower pressure as blood vessels divide into arteries, arterioles, and capillaries such that flow is slow and smooth for gases and nutrient exchange.

Arterioles have the greatest collective influence on both local blood flow and on overall blood pressure.

Constriction of the afferent arterioles causes a decrease in the intraglomerular pressure, reducing GFR proportionally.

If the pressure in the arteries falls, the opposite reflex is elicited: constriction of the arterioles, and a speeding up of the heart rate (called tachycardia ).

Lymphatic capillaries are slightly larger in diameter than blood capillaries, and have closed ends (unlike the blood capillaries open at one end to the arterioles and open at the other end to the venules).

Mean arterial pressure (MAP) is highest in the aorta and the MAP decreases across the circulation from aorta to arteries to arterioles to capillaries to veins back to atrium.

The afferent arterioles form the glomerulus (network of capillaries enclosed in Bowman's capsule ).

The arterioles are the main resistance vessels in the arterial tree, and small changes in diameter cause large changes in the resistance to flow through them.

The reverse occurs at high ambient temperatures, arterioles dilate to increase heat loss.

This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in the body (especially the heart, lungs and brain).

Travelling through the arterioles blood moves into the capillaries where gas exchange can occur.

When ambient temperatures are low, heterotherms will constrict their arterioles to reduce heat loss along skin surfaces.

When the arterial blood pressure rises the arterioles are stimulated to dilate making it easier for blood to leave the arteries, thus deflating them, and bringing the blood pressure down, back to normal.