A Jedi's Strength Flows from the Force.

GE 345: Week 2

Circulatory System -- Circulation and Local Control of Blood Flow

Updated by Tracey 20 June 02

Functions: arteries transport blood under high pressure to the tissues & have strong walls through which blood flows swiftly; arterioles control blood flow to capillaries; capillaries exchange fluid, nutrients, hormones & other substances between blood and interstitial spaces or alveoli in lungs; venules (aka venioles) collect blood from capillaries; veins transport blood back to heart -- by this time pressure is low, so walls are relatively thin.

Tissues control local blood flow proportionally to needs. Typically based on metabolism and oxygen or nutritional need: glandular organs have relatively large blood flow, while muscles at rest have relatively decreased needs and therefore less blood flow.

Flow may be determined in two time frames:

Acute: rapid changes occuring in seconds to minutes.
Thought to be controlled through vasodilation and vasoconstriction or control of the precapillary sphincters.
Long-Term: slow changes over days, weeks or months.
Controlled through changes in tissue vascularity, angiogenesis factor release (causes new vessel growth).

Regulation

The autonomic nervous system operates subcosciously to control internal body functions. It controls the sympathetic and parasympathetic nervous systems.

Sympathetic System
Increases heart rate and resistance to blood flow.
Parasympathetic System
Decreases heart rate and contractility.

Humoral Regulation is controlled by substances in body fluids, such as hormones, ions, etc. Some are formed in special glands and transported through the body. Others are formed in local tissues, causing only local effects. Vasoconstrictors include:

Epinephrine and Norepinephrine secreted by the Adrenal Medullae
Stimulated by the sympathetic system. Norepinephrine is a vasoconstrictor. Epinephrine has similar effects in most areas, although it often causes mild dilation in skeletal and cardiac muscle.
Angiotensin
Powerful vasoconstrictor. Decrease in arterial pressure causes kidneys to secrete renin, which in turn causes angiotensin to split away from a plasma protein, renin substrate.
Vasopressin
Formed in hypothalamus and transported to the pituitary where it eventually secretes into the blood. More powerful constrictor than angiotensin, although only very minute amounts are normally secreted.

Vasodilators include:

Bradykinin
Powerful arteriolar dilator and increases capillary permeability. Believed to help regulate blood flow and capillary leakage of fluids in inflamed tissues as well as helping to regulate blood flow to skin, salivary and other gastrointestinal glands.
Serotonin
Can either dilate or constrict depnding on the condition or area of circulation.
Histamine
Released by damaged tissues. Also can increase capillary porosity, allowing leakage of plasma protein and fluid into the tissues.
Prostaglandin
Most tissues contain small to moderate amounts.

Ions also have specific effects on blood vessels.

Calcium: stimulates smooth muscle contraction, causing vasoconstriction
Potassium: inhibits smooth muscle contraction, causing vasodilation
Sodium: increases osmolality, causing arteriolar dilation
Hydrogen: causes constriction. Slight decrease in Hydrogen ion causes dilation, but intense decrease causes dilation.