Peripheral Vascular System

Function: Carry blood away from the heart.
Pressure: Carries blood under pressure.
Branches: Gives rise to thinner vessels known as arterioles.
Structure:
- Thick walled vessels: Composed of three layers.
- Tunica interna (intima): The innermost layer.
- Tunica media: The middle layer which contains smooth muscle.
- Tunica externa (adventitia): The outermost layer that provides structural support.

Function: Carry blood to the heart.
Pressure: Carries blood under low pressure.
Valves: Contain valves to prevent backflow of blood.
Structure:
- Thin walled vessels: Composed of three layers.
- Tunica intima: The innermost layer.
- Tunica media: The middle layer which is thinner than in arteries.
- Tunica externa: The outermost layer.

Tunica intima (tunica interna):
- Artery: Endothelium appears wavy due to muscle constriction; has internal elastic membrane in larger vessels.
- Vein: Endothelium appears smooth; internal elastic membrane absent.
Tunica media:
- Artery: Normally the thickest layer with smooth muscle cells and elastic fibers that vary by distance from the heart; has external elastic membrane present in larger vessels.
- Vein: Usually thinner than tunica media, consists of smooth muscle and collagen fibers.
Tunica externa (tunica adventitia):
- Artery: Thinner than tunica media in all but the largest arteries, contains collagenous and elastic fibers, and has innervation (nervi vasorum) and blood vessels (vasa vasorum).
- Vein: Normally thicker than tunica media, collagenous and smooth fibers predominate, with nervi vasorum and vasa vasorum present.

Definition: Very small arteries.
Function: Deliver blood to capillaries.
Role: Help regulate blood flow and blood pressure through vasoconstriction and vasodilation.

Definition: Smallest and thinnest blood vessels.
Function: Site of gas and nutrient exchange between blood and interstitial fluid.
Structure: Composed of a single layer of endothelium and a basement membrane.
Types:
- Continuous: Plasma membranes form a continuous ring around the vessel lumen; found in skeletal muscle, smooth muscle, connective tissues, and lungs.
- Fenestrated: Plasma membrane contains pores; found in kidneys and villi of small intestines.
- Sinusoids: Contains spaces between endothelial cells with a missing or incomplete basement membrane; found in liver and spleen.

Process: Gases, nutrients, and wastes exchanged between capillaries and tissues.
Modes of Exchange:
- Diffusion: Movement of ions or molecules from high concentration to low concentration.
- Concentration Gradient: The difference in concentration.
- Bulk Flow: Involves filtration and absorption.
- Vesicular Transport: Includes endocytosis and exocytosis.

Definition: Net movement of particles from an area of high concentration to an area of low concentration.
Conditions for Rapid Diffusion:
- Short distances.
- Large concentration gradients.
- Small size of ions or molecules.

Definition: Removal of solutes as a solution flows across a porous membrane.
Driving Force: Hydrostatic pressure, which forces water and small solutes out of the capillaries.
Process: Occurs at the arterial end where pressures are highest; larger particles such as blood cells and plasma proteins remain.
Net Filtration Example:
- Net filtration pressure: Hydrostatic pressure - Colloid osmotic pressure = Net filtration.
Fluid Exchange: Net average of 3 L/day of fluid filters out of the capillaries to be picked up by lymph vessels and returned to circulation.

Process: Returning fluids and other items into circulation.
Driving Force: Osmosis, which is the movement of water across a selectively permeable membrane due to osmotic pressure.
Osmotic Pressure: Higher solute concentration results in higher osmotic pressure.

Function: True capillaries are the exchange vessels where oxygen and nutrients cross to cells and waste products cross into blood.
Precapillary Sphincters: Control blood flow into capillary beds based on tissue needs.

Components:
- Autoregulation: Local mechanisms respond to changes within tissue.
- Vasodilators: Increase blood flow in response to low oxygen levels, high carbon dioxide, and metabolic acids.
- Vasoconstrictors: Decrease blood flow during injury through constriction of precapillary sphincters.
- Neural Regulation:
- Cardioacceleratory Center: Increases cardiac output via sympathetic stimulation.
- Cardioinhibitory Center: Decreases cardiac output via parasympathetic innervation.
- Hormonal Regulation: Various hormones modulate blood pressure via different mechanisms, such as vasoconstriction, fluid retention, or increasing cardiac output.