Chapter 11 – Part 3The Cardiovascular System

Blood Vessels: The Vascular Blood Vessels: The Vascular SystemSystem

Blood circulates inside the blood vessels, which form a closed transport circuit. Takes blood to

the tissues and back

Blood Vessels: The Vascular Blood Vessels: The Vascular SystemSystem Like a system of roads, the vascular system

has its freeways, secondary roads, and alleys.

1. Blood leaves the heart via arteries

2. It then moves into successively smaller and smaller arteries

3. Blood then moves into arterioles, which feed the capillary beds

4. Blood enters the capillaries

5. Capillary beds are drained by venules

6. Veins empty into great veins, which return the blood to the heart

Blood Vessels: The Vascular Blood Vessels: The Vascular SystemSystem

Blood Vessels: The Vascular Blood Vessels: The Vascular SystemSystem Arteries and veins are simply conducting

vessels. Arteries carry blood away from the heart.

Veins carry blood to the heart.

Capillaries serve the needs of the cells. It is only through

their walls that exchanges between the tissue cells and the blood can occur.

The Vascular SystemThe Vascular System

Three Layers(Tunics) of Blood VesselsThree Layers(Tunics) of Blood Vessels1. Tunic Intima - Innermost layer

Thin layer of endothelium Lines the interior of the vessel Its cells fit closely together and form a slick surface

that decreases friction as blood flows through the vessel.

2. Tunic Media - Bulky middle coat Mostly smooth muscle and elastic Controlled by sympathetic nervous system Responsible for constricting or dilating the vessels

3. Tunic Externa - Outermost tunic Mostly fibrous connective tissue Function is to support and protect the vessels

Differences Between Arteries and Differences Between Arteries and VeinsVeins The walls of arteries are the thickest; the

walls of arteries are thicker than the walls of veins

Arteries are closer to the pumping action of the heart and must be able to expand as blood is forced into them

Artery walls must be strong and stretchy enough to take the continuous changes in pressure

Veins are far from the heart and the pressure in them tends to be low at all times

VeinsVeins The blood pressure in veins

is usually too low to force the blood back to the heart.

Lumens of veins tend to be much larger than those corresponding arteries

The larger veins have valves that prevent backflow of blood

Skeletal muscle “milks” blood in veins toward the heart

CapillariesCapillaries

Walls of capillaries are only one cell layer thick (just the tunica interna) to allow for exchanges between blood and tissue.

The tiny capillaries tend to form interweaving networks called capillary beds.

The flow of blood from an arteriole to a venule (through a capillary bed) is called microcirculation.

Capillary BedsCapillary Beds Capillary beds consist of

two types of vessels:1. Vascular Shunt –

Directly connects an arteriole to a venule

2. True Capillaries – Exchange vessels

O2 and nutrients cross to cells

CO2 and metabolic waste products cross into blood

Capillary BedsCapillary Beds Precapillary Sphincter -

surrounds the root of each true capillary and acts as a valve to regulate the blood flow into the capillary.

When the sphincters are open, blood flows through the capillaries and takes part in exchanges with tissue cells.

When the sphincters are closed, blood flows through the shunts and bypasses the tissue cells.

Varicose VeinsVaricose Veins Common in people who

stand for long periods of time (dentists, hairdressers, etc) and in obese individuals.

Common factor is the pooling of blood in the feet and legs and inefficient venous return resulting from inactivity or pressure on the veins.

The overworked valves give way, and the veins become twisted and dilated.