Blood Vessels

The roads around the body

Some basics

  • Closed circuit of tubes that carries blood from the heart to the cells and back again
  • The force of the heart beat (blood pressure) pushes blood through the first two types of blood vessels. Muscle contraction brings it back.
  • 3 main types

◦Arteries – carries blood away from heart

◦Capillaries – delivers nutrients and other materials to the cells

◦Veins – carries blood to the heart

Arteries

  • Arteries have strong elastic walls adapted to high pressure

◦3 layers

▪Inner layer (tunica interna) made of endothelium like the heart – smooth to help prevent blood clots

▪Middle layer (tunica media) smooth muscle fibers and a thick layer of elastic connective tissue, nerves connect here to help control vasodilation and vasoconstriction

▪Outer layer (tunica externa) thin, connective tissue, attached artery to surrounding area.

  • Branch into thinner vessels called arterioles

Capillaries

  • Start at the end of the smallest arterioles and end at the venules
  • Are the smallest diameter blood vessels

◦Composed of the endothelium that is the inner lining of the arterioles

  • Are semipermeable due to small slits where the endothelial cells overlap

◦Size of the opening depends on the tissue, muscles less, organs more

  • Precapillary sphincters control the blood flow into the capillary.

Veins

  • Veins carries blood back to the artria. Venules are little veins that connect the capillaries to the veins.
  • 3 layer walls once again, but this time the middle layer is not as developed.

◦Therefore less elastic and muscular

  • Many veins have valves

◦2 leaflets that close if the blood moves away from the heart

  • Typically found in the limbs

Neat trick veins...

  • Veins are the blood reservoirs
  • If the blood pressure drops significantly due to blood loss, the veins will constrict and return more blood to the heart.

◦Help to maintain normal blood pressure even loss is 25% of blood volume

Blood Pressure

  • The force the blood exerts on the inner walls of the blood vessels

◦Typically referring to the pressure in the arteries (Arterial blood pressure)

  • Maximum pressure during ventricular constriction is called Systolic pressure

◦120 is average

  • Lowest pressure when the ventricles relax is the Diastolic pressure

◦80 is average

Two pathways

  • Systemic circuit – left ventricle to the rest of the body and then to the right atrium
  • Pulmonary circuit – Right ventricle to the lungs and then to the left atrium

Increased pressure

  • Varicose veins – veins with abnormal dilations

◦Results from increased blood pressure in the saphenous veins (found in the legs) due to gravity.

  • Hypertension – persistently elevated arterial pressure

◦Can contribute to atherosclerosis