Structure of Blood Vessels

Composed of three layers (tunics) Tunica intima – composed of simple squamous

epithelium Tunica media – sheets of smooth muscle

Contraction – vasoconstrictionRelaxation – vasodilation

Tunica externa – composed of connective tissue Lumen

Central blood-filled space of a vessel

Structure of Arteries, Veins, and Capillaries

Structure of Arteries, Veins, and Capillaries

Figure 19.1a

Types of Blood Vessels

Arteries – carry blood away from the heart Capillaries – smallest blood vessels

The site of exchange of molecules between blood and tissue fluid

Veins – carry blood toward the heart

Types of Arteries

Elastic arteries – the largest arteries Diameters range from 2.5 cm to 1 cm Includes the aorta and its major branches Sometimes called conducting arteries High elastin content dampens surge of blood

pressure

Figure 19.2a

Types of Arteries

Muscular (distributing) arteries Lie distal to elastic arteries Diameters range from 1 cm

to 0.3 mm Includes most named

arteries Tunica media is thick Unique features

Internal and external elastic laminae

Figure 19.2b

Figure 19.2c

Types of Arteries

Arterioles Smallest arteries Diameters range from 0.3 mm to 10 µm Larger arterioles possess all three tunics Diameter of arterioles controlled by

Local factors in the tissues Sympathetic nervous system

Capillaries

Smallest blood vessels Diameter from 8–10 µm

Red blood cells pass through single file

Site-specific functions of capillariesLungs – oxygen enters blood, carbon dioxide leaves Small intestines – receive digested nutrientsEndocrine glands – pick up hormonesKidneys – removal of nitrogenous wastes

RBCs in a Capillary

Figure 19.3

Capillary Beds

Network of capillaries running through tissues Precapillary sphincters

Regulate the flow of blood to tissues

Tendons and ligaments – poorly vascularized Epithelia and cartilage – avascular

Receive nutrients from nearby CT

Capillary Beds

Figure 19.4a

Capillary Beds

Figure 19.4b

Capillary Permeabillity

Endothelial cells – held together by tight junctions and desmosomes

Intercellular clefts – gaps of unjoined membrane Small molecules can enter and exit

Two types of capillary Continuous – most common Fenestrated – have pores

Structure of Capillaries – Cross Section

Figure 19.5a

Structure of Capillaries – Cross Section

Figure 19.5b

Routes of Capillary Permeability

Four routes into and out of capillaries Direct diffusion Through intercellular clefts Through cytoplasmic vesicles Through fenestrations

Low Permeability Capillaries

Blood-brain barrier Capillaries have complete tight junctions No intercellular clefts are present Vital molecules pass through

Highly selective transport mechanisms

Not a barrier against Oxygen, carbon dioxide, and some anesthetics

Sinusoids

Wide, leaky capillaries found in some organs Usually fenestrated Intercellular clefts are wide open

Occur in bone marrow and spleen Sinusoids have a large diameter and twisted course

Sinusoids

Figure 19.5c

Veins

Conduct blood from capillaries toward the heart Blood pressure is much lower than in arteries Smallest veins – called venules

Diameters from 8 – 100 µm Smallest venules – called postcapillary venules

Venules join to form veins Tunica externa is the thickest tunic in veins

Figure 19.6

Mechanisms to Counteract Low Venous Pressure

Valves in some veins Particularly in limbs

Skeletal muscle pump Muscles press against

thin-walled veins

Vascular Anastomoses

Vessels interconnect to form vascular anastomoses Organs receive blood from more than one arterial

source

Neighboring arteries form arterial anastomoses Provide collateral channels

Veins anastomose more frequently than arteries

Vasa Vasorum

Tunica externa of large vessels have Tiny arteries, capillaries, and veins

Vasa vasorum vessels of vessels Nourish outer region of large vessels

Inner half of large vessels receive nutrients from luminal blood

Pulmonary Circulation

Pulmonary trunk leaves the right ventricle Divides into right and left pulmonary arteries

Superior and inferior pulmonary veins Carry oxygenated blood into the left atrium