Cardiovascular System Blood Vessels Structure of Blood Vessels The - - PowerPoint PPT Presentation

Cardiovascular System Blood Vessels

Structure of Blood Vessels

• The three layers (tunics)

– Tunica intima – composed of simple squamous epithelium – Tunica media – sheets of smooth muscle

• Contraction – vasoconstriction
• Relaxation – vasodilation

– Tunica externa (adventitia) – composed of connective tissue

• Large vessels contain a vasa vasorum
• Lumen

– central blood‐filled space of a vessel

Structure of Arteries, Veins, and Capillaries

Types of Blood Vessels

• Arteries – carry blood away from the heart

– Types: elastic (conducting), muscular (distributing), and arterioles

• 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 – Thicker tunica intima

• Due to thicker subendothelial

layer

Types of Arteries

• Muscular (distributing)

arteries

– Lie distal to elastic arteries – Diameters range from 1 cm to 0.3 mm – Includes most of the named arteries – Tunica media is thick – Unique features

• Internal and external elastic

laminae

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 tone

Capillaries

• Smallest blood vessels

– Diameter from 8–10 µm

• Red blood cells pass through single file

– Site‐specific functions of capillaries

• In the lungs – oxygen enters blood, carbon dioxide

leaves

• In the small intestines – receive digested nutrients
• In endocrine glands – pick up hormones
• In the kidneys – removal of nitrogenous wastes
• In the liver – removal of toxins, nutrients for metabolic

events…

Capillary Beds

• Network of capillaries running through tissues
• Control of blood in capillary beds

– Precapillary sphincters – regulate the flow of blood to tissues

• Tissues & Structures with little or no blood

flow

– Tendons and ligaments – poorly vascularized – Epithelia and cartilage – avascular

• Receive nutrients from nearby connective tissues

Capillary Beds

Capillary Permeabillity

• Endothelial cells – held together by tight

junctions and desmosomes

• Intercellular clefts – gaps of unjoined

membrane

– Small molecules can enter and exit

• Three types of capillaries

– Continuous – most common – Fenestrated – have pores – Sinusoids – wide porous capillary found in some

• rgans

Cross Section of Continuous Capillaries

Cross Section of Fenestrated Capillaries

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

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

– Other blood barrier?

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

Mechanisms to Counteract Low Venous Pressure

• Valves in some veins

– Particularly in limbs

• Skeletal muscle

pump

– Muscles press against thin‐walled veins

• Respiratory pump

– Causes changes in thoracic vs. abdominal pressure

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

Circulation Routes

• Pulmonary
• Systemic

– Arteries – Capillaries – Veins

• Special Venous Routes

Pulmonary Circulation

Systemic Circulation

• Systemic Arteries

– Carry oxygenated blood away from the heart – Aorta – largest artery in the body

• Capillaries – point of exchange
• Systemic Veins

– Carry deoxygenated blood towards the heart – Venae Cavae – the largest of the arteries that enter into the right atrium

Flow Chart – Main Systemic Arteries

Flow Chart – Main Veins of Systemic Circulation

Figure 19.25

The Basic Scheme of the Hepatic Portal System

Figure 19.22

Veins of the Hepatic Portal System

Figure 19.23

Blood Vessels Throughout Life

• Fetal Circulation

– All major vessels in place by month 3 of development – Differences between fetal and postnatal circulation

• Fetus must supply blood to the placenta
• Very little blood is sent through the pulmonary circuit

Vessels to and from the Placenta

• Umbilical vessels run in

the umbilical cord

– Paired umbilical arteries – Unpaired umbilical vein

• Ductus venosus
• Ligamentum teres
• Ligamentum venosum
• Medial umbilical

ligaments

Shunts Away from the Pulmonary Circuit

• Foramen ovale
• Ductus arteriosus

Fetal and Newborn Circulation Compared

Figure 19.26b