Cardiovascular system -Cardiovascular system -Blood VesselsBlood VesselsChapter 13Chapter 13

Basic Anatomy of Circulatory routes

Arteries

Arterioles

Capillaries

Venules

VeinsCarry blood away from the heart

Control blood flow into capillaries & help regulate BP

Allow for “exchange” of O2/CO2, nutrients/wastes

Connect capillaries to veins

Carry blood towards the heart

Structure of Blood Vessels

Arteries & Veins

Both are comprised of 3 layers of tissue - tunica interna, tunica media & tunica externa - surrounding “lumen” through which blood will flow:

All structural difference between arteries & veins primarily due to differences in pressure of blood flowing within

Lumen – larger diameter in veins compared to corresponding artery

Arteries & Veins

Tunica Interna – innermost endothelium of simple squamous epithelium + basement membrane

Arteries – have an “internal elastic lamina” of elastic CT to allow for expansion under pressure

Veins – may have “valves” (folds of endothelium + CT) to prevent backflow of blood due to low pressure

Arteries & VeinsTunica Media – middle layer containing smooth muscle (for contractility/vasoconstriction) & elastic CT (for elasticity)

Arteries – have relatively thick tunica media allowing for significant vasoconstriction & elasticity

Veins – relatively thin tunica media therefore no significant constriction/elasticity

Arteries & Veins

Tunica Externa (a.k.a. adventitia) – made of collagenous CT

Arteries – thin layer

Veins – thickest layer of vein, trying to support against gravity & low pressure

Venous return is aided by:

Valves

Muscular compression – “milking of veins through skeletal muscle contraction

Respiratory pump – breathing changes in pressure in abdominal/thoracic cavities

As blood moves through CV system from arteries veins, pressure within the vessels decreases significantly

Blood also has to move against gravity in many veins

Arterioles & Venules

Very small, almost microscopic vessels with only 2 layers of tissue surrounding lumen

Arterioles – endothelium (tunica interna) + very thin layer of smooth muscle cells (tunica media); regulate blood flow to tissues & affect arterial blood pressure

Venules – endothelium (tunica interna) + thin layer of CT (tunica externa)

Capillaries

Microscopic, very thin-walled vessels comprised of endothelium with basement membrane

Found in all tissues of the body except for those that are “avascular”

Usually form branching networks (“capillary beds”) within tissues for increased surface area

Capillaries

Structure of capillaries allows for

filtration - at arterial end due to high “capillary hydrostatic pressure” (CHP)

reabsorption - at venous end due to high “blood osmotic pressure” (BOP)

Physiology of Circulation

Primary function of CV system is to maintain adequate blood flow to capillaries of tissues

Under normal circumstances, capillary blood flow is directly related to cardiac output (increased C.O. increased blood flow, and vice-versa)

Capillary blood flow is also affected by pressure & resistance

Physiology of CirculationPressure:

“Blood pressure” - the “force” exerted on the wall of a vessel from the blood within; related to the “beating” of the heart

Systolic pressure – pressure within artery when ventricle contracts

Diastolic pressure – pressure remaining within artery when ventricle relaxes

“Circulatory pressure” – the difference in pressures throughout the circulatory system (high pressure in arteries (avg.100mm Hg at aorta) low pressure in veins (avg. 2mm Hg at IVC))

Blood flow is directly related to circulatory pressure – blood will move from high to low pressure. If arterial blood pressure increases, capillary blood flow will increase, & vice-versa

Physiology of Circulation

Resistance

Resistance – any force that opposes movement

Peripheral resistance – refers to resistance of blood flow in arterial side of system; mainly occurs at arterioles

Peripheral resistance primarily due to friction between blood & BV walls

As peripheral resistance increases (ie. with vasoconstriction of arterioles), capillary blood flow decreases (inverse relationship)

Circulatory Routes - overview

Circulatory Routes – Pulmonary Circuit

Circulatory Routes – Systemic circuit

Ascending aorta (gives off coronary arteries)

Aortic arch

Brachiocephalic trunk

Left common carotid artery

Left subclavian artery

Thoracic (descending) aorta

Abdominal aorta

Common iliac arteries

Arterial blood from left ventricle into ascending aorta

Venous return to right atrium through SVC, IVC & coronary sinus