Cardiovascular system - Blood Vessels Chapter 13

Basic Anatomy of Circulatory routes Arteries Arterioles Capillaries Venules Veins Carry blood towards the heart Carry blood away from the heart Connect capillaries to veins Control blood flow into capillaries & help regulate BP Allow for “exchange” of O2/CO2, nutrients/wastes

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 & Veins Tunica 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

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 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

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 Circulation Pressure: “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