Blood Vessels and Circulation

The Anatomy of Blood Vessels

Structure of vessel walls

•Walls of arteries and veins contain three distinct layers

•Tunic intima

•Tunica media

•Tunica externa

Differences between arteries and veins

•Compared to veins, arteries

•Have thicker walls

•Have more smooth muscle and elastic fibers

•Are more resilient

Arteries

•Undergo changes in diameter

•Vasoconstriction – decreases the size of the lumen

•Vasodilation – increases the size of the lumen

•Classified as either elastic (conducting) or muscular (distribution)

•Small arteries (internal diameter of 30 um or less) are called arterioles

Capillaries

•An endothelial tube inside a basal lamina

•These vessels

•Form networks

•Surround muscle fibers

•Radiate through connective tissue

•Weave throughout active tissues

•Capillaries have two basic structures

•Continuous

•Fenestrated

•Flattened fenestrated capillaries = sinusoids

Capillary Beds

•An interconnected network of vessels consisting of

•Collateral arteries feeding an arteriole

•Metarterioles

•Arteriovenous anastomoses

•Capillaries

•Venules

Veins

•Collect blood from all tissues and organs and return it to the heart

•Are classified according to size

•Venules

•Medium-sized veins

•Large veins

Venous Valves

•Venules and medium-sized veins contain valves

•Prevent backflow of blood

Distribution of blood

•Total blood volume is unevenly distributed

•Venoconstriction maintains blood volume

•Veins are capacitance vessels

•Capacitance = relationship between blood volume and pressure

Cardiovascular Physiology

Circulatory Pressure

•Circulatory pressure is divided into three components

•Blood pressure (BP)

•Capillary hydrostatic pressure (CHP)

•Venous pressure

Resistance (R)

•Resistance of the cardiovascular system opposes the movement of blood

•For blood to flow, the pressure gradient must overcome total peripheral resistance

•Peripheral resistance (PR) is the resistance of the arterial system

Overview of Cardiovascular Pressures

•Factors involved in cardiovascular pressures include

•Vessel diameter

•Cross-sectional area of vessels

•Blood pressure

•Blood viscosity

Arterial blood pressure

•Arterial blood pressure

•Maintains blood flow through capillary beds

•Rises during ventricular systole and falls during ventricular diastole

•Pulse is a rhythmic pressure oscillation that accompanies each heartbeat

•Pulse pressure = difference between systolic and diastolic pressures

Mean arterial pressure (MAP)

Capillary Exchange

•Flow of water and solutes from capillaries to interstitial space

•Plasma and interstitial fluid are in constant communication

•Assists in the transport of lipids and tissue proteins

•Accelerates the distribution of nutrients

•Carries toxins and other chemical stimuli to lymphoid tissues

Processes that move fluids across capillary walls

•Diffusion

•Filtration

•Hydrostatic pressure (CHP)

•Reabsorption

Forces acting across capillary walls

•Capillary hydrostatic pressure (CHP)

•Blood colloid osmotic pressure (BCOP)

•Interstitial fluid colloid osmotic pressure (ICOP)

•Interstitial fluid hydrostatic pressure (IHP)

Filtration and reabsorption

•Processes involved in filtration and reabsorption include

•Net hydrostatic pressure

•CHP - IHP

•Net colloid osmotic pressure

•BCOP - ICOP

Venous pressure and venous return

•Assisted by two processes

•Muscular compression

•The respiratory pump

Cardiovascular Regulation

•Autoregulation

•Neural mechanisms

•Endocrine mechanisms

Autoregulation of blood flow within tissues

•Local vasodilators accelerate blood flow in response to:

•Decreased tissue O₂ levels or increased CO₂ levels

•Generation of lactic acid

•Release of nitric acid

•Rising K⁺ or H⁺ concentrations in interstitial fluid

•Local inflammation

•Elevated temperature

Neural Mechanisms

•Adjust CO and PR to maintain vital organ blood flow

•Medullary centers of regulatory activity include

•Cardiac centers

•Vasomotor centers control

•Vasoconstriction via adrenergic release of NE

•Vasodilation via direct or indirect release of NO

Reflex control of cardiovascular function

•Baroreceptors reflexes monitor stretch

•Atrial baroreceptors monitor blood pressure

•Chemoreceptor reflexes monitor CO₂, O₂, or pH levels

Hormones and cardiovascular regulation

•Antidiuretic hormone – released in response to decreased blood volume

•Angiotensin II – released in response to a fall in blood pressure

•Erythropoietin – released if BP falls or O₂ levels are abnormally low

•Natriuretic peptides – released in response to excessive right atrial stretch

Patterns of Cardiovascular Response

Exercise and the Cardiovascular System

•Light exercise results in

•Extensive vasodilation

•Increased venous return

•A rise in cardiac output

•Heavy exercise results in

•Increased blood flow to skeletal muscles

•Restriction of blood flow to nonessential organs

Cardiovascular response to hemorrhaging: short term

•Carotid and aortic reflexes increase CO and peripheral vasoconstriction

•Sympathetic nervous system elevates blood pressure

•E and NE increase cardiac output and ADH enhances vasoconstriction

Cardiovascular response to hemorrhaging: long term

•Decline in capillary blood pressure recalls fluids from interstitial spaces

•Aldosterone and ADH promote fluid retention

•Increased thirst promotes water absorption across the digestive tract

•Erythropoietin ultimately increases blood volume and improves O₂ delivery

Special circulation

•The brain

•Four arteries which anastomose insuring constant blood flow

•The heart

•Coronary arteries arising from the ascending aorta

•The lungs

•Pulmonary circuit, regulated by local responses to O₂ levels

•Opposite other tissues (declines in O₂ cause vasodilation)

The Distribution of Blood Vessels: An Overview

The distribution of blood: General functional patterns

•Peripheral distribution of arteries and veins is generally symmetrical

•Except near the heart

•Single vessels may have several names as they cross anatomical boundaries

•Arteries and corresponding veins usually travel together

The Pulmonary Circuit

Pulmonary circuit consists of pulmonary vessels

•Arteries which deliver blood to the lungs

•Capillaries in the lungs where gas exchange occurs

•Veins which deliver blood to the left atrium

The Systemic Circuit

Systemic arteries

•Ascending aorta

•Right and left coronary arteries originate from base of aortic sinus

•Aortic arch and branches

•Brachiocephalic

•Left common carotid

•Left subclavian arteries

•Descending aorta and its branches

•Thoracic and abdominal aortas

Systemic Veins

•Superior vena cava

•Drains blood from the head and neck

•Inferior vena cava

•Drains blood from the remainder of the body

Hepatic Portal System

•Contains substance absorbed by the stomach and intestines

•Delivers these compounds to the liver for

•Storage

•Metabolic conversion

•Excretion

Fetal Circulation

Placental Supply

•Fetal blood flow to the placenta is supplied via paired umbilical arteries

•A single umbilical vein drains from the placenta to the ductus venosus

•Collects blood from umbilical vein and liver

•Empties into the inferior vena cava

Fetal Circulation of the Heart and Great Vessels

•No need for pulmonary function in the fetus

•Two shunts bypass the pulmonary circuit

•Foramen ovale

•Ductus arteriosus

Cardiovascular Changes at Birth

•Lungs and pulmonary vessels expand

•Ductus arteriosus constricts and becomes ligamentum arteriosum

•A valvular flap closes the foramen ovale

Aging and the Cardiovascular System

Age-related changes in blood may include

•Decreased hematocrit

•Constriction or blockage of peripheral veins by a thrombus

•Pooling of blood in the veins of the legs

•Vessels are less elastic, prone to Ca²⁺ deposits and thrombi formation