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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