The Cardiovascular System: Blood Vessels and Circulation

Chapter 16

5 Types of Vessels

• Arteries– Carry blood away from the heart to body tissues– Aorta and pulmonary trunk

• Arterioles– Within a tissue or organ branch into numerous microscopic

vessels called capillaries

• Capillaries– Groups of capillaries reunite to form small veins:

• Venules– Merge to form larger vessels:

• Veins– Blood vessels that carry blood from the tissues back to the heart

• Blood reservoirs– Certain veins function as reservoirs b/c contain so much of blood

Arteries and Arterioles

• Arteries have 3 tissue layers surrounding lumen (hollow space)

• Endothelium– Inner layer composed of simple squamous epithelium,

a basement membrane, and elastic tissue (internal elastic lamina)

– Middle layer: smooth muscle and elastic tissue– Outer layer: mainly of elastic and collagen fibers

• Vasoconstriction– Decrease in diameter of the lumen of a blood vessel – Increase in sympathetic stimulation

Arteries and Arterioles

• Vasodilation– Decrease in sympathetic stimulation– Increase in lumen diameter

• Elastic arteries– High proportion of elastic fibers– Help propel blood while ventricles are relaxing b/c relax– Stretch when blood ejected from heart– Ex. Aorta and braciocephaliic, common carotid, subclavian,

vertebral, pulmonary, and common iliac arteries• Muscular arteries

– Great vasoconstriction and vasodilation– Ex. Brachial artery (arm) and radial artery (forearm)

Arteries and Arterioles

• Arteriole– A very small, microscopic artery that delivers

blood to capillaries– Big role in blood flow from arteries

capillaries

Capillaries

• Are microscopic vessels that connect arterioles to venules

• Exchange vessels b/c exchange of nutrients and wastes b/w the body’s cells and blood

• Body tissues w/high metabolic requirements (muscles, liver, kidneys, NS) have many capillary networks

Capillaries

• Very thin walls, allows many substances to pass through

• Precapillary sphincters: rings of smooth muscle at the point where capillaries branch from arterioles

• Numerous, so blood flows slowly through them

• Capillary exchange: the movm’t of substances into/out of capillaries

Capillary Exchange

• 1. Diffusion– O2, CO2, glucose, amino acids, and hormones

by simple diffusion– Plasma solutes – Sinusoids: gaps b/w endothelial cells in

smallest blood vessels in the liver• But too large to allow proteins like fibrinogen and

albumin to enter bloodstream

Capillary Exchange

• 2. Bulk flow: filtration and reabsorption– Bulk flow: is a passive process by which

large numbers of ions, molecules, or particles move together in same direction

– From area of high pressure low pressure– Continues if pressure difference exists– Two opposing pressure drive bulk flow:

• 1. capillary blood pressure• 2. colloid osmotic pressure

Capillary Exchange: Bulk Flow

• 1. Capillary blood pressure– Pressure of blood against the walls of

capillaries pushes fluid out of capillaries into interstitial fluid

• 2. Colloid osmotic pressure– Opposing pressure– Pulls fluid into capillaries– Presence of proteins in plasma and their

virtual absence in interstitial fluid gives blood the higher osmotic pressure

Capillaries: Bulk Flow

• Filtration: water and solute flow out of the blood capillary into the surrounding interstitial fluid via bulk flo– Capillary blood pressure higher than blood

osmotic pressure for first half of capillary– Midway through capillary, pressure drops

reabsorption• Reabsorption

– Water and solutes move via bulk flow from interestitial fluid into the blood capillary

Capillaries: Bulk Flow

• Autoregulation: the ability of a tissue to automatically adjust its blood flow to match its metabolic demands– Regulation of vasodilation and

vasoconstriction

Venules and Veins

• When several capillaries unite venules• Venules: receive blood from capillaries

and empty blood into veins return blood to heart

Venules and Veins: Structure

• Venules = little veins– Similar to arterioles– Walls thinner near capillary end and thicker towars

the heart• Veins

– Structure similar to arteries– Outer layer is thickets, and lumen wider than artery– In some veins, inner layer folds in = valves (prevent

backflow of blood)– Varicose veins: due to weak venous valves

• Blood backflows, causing increase in blood pressure, pushes vein wall outwards

• Loss of elasticity and stretch

Venous Return

• The volume of blood flowing back to the heart through systemic veins– 1. Contractions of the heart– 2. the skeletal muscle pump: (p.395 in leg)

• 1. while standing, venous valves closer and farther from heart closed, blood flow to heart

• 2. contraction of leg muscles, compress veins– Pushes blood through valve closer to heart milking– At same time, valve farther from heart is uncompressed and

closes as blood pushed against it• 3. muscle relaxation: pressure falls in compressed section of

veins, causes valve closer to heart to close– Valve farther from heart opens b/c blood pressure of foot is

higher than in leg

Venous Return

• 3. Respiratory Pump: based on alternating compression an decompression of veins– During inhalation the diaphragm moves downward

decrease pressure in thoracic cavity and increase pressure in abdominal cavity abdominal veins compressed greater volume of blood moves from compressed abdominal veins to decompressed thoracic veins and then into right atrium

– Exhalation: valves in veins prevent backflow of blood from thoracic veins to abdominal veins

Cardiovascular System

Circulatory I

Blood Pressure

• Blood flows from high pressure low pressure

• Blood pressure (BP): the pressure exerted by blood on the walls of a blood vessel– Highest in aorta and large

systemic arteries– Falls as distance from left

ventricle increases

Resistance

• Vascular Resistance: opposition to blood flow due to friction b/w blood and the walls of blood vessels

• Depends on:– 1. size of lumen– 2. Blood viscosity– 3. Total blood vessel

length

Vascular Resistance

• 1. Size of lumen– Smaller the lumen,

greater is resistance to blood flow• Increase in BP

Vascular Resistance

• 2. Blood viscosity– Viscosity (thickness)– Higher blood viscosity,

higher resistance– Increases if

• dehydration or • polycythemia (high

number of red blood cells) – Decreases if

• depletion of plasma proteins or RBCs b/c of anemia or hemorrhage decrease BP

Vascular Resistance

• 3. Total blood vessel length– Resistance increases when total length of all blood vessels

increases– Longer the length greater contact of blood and vessel wall

causes friction– higher BP

Measurement of Blood Pressure:

• Brachial artery in left arm

• Device: sphygmomanometer

• Pressure cuff inflated, artery compressed and blood flow stops

• Stethoscope below cuff, then cuff deflated

Blood Pressure• Systolic Blood Pressure

(SBP)– First sound as cuff is

deflated as artery is opening

– Force with which blood is pushing against arterial walls during ventricular contraction

• Diastolic Blood Pressure (DBP)– Sounds become faint– Represents force exerted

by blood remaining in arteries during ventricular relaxation