Cardiovascular System

Blood Vessels and Hemodynamics

Dr. Michael P. Gillespie

Cardiovascular System

Transports and delivers blood to the body to deliver oxygen, nutrients, and hormones as well as carries away wastes.

Blood vessels form a closed system of tubes, which carries blood away from the heart, transports it to the tissues of the body, and then returns it to the heart.

Hemodynamics

Hemo – blood. Dynamics – power.

Main Types Of Blood Vessels

Arteries – carry blood away from the heart. Arterioles – very small arteries. Capillaries – tiny vessels which allow

exchange of substances between the blood and body tissues.

Venules – very small veins. Veins – carry blood back to the heart.

Vaso Vasorum

Larger blood vessels require oxygen and nutrients just like other tissues of the body.

Vaso vasorum (vasculature of the vessels) are located within the walls of larger vessels and supply them.

Tunics (Coats) Of Arteries

Tunica interna (intima) – contains a lining of endothelium which makes contact with the lumen and blood.

Tunica media – thickest layer and has high compliance (stretches).

Tunica externa – outer coat, elastic and collagen fibers.

Changes In Vascular Diameter

Vasoconstriction – a decrease in the diameter of the lumen of a blood vessel. Sympathetic stimulation causes the smooth

vessels of the vessels to contract, squeezing the vessel wall and narrowing the lumen.

Occurs when an artery or an arteriole is damaged, producing vascular spasm and limiting the blood flow to reduce blood loss.

Changes In Vascular Diameter

Vasodilation – an increase in the diameter of the lumen of a blood vessel. Occurs when sympathetic stimulation decreases

or when nitric oxide, K+, H+, and lactic acid are present.

Elastic Arteries

Elastic arteries propel blood forward while the ventricles are relaxing.

Blood is ejected from the heart and stretches the walls of the elastic arteries.

The stretch of the arteries stores mechanical energy and act as a pressure reservoir.

The vessels recoil and convert stored (potential) energy in the vessel into kinetic energy of the blood.

Muscular Arteries

Medium sized arteries are muscular arteries. They contain more smooth muscle and fewer elastic

fibers than elastic arteries. They are capable of greater vasoconstriction and

vasodilation. They are called distributing arteries because they

distribute blood to various parts of the body.

Arterioles

A very small (almost microscopic) artery that delivers blood to capillaries.

Arterioles regulate resistance. Vasoconstriction of arteriole walls increases

resistance to capillaries and vasodilation of arteriole walls decreases resistance.

Resistance regulates blood flow to the capillaries.

Capillaries

Microscopic vessels that connect arterioles to venules.

The flow of blood from arterioles to venules is microcirculation.

Tissues with high metabolic requirements, such as muscles, liver, kidneys, and nervous system, have more capillaries.

Tissues with lower metabolic requirements, such as tendons and ligaments, contain fewer capillaries.

Capillaries

Capillaries are absent in a few tissues, such as covering and lining epithelia, the cornea of the lens of the eyes, and cartilage.

Exchange vessels – exchange nutrients between blood and tissue cells through the interstitial fluid.

Capillaries

Single layer of endothelial cells. Branch extensively to increase surface area

for exchange. Usually only a small part of the capillary

network is active; However, when a tissue is active (i.e. Contracting muscle) the entire network fills with blood.

Metarteriole

A metarteriole (met = beyond) – is a vessel that emerges from an arteriole and supplies a group of 10 – 100 capillaries (capillary bed).

The proximal end of the metarteriole is surrounded by smooth muscle fibers, which regulate blood flow through the capillary bed.

The distal end of the metarteriole has a thoroughfare channel, which bypasses the capillary bed.

True Capillaries

True capillaries emerge from arterioles or metarterioles.

Precapillary sphincter – ring of smooth muscle that controls blood flow into a true capillary.

Vasomotion – intermittent contraction and relaxation of precapillary sphincters and metarteriole smooth muscle (5-10 times per minute).

Types Of Capillaries

Continuous capillaries - Continuous tube interrupted only by intercellular

clefts. Found in smooth muscle, connective tissue, and

lungs.

Types Of Capillaries

Fenestrated capillaries (fenestr = window) - The plasma membranes have fenestrations (small

pores). Located in the kidneys, villi of the SI, choroid

plexus of the ventricles of the brain, cilary processes of the eyes, and endocrine glands.

Types Of Capillaries

Sinusioids – Wider and more winding than other capillaries. Unusually large fenestrations which allow

protein and blood to pass from the tissues into the bloodstream.

Found in the liver, spleen, anterior pituitary, and parathyroid glands.

Venules

Small veins formed when several capillaries unite.

The walls of the smallest venules (closest to the capillaries) are very porous and serve as a site of emigration for white blood cells.

Veins

Veins contain the same three coats as arteries.

The lumen of a vein is larger than that of a comparable artery.

Veins

Many veins also contain valves (especially in the lower limbs).

The valves are thin folds of the tunica interna. The cusps point toward the heart.

The valves prevent backflow of returning blood in the lower pressure venous system.

Vascular (Venous) Sinus

A vascular (venous) sinus is a vein with a thin endothelial wall that has no smooth muscle to alter its diameter.

Examples: Dural venous sinuses (supported by dura mater). Coronary sinus of the heart.

Varicose Veins

Leaky valves can cause veins to become dilated and twisted in appearance.

This is most common in the esophagus and veins of the lower limb, although it can occur in any veins.

Hemorrhoids are varicose veins in the anal canal. Deeper veins are not as susceptible because

surrounding skeletal muscles prevent their walls from stretching.

Anastomoses

The union of the branches of two or more arteries supplying the same body region is called an anastomosis.

Anastomoses between arteries provide alternate routes for blood to reach a tissue or an organ.

The alternate route of blood flow is known as collateral circulation.

Arteries that do not anastomose are known as end arteries.

Blood Distribution

The largest portion of your blood volume at rest is in the veins (60%).

Systemic capillaries hold about 5%. The veins and venules function as a blood reservoir. Blood can be diverted quickly if the need arises

through venoconstriction. The veins of the abdominal organs and skin serve

as principal blood reservoirs.

Capillary Exchange

The mission of the cardiovascular system is to keep blood flowing through the capillaries to allow for exchange of nutrients and waste products with the interstitial fluid.

Substances enter and leaved the capillaries through three basic mechanisms: Diffusion. Transcytosis. Bulk flow.

Diffusion

Substances diffuse down their concentration gradients (from areas of high concentration to low).

All plasma solutes except proteins pass easily across most capillary walls.

Water soluble substances such as glucose and amino acids pass easily through either fenestrations or intercellular clefts.

Diffusion

Lipid-soluble materials (O2, CO2, & steroid hormones) pass through the lipid bilayer.

Liver capillaries have large gaps which do allow proteins to pass through. Hepatocytes synthesize proteins such as fibrinogen (clotting) and albumin (osmotic pressure), which diffuse into the blood.

Brain capillaries have tight junctions, which allow only a few substances to enter and leave. This forms the blood-brain barrier.

Transcytosis

Substances within the blood plasma are enclosed in tiny pinocytic vesicles that enter endothelial cells by endocytosis.

They move across the membrane and exit the other side by exocytosis.

Transcytosis

This method of transport is utilized for large, lipid-insoluble molecules that cannot cross the capillary walls in any other way.

Insulin enters the blood this way and some maternal antibodies enter the fetal circulation this way.

Bulk Flow: Filtration & Reabsorption

Bulk flow is a process by which large numbers of ions, molecules, or particles in a fluid move together in the same direction.

It occurs from an area of high pressure to an area of low pressure at a rate faster than diffusion would produce alone.

Regulates relative volumes of fluids rather than concentrations of solutes.

Bulk Flow: Filtration & Reabsorption

Continues as long as pressure variances exist. Pressure driven movement of fluid and solutes

from blood capillaries to interstitial fluid is termed filtration.

Pressure driven movement of fluid and solutes from interstitial fluid into blood capillaries is called reabsorption.

Pressures Involved In Filtration And Absorption

Blood hydrostatic pressure (BHP) – pressure from the pumping action of the heart promotes filtration.

Interstitial fluid osmotic pressure filters blood promotes filtration.

Pressures Involved In Filtration And Absorption

Blood colloid osmotic pressure (BCOP) promotes reabsorption.

Interstitial fluid hydrostatic pressure promotes reabsorption.

Net filtration pressure is the balance of these pressures (NFP).

Starling’s Law Of The Capillaries

The volume of fluid and solutes reabsorbed normally is almost as large as the volume filtered.

Edema

If filtration greatly exceeds reabsorption, the result is edema (swelling), an abnormal increase in interstitial fluid volume.

Excess Filtration

Increased capillary blood pressure. Increased permeability of capillaries which

allows plasma proteins to escape. Chemical, bacterial, thermal, or mechanical agents can damage capillary walls.

Inadequate Reabsorption

Decreased concentration of plasma proteins associated with liver disease, burns, malnutrition, and kidney disease.

Hemodynamics

Hemodynamics refer to the factors that affect blood flow.

Blood flow is the volume of blood that flows through any tissue in a given period of time.

Cardiac output (CO) is the total blood flow. Cardiac output (CO) – heart rate (HR) *

stroke volume (SV).

Factors That Determine Distribution Of Cardiac Output

Pressure difference drives blood flow through a tissue. Blood flows from regions of higher to lower

pressure. Resistance to blood flow in specific blood

vessels. The higher the resistance, the smaller the blood

flow.

Blood Pressure

Contraction of the ventricles generates blood pressure (BP).

Systolic blood pressure is the highest pressure attained in the arteries during systole.

Blood Pressure

Diastolic blood pressure is the lowest arterial pressure during diastole.

Mean arterial blood pressure (MABP) is the average pressure in the arteries. MABP = diastolic BP + 1/3 (systolic BP – diastolic BP).

Blood pressure also depends on the total volume of blood in the cardiovascular system.

Resistance

Vascular resistance is the opposition to blood flow due to friction between blood and the walls of blood vessels.

Vascular resistance depends upon: The size of the blood vessel lumen. Blood viscosity. Total blood vessel length.

Systemic vascular resistance (SVR) is the total peripheral resistance from all factors combined.

Venous Return

Venous return to the heart is caused by the following: Pressure generated from contractions of the

heart’s left ventricle. Skeletal muscle pump. Respiratory pump.

Velocity Of Blood Flow

The speed or velocity of blood flow is inversely related to the cross-sectional area.

Each time an artery branches, the cross sectional area increased and the velocity decreases.

Each time a venule merges to form a vein, the cross sectional area decreases and the velocity increases.

Syncope

Syncope, or fainting, is a sudden, temporary loss of consciousness that is not due to head trauma.

Syncope

It is commonly due to cerebral ischemia. Causes:

Vasodepressor syncope – sudden emotional stress. Situational syncope – pressure stress associated with

urination, defecation, or severe coughing. Drug-induced syncope – antihypertensives, diuretics,

vasodilators, & tranquilizers. Orthostatic hypotension – an excessive decrease in blood

pressure that occurs upon standing up.

Control Of Blood Pressure & Blood Flow

Negative feedback systems control blood pressure by adjusting the following factors: Heart rate. Stroke volume. Systemic vascular resistance. Blood volume.

Cardiovascular Center

The cardiovascular (CV) center of the medulla oblongata regulates heart rate and stroke volume.

Sympathetic nerves reach the heart via the cardiac accelerator nerves. Sympathetic stimulation increases the heart rate and contractility.

Cardiovascular Center

Parasympathetic stimulation decreases the heart rate and is conveyed by the vagus nerves (cranial nerve X).

The CV center sends impulses to smooth muscle in blood vessel walls via vasomotor nerves. They moderate the rate of vasoconstriction (vasomotor tone).

Neural Regulation Of Blood Pressure

Baroreceptor Reflexes – baroreceptors are pressure-sensitive receptors located in the aorta, internal carotid arteries, and other large arteries of the neck and chest. Carotid sinus reflex – carotid sinuses are small widenings

of the right and left internal carotid arteries. Pressure stretches the wall of the carotid sinus. Signals are sent to the CV center via the glossopharyngeal nerves (CN IX).

Aortic reflex – signals are sent to the CV center via the vagus nerves (CN X).

Carotid Sinus Massage & Carotid Sinus Syncope

Carotid sinus massage involves massaging the neck over the carotid sinus, to slow the heart rate in a person who has paroxysmal superventricular tachycardia (originates in the atria).

Carotid sinus syncope – fainting due to excessive pressure on the carotid sinus from hyperextension of the head or tight collars.

Chemoreceptor Reflexes

Chemoreceptors monitor the chemical composition of the blood.

They are located close to the baroreceptors in carotid bodies and aortic bodies.

Chemoreceptor Reflexes

They detect changes in blood level of O2, CO2, and H+.

Hypoxia, acidosis, or hypercapnia stimulates the chemoreceptors to send impulses to the cv center producing sympathetic stimulation and vasoconstriction.

Hormonal Regulation Of Blood Pressure

Renin-angiotensin-aldosterone (RAA) system raises blood pressure. Angiotensin II is a vasoconstrictor and stimulates

aldosterone which increases absorption of Na+ ions by the kidneys.

Epinephrine and norepinephrine raise blood pressure. Increase cardiac output by increasing heart rate. Cause vasoconstriction of arterioles in the skin and

abdomen and vasodilatation of arterioles in cardiac and skeletal muscles.

Hormonal Regulation Of Blood Pressure

Antidiuretic hormone (ADH) raises blood pressure. Causes vasoconstriction.

Atrial natriuretic peptide (ANP) lowers blood pressure. Causes vasodilatation and promotes loss of salt

and water in the urine.

Autoregulation Of Blood Pressure

Physical changes. Warming promotes vasodilation and cooling

causes vasoconstriction. Myogenic response – smooth muscle contracts

more forcefully when it is stretched and relaxes when stretching lessens.

Autoregulation Of Blood Pressure

Vasodilating and vasoconstricting chemicals. Vasodilating chemicals include K+, H+, lactic

acid, ATP, and nitric oxide (NO). Kinins and histamine are released from tissue trauma and cause vasodilation.

Vasoconstricting chemicals include thromboxane A2, superoxide radicals, serotonin (from platelets), and endothelins.

Checking Circulation