Cardiovascular System

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.

Cardiovascular System