Document 45 - Chapter 21: Blood Vessels and Circulation PDF

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Chapter 21: Blood Vessels and Circulation...

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Blood Vessels and Circulation Introduction •Structure and function of blood vessels •Blood Circulation •Factors controlling the flow of blood •Major circulatory routes

STRUCTURE AND FUNCTION OF BLOOD VESSELS •Blood vessels form a network in our body taking the blood from the heart to the tissues of the body and returning it back to the heart. •The direction of the flow of the blood in different vessels of the body is:

Heart ->Arteries ->Arterioles -> Capillaries ->Venules ->Veins ->Heart

Arteries •Are blood vessels that take the blood away from the heart. •The wall of an artery consists of three major layers – Tunica interna (intima) – innermost layer •simple squamous epithelium known as endothelium •basement membrane •internal elastic lamina–Tunica media – middle layer •smooth muscle •Capable of vasoconstriction and vasodilation–Tunica externa – outermost layer •elastic & collagen fibers

Arteries – Elastic and Muscular arteries •Elastic (conducting) arteries –Found closer to the heart

–Have more elastic fibers and less smooth muscle, are able to receive blood under pressure –Help to conduct blood from heart to the muscular arteries •Muscular (distributing) arteries –Found closer to the organs and tissues. –Have more smooth muscle tissue –Capable of vasoconstriction and vasodilation, control flow of blood

Aging of the Arteries Arteriosclerosis: is defined as thickening and toughening of the arterial walls. Complications associated with arteriosclerosis can lead to heart attack and stroke. Two main types: Focal calcification: deposition of calcium salts, replacing the smooth muscle tissue of the arterial wall. Can be due to aging, or a consequence of diabetes mellitus.

Atherosclerosis: deposition of lipids (plaques) in the tunica media.

•small microscopic arteries delivering blood to capillaries. •an arteriole has tunica interna, tunica media and very thin tunica externa. •through vasoconstriction and vasodilation regulate the amount of blood entering into the capillaries of an organ or tissue.

Capillaries •microscopic vessels which connect arterioles and venules. •Flow of blood from arterioles --- > to capillaries --- > to venules, is termed microcirculation. •Capillaries are made up of an endothelial and basement layer only.

•Primary function - permit the exchange of nutrients and gases between the blood and the tissues. •Distribution of capillaries in tissues of the body varies depending on the metabolic activity of the body.

Types of Capillaries •Continuous capillaries –plasma membrane of the endothelial cells forms a continuous ring around the capillary. Have no pores –e.g. in skeletal & smooth muscles, & lungs •Fenestrated capillaries–plasma membranes have many holes –e.g. in kidneys, small intestine, choroid plexuses, •Sinusoids –very large fenestrations (pores) –incomplete basement membrane –e.g. liver, bone marrow, spleen

Venules and Veins Venules: •formed by joining of small capillaries. •collect blood from capillaries and drain it into veins.

Veins:

•Veins consist of the same three tunics as arteries but differ in the following: –contain less smooth muscle and elastic tissue –thinner walled in comparison to arteries –contain valves to prevent the backflow of blood. –Lumen is larger than arteries.

Varicose veins: are due to weakening of the valves in the veins of the thighs and legs. Blood pools in the veins and become swollen.

Anastomoses and Blood Distribution Anastomoses

–are the union of the branches of two or more arteries supplying the same region. –provide alternate routes for blood to reach a tissue or organ.

Blood Distribution •60% of blood volume at rest is in systemic veins and venules –function as blood reservoir –blood is diverted from it in times of need •15% of blood volume in arteries & arterioles

Factors Affecting Blood Flow •Blood Flow is the volume of blood that flows through any tissue in a given time period. –Under normal conditions, blood flow is equal to cardiac output, when cardiac output goes up, blood flow through the capillary beds increases.

–It depends on two main factors - Pressure (P) and Resistance (R). Flow is directly proportional to pressure and inversely proportional to resistance.

Factors affecting blood flow Cardiovascular pressure is measured in terms of three values: •Blood Pressure (BP): is the hydrostatic pressure exerted by the blood on the walls of arteries. Measured in mmHg. Ranges from 100 mmHg (near the aorta) to 35 mmHg (near the capillaries). •Capillary hydrostatic pressure (CHP): is the pressure exerted by the fluids on the capillary walls. Ranges from 35 mmH to 18 mm Hg. •Venous pressure: is the pressure within the veins. Lowest of the three pressure, approx 18 mmHg. For circulation to happen circulatory pressure must overcome the total peripheral resistance.

FACTORS AFFECTING BLOOD FLOW Blood Flow is the volume of blood that flows through any tissue in a given time period. It depends on the following factors:

Blood Pressure: is the pressure exerted on the walls of a blood vessel.

•caused by contraction of the ventricles •highest in aorta–120 mm Hg during systole & 80during diastole •The pressure falls as the distance from the heart increases. •Pulse pressure is the difference between the systolic and the diastolic pressure. Mean arterial pressure (MAP) is calculated as follows: MAP = diastolic pressure +pulse pressure/3

•Factors that affect blood pressure include cardiac output, blood volume, viscosity, resistance, and elasticity of arteries.

HEMODYNAMICS: FACTORS AFFECTING BLOOD FLOW (CONTD) Venous return: the volume of blood flowing back to the heart from the systemic veins, depends on the

pressure difference from venules to right atrium. Two mechanisms act to return venous blood.

–Skeletal muscle pump: Skeletal muscles surrounding the veins contract --> this exerts pressure on the walls of the veins --> forces the valves open --> blood is pumped up --> skeletal muscle relaxed --> valve close preventing the backward flow of blood. –Respiratory pump: during inspiration diaphragm moves inferiorly. This causes a decrease in pressure in the thoracic cavity and an increase in the pressure of abdominopelvic cavity. As a result blood flows from the veins in abdominopelvic region to veins in thoracic region. During expiration, valves close preventing back flow.

Factors affecting blood flow Resistance: is the friction between the blood and the walls of the vessels which increases BP. Resistance depends on the following factors.

•Vascular resistance: is the force that resists the flow of blood within the blood vessels. it in turn depends on two parameters: –Size of the lumen: smaller the lumen, greater is the resistance to blood flow. –Total blood vessel length: Resistance is directly proportional to the length of the blood vessel.

•Blood Viscosity: Thickness of the Blood is due to ratio of RBCs to plasma and concentration of plasma proteins. Anemia, polycythemia and other disorders change blood viscosity and thus peripheral resistance. •Turbulence: is due to high flow rates, irregular surfaces and sudden changes in vessel diameter (due to blockages). It increases resistance an thereby slows the flow of blood.

Capillary Exchange Substances enter and leave capillaries in three basic ways described below:

•Diffusion: Substances like oxygen, carbon dioxide, glucose, amino acids, hormones diffuse through the capillaries down their concentration gradient. Lipid soluble molecules like gases and certain hormones, pass directly through the phospholipid layer. Water soluble compounds like glucose and amino acids pass through the intercellular clefts. •Filtration: Driven by hydrostatic pressure. Water and small solutes forced through capillary wall, Leaves larger solutes in bloodstream. •Reabsorption: The result of osmotic pressure (OP). Blood colloid osmotic pressure (BCOP) Equals pressure required to prevent osmosis Caused by suspended blood proteins that are too large to cross capillary walls.

•Two pressures promote this movement: hydrostatic pressure (HP) and osmotic pressure (OP). Net Filtration Pressure: Net Hydrostatic Pressure – Osmotic Pressure

•NFP at the arterial end is 10 mm Hg and at the venous end it is -9 mm Hg. •Thus at the arterial end there is net movement of molecules out of the capillary, and at the venous end fluid moves into the capillary from the surrounding tissue. •On average only 85% of the fluid that comes out of the capillaries reenters the capillaries at the venous end. •The extra fluid is returned back to the circulatory system via the lymphatic system. •Edema: When the filtration exceeds the re absorption, it results in an increase in the interstitial fluid volume, which is termed edema.

CONTROL OF BLOOD PRESSURE AND BLOOD FLOW Blood volume and blood flow are constantly regulated as conditions change in the body. Blood

pressure and blood flow to specific areas of the body are under the control of the following factors:

1.Cardiovascular center: is a group of neurons in the medulla of the brain and it regulates heart rate, contractility, and blood vessel diameter. –Receives input from sensory receptors (baroreceptors and chemoreceptors). –Cardiovascular center sends output via sympathetic and parasympathetic fibers. –Sympathetic activation results in increased heart rate and vasoconstriction, whereas parasympathetic activation causes decreased heart rate. Blood volume and blood flow are constantly regulated as conditions change in the body. Blood pressure and blood flow to specific areas of the body are under the control of the following factors:

CARDIOVASCULAR REGULATORY MECHANISM 2.Hormonal Regulation of Blood Pressure

•Renin-angiotensin-aldosterone system –decrease in BP or decreased blood flow to kidney ->release of renin by kidney -> angiotensinogen converted to angiotensin in liver -> release of aldosterone by adrenal cortex -> increased water reabsorption (increase in Blood volume) and vasoconstriction (increase in BP) •Epinephrine & norepinephrine –increases heart rate & force of contraction –causes vasoconstriction •ADH causes vasoconstriction and increased water reabsorption resulting in increase in BP and blood volume. •ANP (atrial natriuretic peptide) lowers BP and blood volume –causes vasodilation & loss of salt and water in the urine

3.Local Regulation of Blood Pressure: –The ability of a tissue to automatically adjust its own blood flow to match its

metabolic demand for supply of O2 and nutrients and removal of wastes is called autoregulation.

–Important for tissues that show major increase in activity such as brain and skeletal muscles.

CARDIOVASCULAR RESPONSE TO EXERCISE Blood, Heart, and Cardiovascular System - Work together as unit - Respond to physical and physiological changes (for example, exercise and blood loss) - Maintain homeostasis The Cardiovascular Response to Exercise Light Exercise Extensive vasodilation occurs increasing circulation Venous return increases with muscle contractions

Cardiac output rises Venous return (Frank–Starling principle) Atrial stretching

Heavy Exercise Activates sympathetic nervous system Cardiac output increases to maximum About four times resting level Restricts blood flow to “nonessential” organs (e.g., digestive system) Redirects blood flow to skeletal muscles, lungs, and heart Blood supply to brain is unaffected of blood flow.

Circulatory Routes •Most common route – heart -> arteries -> arterioles -> capillaries -> venules ->veins –E.g. pulmonary and systemic circulation •Portal system – blood flows through two consecutive capillary networks before returning to heart

•hypothalamus - anterior pituitary •found in kidneys •between intestines – liver •Anastomoses - Point where 2 blood vessels merge –E.g. coronary circulation

Pulmonary Circulation •The pulmonary circulation takes deoxygenated blood from the right ventricle to the air sacs of the lungs and returns oxygenated blood from the lungs to the left atrium. Pulmonary and systemic circulation are different in several ways: –Distance traveled by the blood is smaller –Pulmonary arteries are larger in diameter, thinner walls and hence resistance to blood flow is low. –Hydrostatic pressure in pulmonary capillary is low, tends to prevent pulmonary edema.

Systemic Circulation •The systemic circulation takes oxygenated blood from the left ventricle through the aorta to all parts of the body, including some lung tissue (but does not supply the air sacs of the lungs) and returns the deoxygenated blood to the right atrium. •The

aorta is divided into the ascending aorta, arch of the aorta, and the descending aorta (thoracic and abdominal aorta). Each section gives off arteries that branch to supply the whole body. •Blood returns to the heart through the systemic veins. All the veins of the systemic circulation flow into the superior or inferior vena cavae or the coronary sinus, which in turn empty into the right atrium. •The principal arteries and veins of the systemic circulation are described in the next few slides.

Fetal Circulation •The fetal circulation involves the exchange of materials between fetus and mother. •The fetus derives its oxygen and nutrients and eliminates its carbon dioxide and wastes through the maternal blood supply by means of a structure called the placenta. •Blood passes from the fetus to the placenta via two umbilical arteries and returns from the placenta via a single umbilical vein. •At birth, when pulmonary, digestive, and liver functions are established, the special structures of fetal circulation are no longer needed.

–The ductus arteriosus becomes the ligamentum arteriosum.

–The foramen ovale becomes the fossa ovalis. –The ductus venosus becomes the ligamentum venosum. –The umbilical arteries become the medial umbilical ligaments. –The umbilical vein becomes the ligamentum teres (round ligament).

Age Related Changes in Cardiovascular System •Three Age -Related Changes in Blood –Decreased hematocrit Peripheral blockage by blood clot (thrombus)Pooling of blood in legs Due to venous valve deterioration

•Five Age -Related Changes in the Heart

–Reduced maximum cardiac output Changes in nodal and conducting cells Reduced elasticity of cardiac (fibrous) skeleton Progressive atherosclerosis Replacement of damaged cardiac muscle cells by scar tissue

•Three Age -Related Changes in Blood Vessels –Arteries become less elastic Pressure change can cause aneurysm Calcium deposits on vessel walls Can cause stroke or infarction Thrombi can form At atherosclerotic plaques...