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Chapter 18 - Blood Vessels and Circulation

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Hormones Affecting Blood Volume - Increased Blood Volume

* Angiotensin-II * ADH *Aldosterone

Resistance

* Any impedance to blood flow; blood flow is inversely proportional to resistance. * PR (Peripheral Resistance) - any factor that hinders blood flow through vasculature contributes to OVERALL resistance. * As PR INCREASES, blood pressure also INCREASES.

Hormone Affecting Blood Volume - Decreased Blood Volume

* Atrial Natriuretic Peptide

Hormone Affecting CO - Decreased Rate

* Atrial Natriuretic Peptide

Diffusion through the membranes of endothelial cells

Lipid-soluble substances such as O2, CO2 and certain lipids can generally enter and exit the capillary by diffusing across the membrane of one side of the endothelial cell and out the membrane on the other side. Can enter either blood or interstitial fluid.

How are Blood Flow, Resistance and Blood Pressure related?

Blood flow is directly proportional to the change in pressure and inversely proportional to resistance.

Tissue Perfusion

Blood flow to a tissue through a capillary bed. Tightly regulated to ensure that metabolic needs of all tissues are met at all times.

Capillary Net Filtration Pressure (NFP) #1

COP and HP gradient cause water to move in opposite directions. Which one wins? Both win - just in different locations. On the arteriolar end and calculate it as : HP - COP = Overall force driving water out. (If the solution of the equation is a positive number the water is being forced out of the capillary by filtration.) On the venous end: HP - COP = Net Filtration Pressure drawing water in. (If the solution of the equation is a negative number the water is being drawn INTO the capillary a process called absorption)

#2 Chemoreceptor Stimulation - Short-Term blood pressure regulation

Central Chemoreceptors: Located in the medulla of the brainstem. These receptors respond to the pH of the interstitial fluid of the brain. When the pH of this fluid decreases, another feedback loop is stimulated that indirectly increases the activity of sympathetic neurons, resulting in vasoconstriction and a rise in blood pressure.

Cardiac Output

Change in Pressure = CO x PR Pressure change is caused by altering cardiac output (CO) and/or peripheral resistance (PR) When cardiac output increases, blood pressure increases and vice versa.

Peripheral Resistance

Anything that hinders blood flow through the vasculature contributes to the overall resistance of the circuit. Most resistance is encountered away from the heart, in the body's periphery. PR and blood pressure are directly related. As PR increases, blood pressure increases.

Arteries

Are the distribution system of the vasculature. Travel away from the heart branching into vessels of progressively smaller in diameter. Arteries of the Pulmonary Circuit carry deoxygenated blood. Arteries of the Systemic Circuit carry oxygenated blood.

Capillaries

Are the exchange system of the vasculature. Vessels of very small diameter that form branching networks. Gases, nutrients, wastes and other substances are quickly exchanged between cells and the blood through capillary walls.

Arteries with Special Functions

Baroreceptors - Pressure Receptors found in aorta as well as in Common Carotid Artery, located in neck. Chemoreceptors - in carotid artery and aorta; detect blood oxygen, carbon dioxide, and hydrogen ion concentrations

Tissue Perfusion - Skeletal Muscle

* Blood flow changes dramatically during exercise, increasing as much as 50-fold - hypermia; due to structure of arteries supplying skeletal muscle tissue. * Progressive, stepwise vasodilation allows skeletal muscles to precisely match perfusion to degree of exercise. 1) Terminal arterioles dilate. 2) Arterioles dilate 3) Feed artery dilates.

Myogenic Mechanism

* Counters a change in blood flow by altering arteriolar resistance. * If the velocity of blood flow increases less capillary exchange takes place. If the velocity decreases the flow is inadequate and blood that has already undergone capillary exchange remains in the capillary. * So the myogenic mechanism slows blood flow by increasing resistance when arteriolar pressure rises. Conversely it speeds up blood flow by decreasing resistance when arteriolar pressure lowers.

Hormone Affecting PR - Decreased PR

* Decreased Renin Secretion

Hormones Affecting PR - Increased PR

* Epinephrine * Norepinephrine * Angiotensin-II * Renin Secretion

Tissue Perfusion - Brain

* Extremely intolerant of ischemia (lack of O2 - tissue dies); receives about 15% of total cardiac output. * Auto-regulatory mechanisms, including myogenic and metabolic controls, maintain cerebral blood flow at a nearly constant rate of about 750 ml/min. * Blood flow within brain to different areas varies considerably with neuronal activity.

Capillaries

* Extremely thin vessels, consists only of an endothelium rolled into a tube and a small amount of basal lamina secreted by endothelial cells. * Capillaries generally are found in clusters called capillary beds; wind their way between cells of most tissue in body. * Pericytes - cells found around some capillaries; have contractile filaments and appear to control blood flow through capillary. * Walls of most capillaries consists of 1 - 3 endothelial cells joined by tight junctions; curl around capillary entire circumference.

What are the two basic pressures that drive water movement?

* Hydrostatic Pressure (HP) * Osmotic Pressure (OP)

Blood Pressure Disorders

* Hypertension * Hypotenstion

Long-term Blood Pressure Regulation - Urinary System Details

* If blood pressure INCREASES, water is lost from body as urine, and blood volume and blood pressure DECREASES. * If blood pressure DECREASES, urine production DECREASES and a slight INCREASE in blood volume and blood pressure.

Tissue Perfusion - Skin

* Largest organ in the body; O2 and nutrients must diffuse from capillaries in dermis to epidermis. * Local auto-regulation of skin's blood flow respond to temperature. * Most important control over skin's blood flow is via sympathetic nervous system; neurons regulate blood flow in skin as part of temperature regulation. * Sympathetic nervous system alters skin perfusion via vasoconstriction to CONSERVE heat or vasodilation to RELEASE heat.

Metabolic Controls

* Mediated by matabolites (O2, CO2 and H+) present in interstitial fluid surrounding the capillaries. * Cells consume O2 to turn ADP and inorganic phosphate into ATP while generating CO2 --> The CO2 diffuses into the interstitial fluid and reacts with water to yield carbonic acid. * So the actively metabolizing cell will contain a low concentration of O2 and high concentration of CO2 and H+. All three of these conditions cause the smooth muscle cells of the local arterioles to relax, dilating the arterioles. This increases perfusion and ensures adequate O2 and nutrient delivery to the actively metabolizing cell. * The reverse: Tissues whose cells are producing ATP slowly will have a high concentration of O2 and low CO2 and H+. These conditions cause constriction of local arterioles and a decrease in tissue perfusion.

What are the two main types of local regulatory control of tissue perfusion?

* Myogenic * Metabolic

What determines Blood Pressure?

* Peripheral Resistance * Cardiac Output * Blood Volume

Tissue Perfusion - Heart

* Receiving about 5% of total cardiac output via coronary circulation. * Perfusion pattern is opposite of systemic circuit; heart tissue perfusion DECREASES during systole. * Main local auto-regulatory mechanism appears to by metabolic control, in particular concentration of O2 in cardiac interstitial fluid. * Low interstitial fluid O2 level triggers production of chemicals called vasodilators.

What determines Cardiac Output?

* Stroke Volume * Heart Rate

Blood Pressure

* The outward force that blood exerts on walls of blood vessels. * Varies dramatically in different parts of vasculature; highest in large systemic arteries - lowest in large systemic veins. * Three main factors that influence blood pressure: Resistance, Cardiac Output and Blood Volume.

Hormones Affecting CO - Increased Rate & Force of Contraction

* Thyroid Hormone * Epinephrine * Norepinephrine

What determines Peripheral Resistance?

* Vessel Radius - resistance varies inversely with vessel's radius. * Blood Viscosity - the inherent resistance that all liquids have to flow. * Blood Vessel Length - the longer the vessel the greater the resistance. * Obstructions in vessels

Blood Flow

* Volume of blood that flows per minute. * Generally blood flow matches cardiac output; average of 5-6 liters/min. * Blood flow is DIRECTLY PROPORTIONAL to pressure gradient. Which means that blood flow INCREASES when pressure gradient INCREASES and vice versa.

What determines Blood Volume?

* Water Loss * Water Gain

Long-term Blood Pressure Regulation - Hormones Details

* When blood pressure INCREASES atrial cells secrete ANP, which causes kidneys to excrete more water. * When blood pressure DECREASES, ADH secretion triggers thirst and increases amount of water retained. * Renin secretion is triggered when blood pressure DROPS, which activates angiotensin-II; induces thirst, and causes sodium ion retention. * Angiotensin-II also triggers secretion of aldosterone which causes retention of sodium ions.

Fetal Blood Circulation - Changes after birth

1) The Foramen Ovale and the Ductus Arteriosus (shunts) close and the regular circulatory pattern begins. 2) Usually within a year after birth the flaps in the interatrial septum seal and the Foramen Ovale becomes a depression called the Fossa Ovalis. 3) The Ductus Arteriosus close within three months after birth due to the pressure changes in the pulmonary trunk and aorta. The remnant of the Ductus Arteriosus becomes the Ligamentum Arteriosum. 4) Ductus Venosus degenerates and becomes the fibrous Ligamentum Venosum. 5) The Umbilical Arteries become the Medial Umbilical Ligaments. 6) The Umbilical Vein becomes the Ligamentum Teres (also called the round ligament of the liver)

Hypotension

Abnormally low blood pressure (Systolic pressure lower than 90 and/or diastolic lower than 60) Symptoms: Mild cases show mild dizziness, lightheadedness. Severe cases (circulatory shock) show loss of consciousness and organ failure Causes: Reduced blood volume, Decreased cardiac output and Vasodilation (excessive vasodilation)

At rest how much of the body's capillary beds are fully open?

About 25%

Arteries vs. Veins

Arteries carry blood away from the heart Veins carry blood to the heart Arteries have a thicker Tunica Media

Mean Arterial Pressure (MAP)

Average pressure in the systemic arteries during an entire cardiac cycle. The MAP generally measures about 95 mm Hg. The equation for calculation of MAP is: MAP = Diastolic Pressure + 1/3 (Pulse Pressure)

Precapillary Sphincter

Controls the amount of blood flowing into the capillaries.

Short-Term Blood Pressure Regulation (PNS)

Done by the Parasympathetic Nervous System: Parasympathetic neurons via the vagus nerve, decrease heart rate, which decreases cardiac output. Autonomic centers inhibit sympathetic neurons during periods of elevated parasympathetic activity. A decrease in sympathetic stimulation causes vasodilation and lower peripheral resistance. Both result in a DECREASE in blood pressure.

Short-Term Blood Pressure Regulation (SNS)

Done by the Sympathetic Nervous System: Sympathetic neurons increase heart rate and contractility, which increases cardiac output. Sympathetic neurons cause vasoconstriction of arterioles, which increases peripheral resistance. Both result in an INCREASE in blood pressure.

Classes of Arteries

Elastic Arteries Muscular Arteries Arterioles

Blood Flow through capillary beds

Flow of blood that takes place within body's capillary beds is collectively called Microcirculation. Microcirculation involves two types of vessels: True Capillaries where material is exchanged and a small Central Vessel.

True Capillaries

Form interweaving networks with multiple anastomoses; fed by proximal end of central vessel, which is formed by either a small terminal arteriole or a metarteriole.

Veins

Functions as the collection system of the vasculature. Drain blood from capillary beds and return it to the heart. Small merge with others to become progressively larger as they get closer to the heart. Pulmonary Circuit - Carry oxygenated blood Systemic Circuit - Carry deoxygenated blood

Hypertension

High blood pressure. Two Types: Essential (Primary) and Secondary. Essential - no identifiable cause - 95% of cases. Secondary - narrowing of the arteries serving the kidneys.

Circulatory Shock

Hypovolumetric Shock Cardiogenic Shock Anaphylactic Shock Septic Shock

Endothelium

Innermost lining of blood vessel.

Muscular Arteries (Distributing Arteries)

Intermediate in diameter; including most named arteries that SUPPLY organs.

Osmotic Pressure (OP)

Is the force we must apply to a solution to prevent water from moving into it by osmotic pressure. * Water follows solutes. * With higher osmotic pressure - pulls water into the capillary.

Transcytosis

Larger substances must cross the endothelial cells by transcytosis. Substances are taken into the cell by endocytosis and then leave the other side of the cell by exocytosis.

Elastic Arteries (Conducting Arteries)

Largest diameter Nearest to the heart Under highest pressure

Capillary Exchange

Main Function of Capillaries - Nutrients, gases, ions and wastes must be able to cross the wall and travel between the blood in the capillary and tissue cells. * Diffusion and osmosis through gaps and fenestrations. * Diffusion through the membranes of endothelial cells. * Transcytosis

#1 Chemoreceptor Stimulation - Short-Term blood pressure regulation

Peripheral Chemoreceptor: Located near the baroreceptors in the aortic arch and carotid artery. These receptors respond mostly to the O2 levels in the blood. A significant decrease in blood O2 concentration triggers a series of feedback loops that indirectly stimulate an increase in heart rate and cause vasoconstriction.

Fetal Blood Circulation

Placenta > Umbilical Vein (One) > Ductus Venosus > Inferior Vena Cava > Right Atrium > Foramen Ovale > Left Atrium > Left Ventricle > Aorta > Umbilical Arteries (Pair) > Placenta Placenta > Umbilical Vein (One) > Ductus Venosus > Inferior Vena Cava > Right Atrium > Right Ventricle > Pulmonary Trunk > Ductus Arteriosus > Aorta > Umbilical Arteries (Pair) > Placenta

Arterial Pressure

Pressure of blood against arterial walls measured by sphygmomanometer. There will be two separate pressures due to the ventricular systole and diastole. The systolic pressure will be the larger of the two numbers and the diastolic will be the smaller of the two numbers. Average Systolic is 110 - 120 mm Hg. Average Diastolic is 70 - 80 mm Hg.

Arterioles

Smallest arteries Contain all three layers of blood vessel walls, but the layers are extremely thin, and the tunica media contains only 1 - 3 layers of smooth muscle cells

Vasomotor Nerves

Stimulate the smooth muscle cells of the Tunica Media.

Baroreceptor Reflex - Decrease in blood pressure

Stimulus: Blood Pressure decreases below normal range. Receptor: Baroreceptors in the carotid sinus detect the decreased pressure and lower their rate of firing. Control Center: The impulses travel to the medulla of the brainstem for integration. Effector/Response: Medulla oblongata increases sympathetic output and decreases parasympathetic output, increasing heart rate and contractility and allowing vasoconstriction. In Homeostatic Range: Blood pressure increases, and feedback returns the medullary response to normal.

Baroreceptor Reflex - Rise in blood pressure

Stimulus: Blood pressure increases above normal range. Receptor: Baroreceptors in the carotid sinus detect the increased pressure and fire action potentials at a faster rate. Control Center: The impulses travel to the medulla of the brainstem for integration. Effector/Response: Autonomic centers in the medulla oblongata inhibit sympathetic activity, inducing vasodilation and decreased heart rate, lowering cardiac output. In Homeostatic Range: Blood pressure decreases, and feedback decreases response from the medulla.

Fenestrated Capillary

Structure: Contain fenestrations in the endothelial cells. Location: Kidneys, Endocrine Glands, Small Intestine Function: Moderately leaky - allow large volumes of fluid and larger substances to cross capillary walls.

Sinusoidal Capillaries

Structure: Discontinuous sheet of endothelium, irregular basal lamina, very large pores. Location: Liver, Lymphoid Organs, Bone Marrow, Spleen. Function: Leakiest - allow large substances such as cells to cross the capillary walls.

Continuous Capillary

Structure: Endothelial cells joined by tight junctions. Location: Skin, Muscle Tissue, Most nervous and connective tissue. Function: Least "leaky" - permit a narrow range of substances to cross the capillary walls.

Elastic Arteries - Structure & Function

Structure: Large arteries with well-developed elastic laminae Function: Conduct blood under high pressure to organs

Venules - Structure & Function

Structure: Small venules have only a tunica intima; larger venules have all three tunics. Function: Drains capillary beds. Veins can function as blood reservoirs because blood can be diverted from veins to other parts of the system.

Muscular Arteries - Structure & Function

Structure: Thick-walled arteries with a well-developed tunica media. Function: Controls blood flow to organs and Regulates blood pressure

Arterioles - Structure & Function

Structure: Thin walls with all three tunics. Function: Controls blood flow to tissues and Feeds capillary beds.

Veins - Structure & Function

Structure: Thin-walled vessels with a large lumen , little smooth muscle, and valves. Function: Returns blood to the heart.

Pulse Pressure (PP)

The difference between the Systolic and Diastolic pressures. PP = SP - DP.

Colloid Osmotic Pressure (COP)

The difference in osmotic pressure between the blood and the interstitial fluid. To determine COP with this formula: Capillary OP - Interstitial OP = COP

Hydrostatic Pressure (HP)

The force that a fluid exerts on the wall of its container. (Same thing as blood pressure). * Follows pressure gradient. Fluid will move from higher to lower pressure.

Tunica Intima

The innermost layer of a blood vessel. Consists of a sheet of simple squamous epithelium and it's basal lamina.

Long-term Blood Pressure Regulation - General

The long-term maintenance of blood pressure falls to the urinary system and certain hormones of the endocrine system. These systems control blood pressure by increasing or decreasing the amount of body water lost as urine, which affects blood volume. Endocrine system regulates blood volume through the release of hormones.

Tunica Media

The middle layer of the blood vessel wall. Two layers - Smooth Muscle Cells arranged in a circular manner around the lumen. Another layer of elastic fibers.

Tunica Externa

The outermost layer of the blood vessel wall. Made of Dense Irregular Collagenous Connective Tissue. Supports the blood vessel and prevents overstretching.

Baroreceptor Reflex - In general

The overall purpose of the baroreceptor reflex is to protect the body from sudden increases or decreases in blood pressure from moment to moment. It adjusts blood pressure with changes in position, such as when you get up from bed in the morning.

Venous Valves

These valves in the veins are extensions of the tunica intima that overlat and prevent blood from flowing backward in the vein. They are especially numerous in the veins of the legs, where blood flow toward the heart is strongly opposed by gravity.

Vaso Vasora

Tiny vessels supplying the Tunica Media and Tunica Externa. Supply oxygen and nutrients to the outer layers of the larger blood vessels, whose cells are too far away from the lumen to receive oxygen and nutrients by diffusion alone.

Capillary Net Filtration Pressure (NFP) #2

To find the Overall Net Filtration Pressure: NHP - NCOP = Overall NFP [Net Hydrostatic Pressure (arteriolar end) - Net Colloid Osmotic Pressure (venous end) = Overall Net Filtration Pressure] To find the NHP: HBP - IHP = NHP To find the NCOP: BCOP - ICOP * If the solution is a positive number - driving water out of the capillary and would be found on the Arterial end of the capillary. * If the solution is a negative number - pulling water into the capillary and would be found on the Venous end of the capillary.

Venous Return

Two Pumps: Skeletal Muscle Pump - When the muscle contracts, blood is squeezed upward, which pushes the valve open. When the muscle relaxes, blood flows backward and the valves close. Respiratory Pump - Helps propel blood through thoracic and abdominal cavity veins. Venous valves prevent backward flow in some veins. Smooth muscle in vein walls can contract under sympathetic nervous system stimulation to increase rate of venous return.

Distribution of blood in the cardiovascular system

Veins typically outnumber arteries, and their lumens have a larger average diameter. For these reasons up to 70% of the total blood in the body is located in the veins at any given moment (this includes both systemic and pulmonary veins). When necessary blood can be diverted from the veins to other parts of the body. Systemic Veins - 55% Right Heart - 6% Pulmonary Arteries - 3% Pulmonary Veins - 15% Left Heart - 6% Systemic Arteries - 10% Capillaries - 5%

Diffusion and osmosis through gaps and fenestrations

Water is able to move freely through these pores by osmosis and small solutes such as monosaccharides and amino acids can move if a concentration gradient is present.

Aorta