Blood Flow and Pressure Exam 3

Net Filtration and Absorption Formula

(Pc + 𝛑if) - (𝛑p + Pif) = NFP. Works for both net filtration and net reabsorption.

Five Main Types of Blood Vessels

1) Arteries 2) Arterioles 3) Capillaries 4) Venules 5) Veins

Three Types of Capillaries

1) Continuous Capillaries 2) Fenestrated Capillaries 3) Sinusoids

Velocity of Blood Flow

1) Dependent on the total cross sectional area of the vessel or group of vessels through which the blood is flowing. 2) Slowest there the total cross-sectional area is the greatest. 3) Each time an artery branches, the total cross-sectional area of all of its branches is greater than the cross-sectional area of the original vessel, so blood flow becomes slower and slower as blood moves further away from the heart, and is slowest in the capillaries. Conversely, when venules unite to form veins, the total cross-sectional area becomes smaller and flow becomes faster. 4) Capillaries are ideal sites of exchange because of their Large Cross Section and Low Velocity.

Lymphatic System Functions

1) Drains excess interstitial fluid. 2) Returns filtered plasma proteins back to the blood. 3) Carries out immune responses. 4) Transports dietary lipids.

Capillary Flow is controlled in two ways

1) Metarteriole: capillaries branch from and then reconnect with these blood vessels that extend from an arteriole to a venule. Serve as shunts that allow blood to bypass the capillaries. 2) Precapillary sphincters: located at the junctions where a capillary branches from an arteriole or a metarteriole, are rings of smooth muscle fibers. Control the flow of blood through the capillaries. Causes Vasomotion: alternating contraction and relaxation of the smooth muscle of precapillary sphincters. Is due in part to local chemical factors released by the endothelial cells.

Hormones Involved in Extrinsic Regulation of Blood Flow

1) Norepinephrine and epinephrine released from the adrenal medulla in response to sympathetic stimulation can affect blood flow. 2) Norepinephrine preferentially activates alpha1 adrenergic receptors to cause vasoconstriction in the arterioles of most tissue and epinephrine preferentially binds to beta2 adrenergic receptors causing vasodilation of arterioles in the heart and skeletal muscle. 3) ADH (vasopressin) and angiotensin II: These hormones are involved in salt and water balance of the ECF. Consequently, they influence blood volume. Both of these hormones are also vasoconstrictors. 4) Atrial Natriuretic Peptide (ANP): Reduces blood pressure.

Arteriolar radius is adjusted to serve two functions

1) Regulate and maintain blood pressure. 2) Distribute cardiac output as needed. Increasing the contractile state of arteriolar smooth muscle causes vasoconstriction, which decreases the vessel radius, increases resistance, and decreases blood flow. By contrast, decreasing the contractile state of arteriolar smooth muscle causes vasodilation, which increases the vessel radius, decreases resistance, and increases blood flow. Arteriolar radius determines distribution of cardiac output (blood flow) to the different organs. Determined by: 1) Neural and hormonal factors (extrinsic control) 2) Local demand (intrinsic controls).

Pressure and Flow

1) The greater the pressure gradient across a blood vessel, the greater the blood flow through that vessel. 2) 𝚫P = P1-P2 where P1 and P2 are the pressures of the vessel inlet and outlet. Ex. Pressure in a vessel is 60mmHg at one end and 40mmHg at the other. Blood flow would be 20mmHg. 3) It is the difference in pressure between the two ends of the blood vessel, and not the absolute pressures within the vessel, that determines the flow rate.

Venous Blood return to the heart is aided by

1) Valves: thin folds of endothelium and connective tissue that form flaplike cusps. Cusps project into the lumen, pointing toward the heart. Aid movement of venous blood to the heart by preventing back flow. 2) Pumps

Factors the promote venous return

1) Venous pressure gradient 2) Venous valves 3) Skeletal muscle pump 4) Respiratory pump 5) Venoconstriction

Several mechanisms maintain the flow of lymph

1. Smooth muscle contractions. When a large lymphatic vessel distends because of the presence of lymph, the smooth muscle in its wall contracts, which helps move lymph from one segment of the vessel to the next. 2. Skeletal muscle pump. The milking action of skeletal muscle contractions compresses lymphatic vessels (as well as veins) and forces lymph toward the venous system. 3. Respiratory pump. Lymph flow is also maintained by pressure changes that occur during inspiration. Lymph flows from the abdominal region, where the pressure is higher, toward the thoracic region, where it is lower. When the pressures reverse during expiration, the valves of the lymphatic vessels prevent backflow of lymph.

Net Reabsorption at venous end of capillaries

17liters per day. Favors reabsorption.

Net Filtration at arterial end of capillaries

20liters per day. Favors filtration.

Local Mediators

A wide variety of local chemical mediators (paracrines) can alter arteriolar radius; some mediators increase arteriolar radius (vasodilation) and others decrease it (vasoconstriction). In addition to acting on arterioles to alter vessel radius, local mediators can act on precapillary sphincters to regulate blood flow through capillary beds. Vasodilators relax precapillary smooth muscle, increasing flow through capillary beds, whereas vasoconstrictors contract precapillary smooth muscle, reducing blood flow through capillaries.

Distribution of Cardiac Output: Percent of cardiac output received by each organ is based upon

A) Metabolic Need B) Organ Function - organs that " recondition" blood receive a higher proportion of CO. Ex.: Digestive tract - adds nutrients, Kidneys - salt and water balance, Liver - detoxifies, and the Skin - temperature regulation.

Total peripheral resistance (TPR)

Also known as systemic vascular resistance. Refers to all of the vascular resistances offered by systemic blood vessels. Major role of arterioles is to control TPR via changing their radius.

Blood flow from organ to organ varies

At rest, conditioning organs receive a large percentage of blood flow. Changes in metabolic demand, redirect blood flow to where it's needed. Brain is the only organ with constant total blood flow. The lungs receive 100% of the cardiac output from the right ventricle; the other organs of the body receive only a fraction of the cardiac output from the left ventricle.

Mean Arterial Pressure

Average blood pressure in the arteries. Drives blood flow through the cardiovascular system. Equal to the diastolic pressure plus one-third of the pulse pressure. If the entire systemic circulation is considered to be a single blood vessel that carries blood away from and back to the heart, then ΔP (pressure gradient) represents the difference between the pressure at the beginning of the vessel (the aorta) and the end of the vessel (the vena cavae at the entrance to the right atrium). Because MAP is the average pressure in the aorta and other arteries and the venous pressure at the right atrium is essentially 0 mmHg, ΔP is equal to MAP. MAP = CO x TPR. Determined by: Cardiac output (CO) and Total peripheral resistance (TPR).

Lymphatic System

Begins in the tissues close to blood capillaries. Consists of: A fluid called lymph. Tubes that are called lymphatic vessels that transport the lymph. Lymphoid organs and tissues, including lymph nodes, bone marrow, thymus, spleen, tonsils, Peyer's patches of the small intestine, and the appendix. Picks up excess fluid filtered by the capillaries. Fluid inside the vessels referred to as lymph.

Laminar Flow

Blood flow that is in a smooth, streamlined manner that is parallel to the vessel axis. So-named because the fluid behaves as if it were comprised of many layers (lamina=layers). As the layers move, they slide past one another. Layer closest to the wall moves very slowly because it adheres to the wall and therefore has the greatest resistance. Each successive layer toward the center of the vessel moves progressively faster as it moves further from the vessel wall. Is quiet and does not produce any sounds.

Filtration

Bulk movement from capillaries into interstitial fluid. Pressure driven movement of fluid and solutes from blood capillaries into interstitial fluid.

Absorption

Bulk movement from interstitial fluid into the capillaries. Pressure driven movement from interstitial fluid into blood capillaries.

Blood Pressure Determined by

Cardiac output, Blood volume, Vascular resistance.

Baroreceptor Sensor Information Sent to Medulla

Cardiovascular centers in the medulla oblongata alter sympathetic and parasympathetic output.

Control of Blood Flow

Changes in flow are required when the metabolism of a tissue changes. Blood flow changes occur by adjusting blood vessel diameter. Vascular tone of arterioles serves as the baseline for constriction and dilation.

Diastolic Blood Pressure

Created by the diastolic phase of the heart. Lowest measured pressure attained in the system (arteries) during diastole.

Systolic Blood Pressure

Created by the systolic phase of the heart. Highest measured pressure attained in the system (arteries) during systole.

Capillary Exchange

Diffusion is the most important method of _________ exchange. In a ________, nutrients, gases, and wastes are exchanged between blood and interstitial fluid. Many different types of molecules are exchanged. Gases, Nutrients, Fluid. Large molecules transported via transcytosis. Mainly used to transport large, lipid-insoluble molecules that cannot cross ___________ walls in any other way.

Circulatory Changes During Dynamic Exercise

Distribution of CO is adjusted as metabolic needs of a given tissue change. For example, during exercise, skeletal muscle and heart receive a higher proportion of CO. This "extra" blood flow is diverted away from the "reconditioning" organs.

Two Types of Arteries

Elastic Arteries and Muscular Arteries

Shock

Failure of the cardiovascular system to deliver enough O2 and nutrients to meet cellular metabolic needs.

Tissue/Capillary Fluid Exchange

Filtration of fluid occurs between the vascular fluid compartment and interstitial fluid as a result of ultrafiltration and reabsorption as blood passes through the capillaries.

Capillaries

Form extensive branching networks that increase the surface area available for rapid exchange of materials. In most tissues, blood flows through only a small part of the capillary network when metabolic needs are low. However, when a tissue is active, such as contracting muscle, the entire capillary network fills with blood. Form beds at a tissue known as Capillary Beds (a network of 10-100 capillaries).

Veins

Function as a blood reservoir. Systemic veins and venules contain the largest volumes of blood at rest. From these reservoirs, blood can be diverted quickly if the need arises. Large veins enter through the right atrium of the heart (superior and inferior vena cava). Have thinner walls and less smooth muscle and elastic tissue compared to arteries. Carry blood back to the heart

Elastic Artery

Function of elastic arteries: Pressure reservoir. Large elastic arteries leave the heart and divide into medium-sized, muscular arteries. Ex. Aorta and the pulmonary trunk. Serve as the pressure reservoirs that maintain the driving force for blood flow while the ventricles are relaxing. As blood is ejected from the heart into elastic arteries during systole, their highly elastic walls stretch due to the increased blood volume. As they stretch, they momentarily store some of the pressure generated by the contraction of the ventricles. Then, while the ventricles are relaxing during diastole, the walls of the elastic arteries recoil as the stored pressure is released. This elastic recoil propels blood onward, ensuring that blood continues to move through the remaining arteries of the circulation even though the ventricles are relaxing and not ejecting blood.

Fenestrated Capillaries

Have fenestrations, cylindrical pores that extend through the endothelial cells. Intercellular clefts are also present between the endothelial cells. Have a higher permeability to water and small solutes. Found in the kidneys, small intestine, and endocrine glands.

Four Pressures of Starling Forces

Hydrostatic Pressure: Capillary Hydrostatic Pressure and Interstitial Fluid Hydrostatic Pressure. Osmotic Pressure: Plasma Colloid Osmotic Pressure and Interstitial Fluid Colloid Osmotic Pressure.

Tonic Activity of Baroreceptor Afferents

Important to provide CNS continuous information about the BP. Increase BP > Increases firing rate. Decrease BP > decreases firing rate.

Lymphatic System Returns filtered plasma proteins back to the blood

Interstitial fluid contains only a small amount of protein because most plasma proteins are unable to be filtered across blood capillary walls. The relatively few proteins that are filtered across blood capillary walls cannot return to the blood by diffusion because the concentration gradient (high level of proteins inside blood capillaries, low level outside) opposes such movement. These proteins can, however, move readily into lymph by entering highly permeable lymphatic vessels called lymphatic capillaries (described shortly). From lymph, the plasma proteins eventually move back into the bloodstream.

There are two methods of controlling the Radii, and therefore resistances, of arterioles

Intrinsic and Extrinsic control. The importance of each depends on the tissue involved.

Blood Pressure

Is hydrostatic pressure exerted by blood on blood vessel walls. Contraction of the ventricles generates BP. BP is highest in the aorta and other large systemic arteries; in a resting, healthy young adult, BP rises to about 110 mmHg during systole (ventricular contraction) and drops to about 70 mmHg during diastole (ventricular relaxation).

Capillary

Known as exchange vessels. Their primary function is the exchange of nutrients and wastes between the blood and tissue cells. They are well suited for this function because: 1) Capillary walls are thin, composed of only a single layer of endothelial cells surrounded by a basement membrane; no smooth muscle or connective tissue is present . Thus, a substance in the blood must pass through just one cell layer to reach the interstitial fluid and tissue cells. 2) Capillary walls contain pores (spaces) that permit passage to certain substances. These pores are located both in the endothelial layer and throughout the extracellular matrix of the basement membrane. The number and size of the pores in a capillary can vary in different tissues.

Blood Flow Through Blood Vessels can be

Laminar or Turbulent

Lymphatic Vessels Begin as capillaries

Located in the spaces between cells, are closed at one end. Lymphatic capillaries are slightly larger than blood capillaries and have a unique structure that permits interstitial fluid to flow into them but not out. Lymphatic vessels begin as lymphatic capillaries, which are larger in size and are more permeable than blood capillaries.

Lymphatic System Drains excess interstitial fluid

Lymphatic vessels drain excess interstitial fluid from tissue spaces and return it to the blood. About 17 liters of the fluid filtered daily from the arterial end of blood capillaries return to the blood directly by reabsorption at the venous end of the capillaries. The excess filtered fluid—about 3 liters per day—passes first into lymphatic vessels and then is returned to the blood. Then interstitial fluid enters into lymphatic vessels, it is known as lymph. The major difference between interstitial fluid and lymph is location: Interstitial fluid is found between cells, and lymph is located within lymphatic vessels.

Lymphatic System Carries out immune responses

Lymphoid organs and tissues initiate immune responses directed against microbes or abnormal cells.

Intrinsic Regulation of Blood Flow (Local Controls)

Metabolic Control Mechanisms within an organ that regulate blood flow by altering the radii of the arterioles that supply the organ. Allows a tissue to adjust blood flow to match its metabolic demands. Allows autoregulation: the ability of a tissue to maintain a relatively constant blood flow in the presence of changing arterial pressure. Local control over-rides sympathetics. There are two types of intrinsic control: 1) Physical Changes 2) Local Mediators.

Venules

Microscopic veins. Drain blood from capillaries and begin the return of blood back toward the heart. Function as a blood reservoir.

Continuous Capillaries

Most capillaries are continuous. Permeable to water and small solutes such as sodium ions and glucose. Found in muscle, connective tissue, and the lungs. Also present in the brain.

Starling Forces

Net Pressure. Bulk Flow across blood capillary walls is determined by four pressures collectively known as _________ Forces. Two promote filtration and the other two promote absorption. Movement of fluid across the capillary walls is dependent upon the forces acting across the wall.

Bulk Flow Through Capillaries

Passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction. The substances move at rates far greater than can be accounted for by diffusion alone. Occurs from an area of higher pressure to an area of lower pressure, and continues as long as a pressure difference exists. Important for regulation of the relative volumes of blood and interstitial fluid. Moves large volumes of solutes in fluid together in the same direction.

Baroreceptors

Pressure sensors that monitor BP, located in the aorta, internal carotid arteries, and other large arteries in the neck and chest. Send input to the CV center to help regulate blood pressure. Important in short-term regulation of BP Located in aortic arch and carotid arteries.

Resistance Equation

R=nL/r4 (R = resistance, n=blood viscosity, L=blood vessel length, r=blood vessel radius).

Vascular Compliance

Refers to the ability of a blood vessel to stretch; veins have a high compliance, whereas arteries have a low compliance.

Blood viscosity

Resistance blood flow is directly proportional to the viscosity (thickness) of blood. The viscosity of blood depends mostly on the ration of erythrocytes to plasma (fluid) volume, and to a smaller extent on the concentration of proteins in plasma.

Blood vessel radius

Resistance to blood flow is inversely proportional to the fourth power of the radius of the blood vessel. The smaller the radius of the blood vessel, the greater the resistance it offers to blood flow. Because it is raised to the fourth power, it is the most significant to blood flow resistance.

Blood vessel length

Resistance to blood flow through a vessel is also directly proportional to the length of the blood vessel. The longer the blood vessel, the greater the resistance.

Nerves Extrinsic Regulation of Blood Flow - ANS

Sympathetic Nervous System. Arterioles in the skin and viscera are innervated by sympathetic nerve endings. Alpha-adrenergic receptors are excitatory. Norepinephrine binds to these receptors causing vasoconstriction. Beta adrenergic receptors are inhibitory. Norepinephrine binds to these receptors causing vasodilation. Responsible for vascular "tone." Ex. of fight or flight response. Increased sympathetic activity causes vasoconstriction in the viscera and skin; and vasodilation in the skeletal muscles. Sympathetics maintain MAP by broad vasoconstriction, which is the driving force for flow.

Pulse Pressure

The difference between systolic pressure and diastolic pressure. Provides information about the condition of the cardiovascular system.

Lymphatic System Transports dietary lipids

The lymphatic system transports absorbed lipids from the gastrointestinal tract to the blood. The lipids first enter the lymphatic system by passing into specialized lymphatic vessels called lacteals. From lacteals the lipids enter larger lymphatic vessels and then move into the bloodstream.

Nervous system is an important regulator of MAP

The nervous system regulates MAP via negative feedback loops that involve the cardiovascular center of the medulla oblongata and two types of reflexes: 1) Baroreceptors reflexes 2) Chemoreceptor reflexes

Blood Pressure Regulation

To maintain adequate blood flow to the tissues, the body monitors blood pressure and adjusts CO and TPR to maintain adequate perfusion pressure. Uses baroreceptors to monitor blood pressure.

Regulation of Mean Arterial Pressure

Two major determinants of MAP are CO and TPR. MAP = CO x TPR (Cardiac Output & Total Peripheral Resistance). All factors of TPR and CO are determinants. Factors of CO: 1) Heart Rate: Parasympathetic activity, sympathetic activity, and hormones from the adrenal medulla. 2) Stroke Volume: Sympathetic activity and hormones from the adrenal medulla, Venous return, and contractility. Factors of TPR: 1) Blood Vessel Radius: vasoconstrictor or vasodilator substances (local mediators) and Sympathetic activity. 2) Blood Viscosity: Number of erythrocytes. 3) Blood Vessel Length: increased body size, i.e. obesity.

Baroreceptor Reflex

Two most important baroreceptor reflexes are the carotid sinus reflex and the aortic reflex. Blood pressure stretches the walls of the carotid sinus or aortic arch baroreceptors. When blood pressure falls, the baroreceptors are stretched less, and the sensory (afferent) neurons associated with the baroreceptors send action potentials at a slower rate to the CV center. In response, the CV center decreases parasympathetic stimulation of the heart by way of motor axons of the vagus nerves and increases sympathetic stimulation of the heart via cardiac accelerator nerves. As a result, heart rate and contractility increase, which causes cardiac output to increase. Another consequence of increased sympathetic stimulation is increased activity of the vasomotor nerves, resulting in greater vasoconstriction and total peripheral resistance. Increased cardiac output and increased total peripheral resistance cause blood pressure to increase to the normal level. Conversely, when an increase in blood pressure occurs, the baroreceptors are stretched more, and the associated sensory neurons send action potentials at a faster rate to the CV center. The CV center responds by increasing parasympathetic stimulation and decreasing sympathetic stimulation. The resulting decreases in heart rate and contractility reduce cardiac output. The CV center also slows the rate at which it sends sympathetic output along vasomotor nerves that cause vasoconstriction. The resulting vasodilation lowers total peripheral resistance. Decreased cardiac output and decreased total peripheral resistance both lower systemic arterial blood pressure to the normal level. Decrease in MAP. Important for quick beat-by-beat regulation Ex.Orthostatic hypotension, lying down to standing up. Increase in MAP

Venous Return

Volume of blood flowing back to the heart through systemic veins. Brings blood back to the heart. Largely determined by the venous pressure gradient - the driving force through systemic veins, equal to the difference in pressure between the venules and the right atrium. If pressure in the right atrium or ventricle increases, venous return will decrease.

Physical Changes: Myogenic Response

Warming promotes vasodilation, and cooling causes vasoconstriction. In addition, smooth muscle in arteriole walls exhibits a myogenic response—it contracts more forcefully when it is stretched and relaxes when stretching lessens. If, for example, arterial pressure increases, the elevated blood pressure stretches the walls of the arterioles. This causes the arteriolar smooth muscle to contract and produce vasoconstriction, which keeps blood flow constant. Important for autoregulation of blood flow to organs. If blood flow decreases, arterioles dilate to maintain adequate blood flow. If blood flow increases, arterioles constrict to protect smaller vessels downstream.

Turbulent Flow

When blood flows through an abnormally constricted area, moves over a rough surface, makes a sharp turn, or exceeds a critical velocity, laminar flow becomes turbulent. Components of blood move at various angles to the axis of the vessel. This causes blood to mix, forming vortices (whorls) that increase the interaction between blood and the vessel wall, resulting in vibrations that are heard as sounds. Greater resistance compared to laminar flow.

Sinusoids

Wider and more winding than other capillaries. Have unusually large fenestrations. Have very large intercellular clefts and an incomplete or absent basement membrane. Permeable to water, small solutes, and relatively large substances in the blood such as proteins and blood cells. Present in bone marrow, the spleen, and the liver.

Vascular Tone

ability of a blood vessels smooth muscle to maintain a state of partial contraction.

Oxygen (O2) can also act as a local mediator

alters arteriolar radius. An important difference between the systemic and pulmonary circulations is their response to changes in O2 level. The walls of arterioles in the systemic circulation dilate in response to low O2. With vasodilation, O2 delivery increases, which restores the normal O2 level. By contrast, the walls of arterioles in the pulmonary circulation constrict in response to low levels of O2. This response ensures that blood mostly bypasses those alveoli (air sacs) in the lungs that are poorly ventilated by fresh air. Thus, most blood flows to better-ventilated areas of the lung.

Hyperemia

an excess of blood in the vessels supplying an organ or other part of the body. Intrinsic control of blood flow is evident when blood flow to a tissue increases. Two Types: Active and Reactive _______. Vasodilation > decreases resistance > increases flow > delivers more O2 and removes metabolic end products. Active _________ and reactive __________ operate via the same mechanisms—release of local mediators that cause vasodilation of arterioles and precapillary sphincters. The difference between the two types of __________ is based on the cause. In active __________, the cause is an increase in metabolic activity; in reactive __________, the cause is a blocked blood supply.

Veins

are blood vessels that carry blood from tissues back to the heart.

Extrinsic Controls

are mechanisms originating outside an organ that regulate blood flow by altering the radii of the arterioles that supply the organ. There are two types of extrinsic control: 1) Nerves 2) Hormones.

Vascular Tone

arteriolar smooth muscle exhibits a state of partial contraction. This establishes a baseline level from which contraction can be increased or decreased.

The sequence of fluid flow is

blood capillaries (blood) → interstitial spaces (interstitial fluid) → lymphatic capillaries (lymph) → larger lymphatic vessels (lymph) → venous circulation.

Vasculature

blood vessels and all arteries that carry blood.

Arteries

carry blood away from the heart. Large-diameter arteries have thick walls composed of several layers of tissue: an endothelium, basement membrane, smooth muscle, fibrous connective tissue, and a high proportion of elastic connective tissue.

Skeletal Muscle Pump

contraction of skeletal muscles in the lower limbs also helps boost movement of blood back to the heart.

Hemodynamics

forces involved in circulating blood throughout the body

Venules

groups of capillaries within a tissue unite to form small veins. These in turn merge to form progressively larger veins.

Active Hyperemia

increase in metabolism causes a local decrease in O2, and an increase in metabolic end-products (CO2, K+, lactic acid, osmolarity, paracrine signals such as adenosine, and prostaglandins). Blood flow to a tissue increases in response to an increase in metabolic activity. When tissues are active, metabolism increases. Cells consume more O2 and release large amounts of metabolites such as CO2, K+, and adenosine. These local mediators trigger vasodilation of nearby arterioles and precapillary sphincters, increasing blood flow to the tissue. The increase in blood flow brings in more O2 and nutrients and removes the metabolites.

Net Exchange (ie, plasma fluid lost, gained, or no change)

is determined by the balance between filtration and reabsorption. Net Exchange = Filtration - Reabsorption. Capillary pressure drops along the length of the capillary. Need to calculate forces at both the arteriole and venule end.

Reabsorption

is the inward movement of fluid, that is the fluid that is being returned to the capillary blood. Venous End to the heart.

Ultrafiltration

is the outward movement of fluid from the capillaries to interstitial fluid through pores in the capillary walls. Arterial End from the heart.

Plasma-colloid osmotic pressure (𝛑p)

is the pressure due to the colloidal suspension in blood of plasma proteins, which are unable to move across capillary walls. Promotes reabsorption by causing osmosis of fluid from the interstitial spaces into the capillaries. = 25mmHg, opposing filtration (movement of water).

Interstitial fluid-colloid osmotic pressure (𝛑if)

is the pressure due to the presence of plasma proteins in interstitial fluid. Promotes filtration by causing osmosis of fluid from blood into the interstitial spaces. = 0mmHg

Interstitial-fluid Hydrostatic pressure (Pif)

is the pressure that water in interstitial fluid exerts against the outer surface of capillary walls. Promotes reabsorption by forcing fluid from interstitial spaces back into capillaries. = 1mmHg, driving force out of capillary.

Arterioles

literally means small arteries; branch into capillaries delivering blood. Are abundant microscopic vessels. Along with capillaries and venules, are known as the microcirculation due to the need of a microscope to view them. Known as resistance vessels. Small diameters provide the greatest resistance to blood flow. Affect blood flow resistance by changing diameter. Vasoconstriction increases resistance and blood pressure. Caused by sympathetic nervous system stimulation. Vasodilation decreases resistance and blood pressure. Caused by a decrease in sympathetic nervous system stimulation or the presence of certain chemicals (nitric oxide and H+ions)

Regulation of Blood Flow

mean arterial pressure is essentially constant from one organ to another, the driving pressure for blood flow is the same for each organ. This means that differences in blood flow are solely determined by changes in resistance. The principal determinant of resistance is blood vessel radius and arterioles are the main blood vessels that contribute to resistance. Therefore, the main way to regulate blood flow to different organs in the body is to alter the radii of the arterioles that supply these organs.

Muscular Artery

medium-sized arteries contain more smooth muscle and less elastic connective tissue in their walls. They distribute blood to the various organs of the body. Have a lower ability to recoil and propel blood compared to elastic arteries. Capable of greater vasoconstriction and vasodilation. Most of the arteries. Makes up the majority of the arteries in the arterial circuit. Distribute blood to the organs of the body. As it enters an organ, it divides into small arteries called arterioles.

Resistance

opposition to blood flow due to friction between blood and the walls of blood vessels. Depends on three factors: 1) Blood viscosity 2) Blood Vessel Length 3) Blood Vessel Radius

Vasodilating chemicals (Local Mediators)

released by metabolically active tissue cells include CO2, K+, H+ (from acids such as lactic acid), and adenosine (from ATP). Another important vasodilator is nitric oxide, a gas released from endothelial cells. Tissue trauma or inflammation also causes release of vasodilators such as bradykinin, histamine, and prostacyclin (a type of prostaglandin).

Vasoconstricting chemicals (Local Mediators)

released from cells include thromboxane A2, superoxide radicals, serotonin (from platelets), and endothelin (from endothelial cells).

Capillaries

smallest blood vessels of the body. Thin, porous walls of capillaries allow the exchange of substances between the blood and body tissues. Microscopic vessels that connect arterioles to venules.

Capillary Hydrostatic blood pressure (Pc)

the pressure that water in blood exerts against the inner surface of capillary walls. It promotes filtration by forcing fluid out of capillaries into interstitial fluid. = 37-25mmHg at arteriole end and = 17mmHg at the venule end. Driving force out of capillary.

Myogenic Control Mechanisms

vascular smooth muscle responds to changes in arterial blood pressure. Important for autoregulation of blood flow to organs. If blood flow decreases, arterioles dilate to maintain adequate blood flow. If blood flow increases, arterioles constrict to protect smaller vessels downstream.

Blood Flow

volume of blood that flows through any tissue in a given time. 1) Depends on the pressure gradient (difference in pressure) that drives blood flow through a tissue. 2) Affected by resistance of blood flow in specific blood vessels. 3) Flow equation: F=𝚫P/R (F=blood flow, 𝚫P=pressure gradient, R=resistance to blood flow) 4) Flow is directly proportional to the pressure gradient 5) Flow is indirectly proportional to the resistance of blood flow. 6) Blood flows from regions of higher pressure to regions lower pressure; the greater the pressure gradient, the greater the blood flow. However, the higher the resistance, the smaller the blood flow.

Reactive Hyperemia

when blood flow is blocked, metabolites build up and cause vasodilation. When the blockage is removed, blood flow is increased. Blood flow to a tissue increases in response to a temporary blockage of the blood supply to that area. When the blood supply to a tissue is blocked, cells run out of oxygen and an oxygen debt accrues. In addition, metabolites released from the cells accumulate because there is no blood flow to remove them. The decreased O2 and built-up metabolites cause vasodilation, but the occlusion prevents blood from entering. However, once the blockage is removed, there is an increase in blood flow to the area. The increase in blood flow continues until the O2 debt is paid back and the metabolites are flushed out.