Blood Vessels And Circulation

Front 1

Three Principle Categories of Blood Vessels

Back 1

• Arteries: efferent vessels of the cardiovascular system- vessels that carry blood away from the heart

• Veins: afferent vessels that carry blood back to the heart

• Capillaries: microscopic thin-walled vessels that connect the smallest arteries to the smallest veins

Front 2

The Vessel Wall: Tunica Interna (Tunica Intima)

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Innermost layer that lines the inside of the vessel and is in contact with the blood (smooth/minimizes friction). It is composed of a simple squamous epithelium called the endothelium and is continuous with the endocardium of the heart.

Front 3

Functions of the Tunica Interna

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o Acts as a selectively permeable barrier to materials entering or leaving the blood stream

o Secretes chemicals that stimulate dilation or constriction of the vessel

o Repels blood cells and platelets so they flow freely and don’t stick to the vessel wall

o When endothelium is damaged, platelets may adhere to it to form a blood clot or when the tissue is inflamed, endothelial cells produce cell adhesion molecules to induce leukocytes to adhere to the surface (defensive actions needed)

Front 4

The Vessel Wall: Tunica Media

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middle layer and usually the thickest. It consists of smooth muscle, collagen, and elastic tissue. The amount of smooth muscle varies greatly from one vessel to another.

Front 5

Functions of Tunica Media

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o Strengthens the vessels and prevents blood pressure from rupturing them (regulated by the SNS)

o Vasomotion: regulates the diameter of a blood vessel- Decrease in SNS: vasodilation- smooth muscle relaxes. Increase: vasoconstriction

o Cold: less diameter in blood vessel= less surface area to keep warm causing the body to vasoconstrict and more blood to the core
o Heat: vessels dilate (vasodilation), core widens, and lose more heat

Front 6

Tunica Externa

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outermost layer that consists of loose connective tissue

Functions:
o Protects and anchors vessel to adjacent tissues
o Provides passage for small nerves, lymphatic vessels, and smaller blood vessels that supply the tissues of the larger vessel.
o Vasa Vasorum: small vessels that that supply blood to the outer wall of larger vessels

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Arteries

Back 7

called the resistance vessels of the cardiovascular system because they are designed to withstand surges of pressure (heart beat causes surge of pressure in arteries as blood is ejected into them). More smooth muscle than veins- can retain their round shape.

Three categories: Conducting arteries, Distributing arteries, and Resistance arteries

Front 8

Conducting Arteries

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largest diameter with a layer of elastic tissue called the internal elastic lamina between the elastic tunica media
Functions:
• Expand as they receive blood during ventricular systole and recoil during diastole- maintains blood flow
• The expansion takes some of the pressure off the blood so that the smaller arteries downstream are subjected to less systolic stress.
• Their recoil between heartbeats prevents the blood pressure from dropping too low while the heart is relaxing and refilling
• Expansion stores potential energy and their recoil releases it to keep the blood flowing (reduces fluctuation in BP)
Examples: aorta, pulmonary trunk, common carotid, subclavian, common iliac

Front 9

Distributing Arteries

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thickest muscular arteries- medium sized and typically have up to 40 layers of smooth muscle

Functions:
• Deliver blood to specific organs and adjust flow based on demand. Becomes more active during vasoconstriction/vasodilation- shunts blood from one place to another
Examples: brachial, femoral, renal, and splenic arteries

Front 10

Resistance Arteries

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small arteries that exhibit up to 25 layers of smooth muscle (varies) and little elastic tissue. Compared to large arteries, they have a thicker tunica media in proportion to the lumen

• Arterioles: smallest resistance vessels that lead to the capillary bed and are the major point of control over how much blood an organ or tissue receives.
• Metarterioles: short vessels that link arterioles to capillaries and provide shortcuts in which blood can bypass the capillaries and flow directly to a venule

Front 11

Functions of Resistance Arteries

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• Primary control of blood flow

• Most responsible for peripheral resistance

• Significantly affects BP

Front 12

Capillaries and Functions

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smallest blood vessels that are sometimes considered the “exchange vessels” of the cardiovascular system. They consist of a single layer of endothelial cells and a small lumen (just large enough for a red blood cell)

Functions:
• Exchange of materials: gas, hormones, nutrients, wastes
• Any cell in the body is only 60-80 micrometers away from the nearest capillary (scarce in tendons/ligaments, cartilage) - Close proximity to cells

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Types of Capillaries: Continuous

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Present in most tissues and their endothelial cells are held together by tight junctions to form a continuous tube. The basal lamina surrounds the endothelium and separates it from the adjacent connective tissue.

Intercellular Clefts: separates the endothelial cells, which allow small solutes such as glucose to pass through these clefts. Most plasma protein and other large molecules are held back, as well as formed elements (platelets and blood cells).

Front 14

Continuous Capillaries: Pericytes

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continuous capillaries may exhibit these cells that lie external to the endothelium. They contain the same contractile proteins as muscle and it is thought that they can contract and regulate blood flow through the capillaries. They also differentiate into endothelial and SM cells, contributing to vessel growth/repair

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Types of Capillaries: Fenestrated

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have endothelial cells with filtration pores (fenestrations): allow for greater permeability, rapid passage of small molecules, but still do not allow formed elements or most proteins through- most important in organs that engage in rapid absorption or filtration
• Examples: Kidneys, endocrine glands, small intestine, choroid plexus

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Types of Capillaries: Sinusoids

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Also called Discontinuous capillaries- irregular, blood filled spaces that conform to the shape of the surrounding tissue. The endothelial cells are separated by wide gaps and have especially large fenestrations through them. Proteins and formed elements can pass through.
• Examples: liver, bone marrow, spleen

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Capillary Beds

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network of capillaries (typically 10-100 capillaries) that exchange materials (materials must pass through the capillary beds supplied by a single arteriole or metarteriole).

• Capillary flow is usually regulated by the dilation or constriction of arterioles upstream from the capillary beds

• Not all are perfused

Front 18

Thoroughfare Channel and Precapillary Sphincters

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Thoroughfare channel: capillaries drain into a distal continuation of the metarteriole, which leads to a venule

Precapillary Sphincter: in capillary beds supplied with metarterioles, there is often a single smooth muscle cell that wraps like a cuff around the opening of each capillary- this regulates blood flow

• If the sphincters are relaxed, the capillaries are well perfused, but if many of the sphincters constrict, blood bypasses the capillaries leaving them less perfused or bloodless- blood takes shortcut through metarteriole and thoroughfare channel directly to nearby venule
• During exercise, capillary sphincters are open (mostly to skeletal muscles)

Front 19

Venules

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Thin walled and flaccid and expand easily to accommodate an increased volume of blood- they have a greater capacity for blood containment than arteries. They are subjected to low blood pressure and blood flow is steady rather than pulsating like the arteries.

Types of veins: Postcapillary venules, muscular venules, medium veins

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Postcapillary and Muscular Venules

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1. Postcapillary venules: smallest of the veins, which receive blood from capillaries directly or by way of the distal ends of the thoroughfare channels
• has a tunica intima with few fibroblasts and no muscle
• they are surrounded by pericytes
• more porous than capillaries causing an exchange of fluid with the surrounding tissues
2. Muscular Venules: has a thin tunica media of one or two layers of smooth muscle and a thin tunica externa
• Receives blood from the postcapillary venules

Front 21

Medium Veins

Back 21

has all three tunics: has a thin tunica media with bundles of smooth muscle, a thick tunica externa, and a tunica intima with an endothelium and the formation of venous valves

• Blood reservoirs: large percentage of total blood supply
• Thin wall vessel: causes BP to be lower and the veins to collapse when it’s empty

Front 22

Venous Valves

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infoldings directed towards the heart of the tunica intima that meet in the middle of the lumen
• The pressure in the veins is not high enough to push all the blood upward against the pull of gravity in a standing or sitting person- the upward flow of blood in vessels depends partly on the massaging action of skeletal muscles and the ability of these valves to keep the blood from dropping down again when the muscles relax. (skeletal muscle pump)
• Abundant where upward flow is opposed by gravity
• Not present in: small or large veins, brain, abdominal or thoracic cavities

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Large Veins

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has some SM in all three tunics: very thin tunica intima, thick tunica externa with bundles of smooth muscle, and thin tunica media

• No veins
• Examples: venae cavae, pulmonary veins, internal jugular veins, and renal veins

Front 24

Circulatory Routes: Simple Pathway and Portal System

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Simple Pathway: simplest and most common flow (one capillary bed): Heart→ arteries→ capillaries→ veins→ heart – blood usually only passes through one network of capillaries from time it leaves the heart until the time it returns
Portal System: blood flows through two consecutive capillary networks before returning to the heart. Examples: kidneys, connecting the hypothalamus to the anterior pituitary gland

Front 25

Circulatory Routes: Ateriovenous Anastomosis (Shunt)

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blood flows from an artery directly into a vein and bypasses the capillaries- shunts occur in the fingers, palms, toes, and ears where they reduce heat loss in cold weather- also makes them more susceptible to frostbite
• Anastomosis: a point of convergence between two blood vessels other than capillaries

Front 26

Circulatory Routes: Venous and Arterial Anastomoses

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Venous Anastomoses: one vein empties directly into another, providing several alternative routes of drainage from an organ (blockage of a vein is rarely as life threatening as blockage of an artery)

Arterial Anastomoses: two arteries merge, providing collateral (alternative) routes of blood supply to a tissue

Front 27

Flow and Perfusion

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• Flow: volume of blood flowing through an organ, tissue, or blood vessel in a given time (mL/min)

• Perfusion: flow per given volume or mass of tissue (mL/min/g)

• Hemodynamics: the study of the physical principles of blood flow, which are based mainly on pressure and resistance

Formula: F= ∆P/R where F=blood flow ∆P=difference in pressure and R=resistance to flow

Front 28

Example of Flow/Perfusion

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a large organ such as the femur could have a greater flow but less perfusion than a small organ such as the ovary because the ovary receives much more blood per gram of tissue

• In a resting individual, flow is constant and equal to cardiac output, but flow through individual organs varies from minute to minute as blood is redirected from one organ to another.

Front 29

Blood Pressure and Influences

Back 29

Blood Pressure: the force that the blood exerts against a vessel wall (mmHg)
• Varies throughout the system
• Highest pressure is immediately after systole (contraction) and proximal to the heart
• Pressure is highest in the arteries→capillaries→ and lowest in the veins
Influences:
• Elasticity of the arteries- recoil maintains pressure and facilitates flow
• Volume of blood forced
• Amount of kinetic energy of blood stretching the aorta

Front 30

Measures of Arterial Blood Pressure and Pulse Pressure

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• Systolic: the peak of arterial BP attained during ventricular contraction
• Diastolic: the minimum arterial BP occurring during the ventricular relaxation between heartbeats
o For a healthy person age 20-30, the normal pressure is around 120/80 mmHg (systolic/diastolic)

• Pulse Pressure (PP): the difference between systolic and diastolic pressure- this is a measure of pressure surges generated by the heart

o Equation: Systolic BP-Diastolic BP: 120-80=40 mmHg

Front 31

Mean Arterial Pressure (MAP)

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the average pressures you would obtain if you took measurements at several intervals throughout the cardiac cycle- because diastolic lasts longer than systole, a close estimate of MAP is obtained by adding diastolic pressure and one-third of the pulse pressure
o Equation: Diastolic+1/3 (PP): 80+40/3=93 mmHg
o It is the mean arterial pressure that most influences the risk of disorders such as syncope (fainting), atheroschlerosis, kidney failure, edema, and aneurysm

Front 32

Determinants of Blood Pressure

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determined by three principle variables:
1. Cardiac Output
2. Blood volume (regulated mostly by the kidneys, which have a greater influence than any other organ on BP)
3. Resistance to flow

• Also depends on coordinated function from: the brain: which in turn, controls the heart, kidneys, and blood vessels

Front 33

Peripheral Resistance

Back 33

Measure of friction that the blood encounters in vessels away from the heart (opposition of flow).
• Pressure and resistance are not independent variables in blood flow (both affect each other)
• Peripheral resistance is generally from smaller vessels and arterioles

Hinges on three variables: Blood viscosity, Blood vessel length, and blood vessel diameter

Front 34

Blood Viscosity

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thickness of the blood- this results from the erythrocyte (RBC) count and albumin concentration
• Directly proportional to resistance and usually remains constant
• A deficiency of RBC or albumin reduces the viscosity and speeds up blood flow and an increase in viscosity causes a decline in flow (inversely proportional to flow)

Front 35

Blood Vessel Length

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Distance the blood must travel: the farther a liquid travels through a tube, the more cumulative friction it encounters, which causes pressure and flow to decline with distance.

• Directly proportional to resistance: longer vessel=greater resistance
• Inversely proportional to flow: longer vessel=lower flow
• Vessel lengths and blood viscosity do not change in the short term and the only significant ways of controlling peripheral resistance from moment to moment is through vasoconstriction and vasodilation

Front 36

Blood Vessel Diameter

Back 36

The diameter changes in a blood vessel through vasoconstriction: the narrowing of a vessel which occurs when the SM of the tunica media contracts and vasodilation: the widening of a vessel occurs with muscle passivity- relaxation of the smooth muscle, allowing blood pressure to expand the vessel. Refer to vasoconstriction and vasodilation as vasoreflexes

• Vasoreflexes are constantly altering peripheral resistance
• The effect of vessel radius on blood flow stems from the friction of the moving blood against the vessel walls.

Front 37

Laminar Flow

Back 37

normal blood flow- smooth, silent: flows in layers. Faster near the center of a vessel, where it encounters less friction, and slower near the walls, where it drags against the vessel

• When a blood vessel dilates (radius larger), a greater portion of the blood is in the middle of the stream, causing a swift average flow.
• When the vessel constricts (less radius), more of the blood is close to the wall and the average flow is slower

Front 38

Regulation of Pressure and Flow- Local Control

Back 38

Autoregulation is the ability of tissues to regulate their own blood supply (flow).
Myogenic control: how the arteries react to an increase or decrease of BP to keep the flow within the blood vessel constant
Metabolic Control: If a tissue is inadequately perfused, it becomes hypoxic and its metabolites (waste products) accumulate (CO2, H, K, lactic acid). These factors stimulate vasodilation, which increases blood flow.
• As the bloodstream delivers oxygen and carries away the metabolites, the vessels reconstrict, causing a homeostatic dynamic equilibrium that adjusts perfusion to the tissue’s metabolic needs

Front 39

Local Control: Vasoactive Chemicals and Reactive Hyperemia

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Vasoactive chemicals: platelets, endothelial cells, and perivascular tissues secrete these chemicals to stimulate vasodilaton under conditions of trauma, inflammation and exercise. They include: histamine, bradykinin, and prostaglandins (vasodilators)
Reactive hyperemia: if a tissue’s blood supply is cut off for a time and then restored- it is an increase above the normal level of flow. Ex. flushed skin after a person comes in from the cold

Front 40

Local Control: Angiogenesis

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a hypoxic tissue can increase its own perfusion- the growth of new blood vessels (also refers to the embryonic development of blood vessels). Several growth factors and inhibitors control angiogenesis
• Important for: regrowth of uterine lining after menstrual period, development of blood capillaries in muscles of athlete, and growth of arterial bypasses around obstructions in the coronary circulation

Front 41

Regulation of Pressure and Flow: Neural Control

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blood vessels are under remote control by the CNS and ANS- the vasomotor center of the medulla oblongata exerts sympathetic control and sends impulses to the smooth muscle.
• Sympathetic nerve fibers stimulate most blood vessels to constrict, but they dilate the skeletal and cardiac vessels to meet the metabolic demands of exercise on the heart
The vasomotor center is an integrating center for three autonomic reflexes: baroreflexes, chemoreflexes, and medullary ischemic reflex

Front 42

Baroreflexes

Back 42

An autonomic, negative feedback response to changes in blood pressure. The changes are detected by baroreceptors of the carotid sinuses. Nerve fibers from these sinuses transmit signals continually to the brainstem.

• When blood pressure rises, signaling rates rise, inhibiting the sympathetic cardiac center and vasomotor center. It excites the vagal fibers to the heart. Thus, it reduces the heart rate and cardiac output, dilates the vessels, and reduces blood pressure
• Barorelexes are important for short-term regulation, not for chronic hypertension

Front 43

Chemoreflexes

Back 43

Autonomic response to chemical changes. It is initiated by chemoreceptors (called aortic bodies) which focuses mostly on a decrease in pH, a decrease in oxygen, and an increase in carbon dioxide.
• Primary role: to adjust respiration to changes in blood chemistry
• Secondary role: vasomotor- hypoxemia (blood oxygen deficiency), hypercapnia (excess CO2) and acidosis (low blood pH) stimulate chemoreceptors and act through the vasomotor center to induce widespread vasoconstriction- this increases BP, which increases perfusion of the lungs and the rate of gas exchange

Front 44

Medullary Ischemic Reflex

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an autonomic response to drop in brain perfusion- the medulla oblongata monitors its own blood supply and activates corrective reflexes when it senses a state of ischemia (insufficient perfusion)
• Within seconds of a drop, the cardiac and vasomotor centers of the medulla send sympathetic signals to the heart and blood vessels that accelerate the heart and constrict the vessels (increases heart rate and contraction)
• Causes widespread vasoconstriction, which raises the BP and restores normal cerebral perfusion
• The cardiac and vasomotor centers also receive input from other brain centers- stress, anger, arousal, and exercise can raise BP

Front 45

Regulation of Pressure and Flow: Chemical/Hormonal Control- Angiotension II, Aldosterone, Atrial Natriuretic Peptide

Back 45

hormones act on vascular smooth muscle and influence blood pressure:

• Angiotensin II: vasoconstrictor that raises BP
• Aldosterone: “salt retaining hormone” that promotes sodium retention by the kidneys- water follows sodium osmotically, so sodium retention promotes water retention and supports BP
• Atrial natriuretic peptide: antagonize aldosterone, increasing sodium excretion by the kidneys, thus reducing blood volume and pressure- also has vasodilator effect to lower BP

Front 46

Chemical/Hormonal Control- Antidiuretic, Epinephrine, Norepinephrine

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• Antidiuretic hormone: promotes water retention- raises blood pressure
• Epinephrine and norepinephrine: these catecholamines bind to receptors of SM of most blood vessels and stimulate vasoconstriction and raises the blood pressure- but in the coronary and skeletal muscle blood vessels they vasodilate during exercise (increase of blood flow to the myocardium)

Front 47

Two Purposes For Vasoreflexes

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1. a generalized raising/lowering of BP throughout the body: requires centralized control- action on the part of the medullary vasomotor center or by hormones that circulate through the system- Vasoconstriction raises BP and vasodilation lowers BP

2. selectively modifying the perfusion of a particular organ and rerouting blood from one region of the body to another: can be achieved by localized vasoconstriction of an artery

Front 48

Redirection of Blood Flow

Back 48

If an artery constricts, pressure downsteam from the constriction drops and pressure upstream from it rises- if blood can travel by either of two routes and one route puts up more resistance than the other, the blood follows the path of least resistance
• this mechanism enables the body to redirect blood from one organ to another (meet the changing metabolic priorities of the body) Ex. during periods of exercise, reduce the flow to the kidneys and digestive tract

*look at pie chart

Front 49

Capillary Exchange

Back 49

two-way movement of materials and fluids-
Delivery: follows the stages of Blood→ capillary→ interstitial space→ to the perivascular tissues Ex. Oxygen, glucose, AA, lipids, minerals and hormones
• The velocity of blood slows into the capillary bed because there is an increase in the cross sectional area of the bed (capillaries are much larger in number than venules), which increases the time for exchange
Removal: follows the stages of perivascular tissue→ interstitial space→ capillary→ blood – this occurs at the distal end of the capillary
Ex. CO2, ammonia, and other wastes

Front 50

Modes of Capillary Exchange- Diffusion

Back 50

The most important mechanism of exchange
• Simple diffusion: requires a concentration gradient- process where a substance passes through a membrane without the aid of a membrane protein
• Lipid soluble substances (steroid hormones) diffuse easily through the plasma membranes
• Insoluble lipids (glucose/electrolytes) must pass through membrane channels, fenestrations (pores in the capillary cell), or intercellular clefts (gaps in tight junctions between cells)
• Large molecules such as proteins are usually held back
• Glucose and oxygen, which are more concentrated in the systemic blood than in the tissue fluid, diffuse out of the blood, whereas CO2 and wastes are more concentrated inside the tissue fluid and diffuse into the blood

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Modes of Capillary Exchange- Vesicular Transport

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• Trancytosis: process in which endothelial cells (vesicles) pick up fluid on one side of the PM by pinocytosis: small particles are brought into the cell, forming an invagination and then are suspended within small vesicles- they are then transported across the cell and discharged on the other side by exocytosis.
• This accounts for only a small fraction of solute exchange across the capillary wall- fatty acids, albumin, and some hormones (insulin) move across the endothelium by this mechanism

Front 52

Modes of Capillary Exchange: Bulk flow

Back 52

The balance of filtration and osmotic forces- fluid typically filters out of the arterial end of a capillary and reabsorption occurs at the venous end of the capillary. This fluid delivers materials to the cells and removes their metabolic wastes (fluid from interstitial space to capillary). This comes about as the result of a shifting balance between hydrostatic and osmotic forces.

Front 53

Influences On Filtration

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• Filtration is depends on forces that keep plasma in the capillary blood vessels (Blood osmotic forces) and forces that push plasma out of the capillary blood vessel (Blood hydrostatic forces)

• Forces outside of the capillary also influence filtration. Interstitial hydrostatic forces oppose capillary filtration, while interstitial osmotic forces enhance capillary filtration

Front 54

Blood and Interstitial Hydrostatic Pressure (Proximal)

Back 54

• Blood hydrostatic pressure: The hydrostatic force which is the mechanical pressure exerted on the fluid of plasma by the pumping of the heart during systole which tends to push water from the capillaries into the interstitial fluid

• Interstitial fluid hydrostatic pressure: The hydrostatic force which is the mechanical pressure exerted on the interstitial fluid which tends to push water from the interstitial fluid back into the capillaries

Front 55

Blood and Interstitial Colloid Osmotic Pressure (Distal)

Back 55

Colloid Osmotic Pressure: portion of the osmotic pressure due to protein

• Blood colloid osmotic pressure: The osmotic force (water concentration gradient) which is the result of differences in water concentration between plasma and interstitial fluid, which tends to pull water from the interstitial fluid and back into the plasma in the capillaries
• Interstitial fluid osmotic pressure: The osmotic force (water concentration gradient) which is the result of differences in water concentration between plasma and interstitial fluid, which tends to pull water from the plasma in the capillaries into the interstitial fluid

Front 56

Net Filtration Pressure

Back 56

The dynamic equilibrium force which may be measured at any point along the capillaries from the arterial to the venous end

NFP= Net Hydrostatic Pressure-Net Colloid Osmotic Pressure

Front 57

Mechanisms of Venous Return- Pressure Gradient

Back 57

Venous Return: the flow of blood back to the heart

Pressure gradient: Pressure generated by the heart, pressure in the venules, and pressure at the point where the venae cavae enter the heart: central venous pressure: venous pressure gradient favors the flow of blood toward the heart
• The pressure gradient and venous return increase when blood volume increases and in the event of widespread vasoconstriction because this reduces the volume of the circulatory system and raises blood pressure and flow

Front 58

Mechanisms of Venous Return- Gravity and Skeletal Muscle Pump

Back 58

Gravity: When you are sitting/standing, blood from the head and neck returns to the heart simply by flowing “downhill” through the large veins above the heart
• The large veins of the neck are nearly collapsed and venous pressure is close to zero. The dural sinuses of the brain cannot collapse and their pressure is as low as -10 mmHg

The skeletal muscle pump: in the limbs, the veins are surrounded and massaged by the muscles- contracting muscles squeeze or “milk” the veins and the valves ensure that this blood can go only toward the heart

Front 59

Mechanisms of Venous Return- Respiratory Pump and Cardiac Suction

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Thoracic respiratory pump: When you inhale, your thoracic cavity expands and its internal pressure drops, while downward movement of the diaphragm raises the pressure in your abdominal cavity- the blood is squeezed upward toward the heart

Cardiac suction: During ventricular systole, the tendinous cords pull the AV valve cusps downward, slightly expanding the atrial space. This creases a slight suction that draws blood into the atria from the vena cavae and pulmonary veins.