Blood Vessel Types and Structure

• Blood vessels create a large interconnected loop in the body.
• There are five main types of blood vessels: arteries, arterioles, capillaries, venules, and veins.

Arteries and Arterioles

• Arteries carry blood away from the heart.
• Large elastic arteries carry blood away from the heart and branch into muscular medium-sized arteries.
• Small arteries branch into arterioles.

Capillaries

• Arterioles branch into tissues and become blood capillaries.
• These are very small in diameter, facilitating exchange between blood and tissues through their thin walls.

Veins and Venules

• Capillaries merge into small veins called venules.
• Venules merge into larger vessels called veins.
• Veins return blood to the heart.

Blood Vessel Structure

• Blood vessels have layers called tunics.
• The tunica interna is an endothelial lining that is in direct contact with the blood
• The tunica media is an intermediate layer of smooth muscle + CT
• The tunica externa is a surrounding connective tissue (CT) layer.

Tunica Interna

• The thin endothelial lining is in direct contact with blood, which facilitates exchange by diffusion and provides a smooth surface for blood flow.
• The endothelium secretes nitric oxide (NO) and other vascular regulators.
• A basement membrane anchors the endothelium to underlying tissues, providing tensile strength and acting as a substratum for cell migration during wound healing.
• Superficial to the basement membrane is a thin sheet of elastic fibers that maintains extensibility of blood vessels.
• This layer contains large pores through which large molecules can diffuse into or out of the blood.

Tunica Media

• This layer varies between vessel types.
• It is a thick layer of smooth muscle and elastic fibers that mediate vasoconstriction (decrease in lumen diameter) & vasodilation (increase in lumen diameter).

Tunica Externa

• The Tunica Externa is mostly made of collagen and elastic fibers with many nerves
• Large blood vessels serviced by many tiny vessels are called the vasa vasorum
• An example location for the vasa vasorum is outside the aorta

The Structure of Blood Vessels

• Large elastic arteries carry blood from the heart and branch into muscular arteries
• Muscular arteries branch into arterioles, which become capillaries at tissues
• Capillaries merge into venules as they exit tissues, then merge into veins.
• Veins carry blood back to the heart
• Arteries and veins have all three tunics, while capillaries possess only the tunica interna and basement membrane.

Types of Arteries

• Elastic arteries are enriched with elastic fibers, have a thin wall of smooth muscle, and are the largest in the body.
• They push blood from the heart during ventricular diastole, with blood stretching the elastic fibers in the tunica interna and media to create a pressure reservoir and routing blood from the heart.
• Muscular Arteries have a thicker layer of smooth muscle than elastic arteries.
• They have a looser tunica externa because stretching is permitted during vasoconstriction or vasodilation.
• Muscular arteries are also known as distributing arteries because they move blood into multiple small arterioles.

Anastomoses

• Anastomoses are points where vessels of related functions join one another.
• They provide bypasses for blood during collateral circulation, movement compresses an artery, and restricts blood flow. -Anastomoses can occur between arteries, veins, arterioles, and venules.
• Arteries without anastomoses are called end arteries, and a cessation of circulation through these arteries can lead to tissue death or necrosis.

Arterioles

• Arterioles are microscopic arteries with a diameter of 15–300 μm.
• The vessel wall thickness is 50% of the vessel diameter.
• They regulate blood flow into capillaries
• Arterioles feature a pours elastic lamina in tunica interna
• They contain a tunica media with 1-2 layers of ring-shaped smooth muscles
• They have narrowed sections called metarterioles that enter into capillary beds with distal muscle to form a precapillary sphincter
• Nerves in tunica externa of metarterioles
• They are critical to arteriole funtion so blood flow can be regulated

Arteriole Function

• Arterioles modulate resistance
• Blood friction experiences as it flows over blood wall
• If vasoconstriction increases, blood contact with walks increases, so friction will increase, and friction increases resistance.

Capillaries

• Capillaries are the smallest blood vessels (5–10 μm diameter).
• Extensive branching networks ensure all body cells have access to nutrients, signals, and a way to eliminate wastes.
• Organs with high metabolic function have higher capillary density (e.g. brain > tendons)
• Primary function = exchanging with interstitial fluid
• They lack both tunica externa and tunica media.
• They only contain endothelium + basement membrane allowing diffusion across a single cell layer
• Many branches increase surface area
• Erythrocytes (8 um diameter) are able to travel through capillaires

Capillary Beds

• Branching from metarterioles will form 10-100 capillaries
• They form two routes for blood to flow
• One route is through capillaries (enters and exits post capillary venules (microcirculation))
• There is a rate in precapillary sphincter contraction, 5-10 times per minute, called vasomotion, and regulated by hormones (e.g. NO)
• Only 25% of blood flows through capilaries at a given time so another route occurs
• Another route is through a thoroughfare channel, which permits direct flow through precapillary sphincters to the venules

Metarteriole Question

• Metarteriole are a sphincter that control blood flow into/out of capillary beds

Types of Capillaries

• Continuous capillaries have a smooth and continuous endothelium, interrupted only by intercellular clefts between adjacent endothelial cells.
• This type is found in the organs of the central nervous system (CNS), lungs, muscle, and skin.
• Fenestrated capillaries have endothelial cells with "windows" or pores called fenestrations, which permit the diffusion of proteins and other molecules and are found in kidneys, small intestinal villi, eyes, and endocrine glands.
• Sinusoids are wider and "twistier” than other types, lack or have little basement membrane
• Sinusoids have large fenestrations and intercellular clefts, where reticulocytes enter circulation from red bone marrow.
• These capillaries line organs like the liver, spleen, anterior pituitary, parathyroid, and adrenal glands.

Venules and Veins

• The following structures have thin walls that lose shape easily and cannot withstand the same pressures as arteries or arterioles.
• Muscular venules are distensible and accumulate large blood volumes (record = 360% volume!)
• Veins change shape as they merge into larger vessels but all are thin-walled
• Walls are 10% of vessel diameter
• Diameter ranges from 0.5 µm to 3 cm
• They contain valves to prevent backflow of blood within a thick tunica externa contributing to distensibility with large lumens that appear collapsed
• The systemic veins and venules act as blood reservoirs and blood volume can be redirected
• Valves in veins prevent backflow where the point is to maintain blood flow

Capilary Exchange

• The heart pumps blood so that the blood can exchange materials with the tissues through capilaries
• Capillary exchange facilitates the movement of substances between blood and interstitial fluid
• This occurs through 3 mechanisms:

3 Mechanisms for Capillary Exchange

• Diffusion occurs through fenestrations, intercellular clefts, or endothelial cell membranes.
• Examples include gases, solutes, amino acids, glucose, and other hormones. -Capillaires that permit passage of proteins like fibrinogen are called sinusoids
• most capillaries in the brain are continuous capillaries with tight intercellular junctions creating the blood-brain barrier and an end result of high selective permeability
• Transcytosis is a relatively rare method of exchange used in pinocytosis to bulk uptake materials
• For examples, pinocytic vesicles containing large molecules can cross this way for instances such as Insulin entering the bloodstream or maternal antibodies crossing the placents into to fetal blood
• Bulk flow is the collective movement of large volumes that regulates the relative volumes of blood and interstitial fluid
• Bulk flow moves from high to low pressures

Fluid Pressures

• Movement of fluid through blood/interstitial is regulated by fluid pressure
• This movement is driven by net filtration pressure NFP
• Where NFP determines relative blood and interstitial fluid volumes change
• Infiltration is the movement from blood into interstitial fluid
• Reabsorption is the movement from interstitial fluid into blood
• The volume of reabsorbed fluid should equal the volume of filtered fluid
• Filtration consits of a pressure that pushes

Other Pressures

Blood hydrostatic Pressure (pushing)

• BCOP is the pressure of water pushing on the endothelium The pressure increases if forcefully ejected from the heart’s ventricles It is higher at the arterial end then venous end of capillaires and contributes to filtration IFHP is fluid resisting (suction)
• Interstitial fluid osmotic pressure (IFOP) is very weak that equals to 1mmHg, that will have constant pressure the entre way and only will contribute to reabsorption

Filtration Review

Blood hyrdostatic pressure exists, intersitial hyrdostatic pressure, blood colliod osmotic pressure, intersitial fluid osmotic pressure, and the net filtration process

The Fate of Filtered Fluid

• 85% of fluid in capillaires is reabsorbed
• The remaining 15$ is taken up by the nymph vessals, returning to the circulation via their subclavian veins

Effects of Build Up

• Excess filtered fluid builds up which triggers edema which is an increase in the volume of interstitial fluid due to the lyphatic system failing to filter properly
• This couls happen with protein deficiency

Summary of Filtration and Blood flow

Substances flow at pressure gradients NPF is always calculated for Blood - Osmotiuc forces

Sample Questions:

• Which way does absorption primarily follow? - BCOP

Hemodynamics

• Hemodynamics is the study of forces that affect blood flow in the body.
• Blood flow is the volume of blood flowing through a tissue at a particular time (mL/min)
• total blood flow equals to cardiac output The rate of blood flow is called perfusion: blood pressure, vascular resistance modulate the blood flow

Blood Pressure

BP exerts pressure that blood exerts on blood vessel walls and is generated through contraction of veintrcles and is hihgest in the aorta Sysolic is highest blood pressure in arteries during cardiac systole, diastolic is lowest in the arteries during diastile Blood flos from High to low pressures, but small effect on blood volume

Vascular Resistance

• Is what poses the forces of frition
• Resistance is the force that acts on friction that stops the flow of blood Vascular resistance changes and regulates:
• Diameter of vessel lumen: vasoconstriciton increases resistance
• Viscosity: thickening blood increases resistance of friction, polynthemia
• Total vessel length: long vessels increases friction due to the surface area

Factors Affecting

• Total pressure depends Blood is determined by pressure going out and returning The most venous of return is muscle pumps, where valves prevent the backflow and maintain pressure

The Respiratory System:

• The diaphram separates the body cavities in the thoracic area -The diaphram creates inhalation, contration, increasses pressure -When the diaphram expands, it increases abdonimal pressure
• Durign exhalation: venal valves press down and prevent backflow of blood
• The body’s primary means of changing caridac output is through altering heart rate, which alters blood pressure.
  Increased or contracted through vasodilitation -The body’s response to high pressure: The vascular centre will signa

Autoregulation

• Vessel Autoregulate by means of physcial stretching If vessel will act as stretchy, if an activity occurs, leads for for bllod to go to that organ Vessles use vasodilators and vasoconstrictons to change the flow of blood

Vasodilation and vasoconstriciton

• Substances create increase / decrease through NO - Nitric oxide If the systemic blood volume is low, to keep the body alive and active, increases the vessels that dialate Pulmonary The undamanged alveoli, body wants vasodilation in those areas more then area not functioning well, low oxygen

Other Regulatory Systems

• To restore oxygen and nutrients, the body functions by:

1. stimulating the RAA pathway to increase the blood volume (Lowers ) 2)Realeasing ADH to increased blood volume and signals 3
2. increases the blood volume

Types of Hormones

4. Anti - Dialectic Hormone (lowers ADH) In a system running low, there is in the RAA

Homeostatic Imbalances

• Insufficient delivery of oxygen and nutrients means less metabolic reaction -Hypovolenimic = (sweating - Dehydration) -Cardio : damage to the heart - Blood cannot pump

More Types of Shocks

• Anaphylactic shock - production with histamine
• Neurogenic shock - Heaf Trauma
• Sepsis Shock
• Obstructive shock

All in all: vasocondialtors occur more than in a heatlhier state

Blood Velocity

• Refers to its speed is inversely proportional to total cross sectional diameter
• Vascular resistance strongly affects

-The cardiovascular centre has other functions as well that signal vasoconstriction or disolatino

Autoregulation

• Blood Vessles also autoregulate the flow

Hormones Regulation

• RAA (decreases unrination) Epe and Norepine : increases systolic calcium Adh: vasoconditions

Anp (vasodialtion)

Two Main Circulatory Routes

• Systemic
• includes conrnary (HEART TISSUE), cerebral (BRAIN), heartic potral (organs)
• Pulmonary
• Deoxited blood flow from LUNGS Arteries supply the head Number for head

Cerebral Arterial Circle

• A Number for anastomoses that ensure CONSTENTLY PERFUSED
• Anterior Arteries (frontal lobe) Basiller arteries make form cerbratal

If there is right side blockage but there is cerebral area The Brain - It WILL be perfused

Venues

• Venel and the arteries follow similar
  • There is internal jugular where veins come out in the head

Portal Circulation / Hepatic

A portal vein carries 2 capillary networks in blood, 30 SECONDS Nutrient passes by portal blood/portal circulation, more common than systemic