Homeostasis and Blood Flow Regulation

• Blood vessels and blood pressure are the main focus
• Blood is supplied to all parts of the body through a system of blood vessels
• Fresh supplies get delivered and wastes get removed by the blood
• Reconditioning organs get excess blood flow to maintain homeostasis
• Extra blood is adjusted to achieve homeostasis

Cardiac Output

• Lungs receive all blood pumped from the right side of the heart
• Systemic organs receive blood pumped from the left side of the heart
• The percentage of pumped blood by organs at rest can adjust easily, and on demand

Blood Flow

• The pressure gradient affects blood flow through a vessel
• The bigger the pressure gradient, the easier the blood flows
• Blood flow is also dependent on vascular resistance
• Vascular resistance hinders flow from friction between moving liquid and stationary vessel walls
• Vascular resistance is directly related to:
  • Viscosity of blood
  • Vessel length
  • Is inversely to vessel diameter

Pressure gradient

• Flow is related to the pressure gradient in a vessel
• As the difference in pressure between the two ends of a vessel increases, so does flow rate
• Flow rate depends on the difference in pressure between the two ends of a vessel
• The actual magnitude of the pressures at each end is not what matters

Flow Rate

• Volume of blood passing through per unit of time
• Directly proportional to the pressure gradient, the bigger the gradient, the greater the flow
• Inversely proportional to vascular resistance

Poiseuille's Law

• Integrates factors affecting flow rate through a vessel

Vessel Radius

• Smaller-radius vessels offer more resistance to blood flow
• Blood "rubs" against a larger surface area

Radius Size

• Doubling the radius decreases resistance to 1/16
• Flow increases 16 times because resistance is inversely proportional to the fourth power of the radius

Vascular Tree

• Consists of arteries, arterioles, capillaries, venules, and veins
• Arteries transport blood from heart to organs
• Arterioles control blood flow through each organ
• Capillaries are vessels where materials are exchanged between blood and tissue cells
• Veins return blood to the heart

Arteries as Passageways

• Arteries serve as rapid-transit passageways to the organs and as a pressure reservoir
• Heart contraction pumps blood into arteries, relaxation allows arteries to refill with blood from veins

Arterial Pressure

• Arterial blood pressure fluctuates in relation to ventricular systole and diastole
  • Systolic pressure averages 120 mm Hg
  • Diastolic pressure averages 80 mm Hg
  • Mean arterial pressure reminder is afterload

Elastic Arteries

• Elastic arteries distend during cardiac systole as more blood is ejected into them
• Drains off into the narrow, high resistance arterioles downstream
• Elastic recoil of arteries during cardiac diastole continues driving blood forward when the heart is not pumping

Arterial Pressure and Blood Flow

• Mean arterial pressure is the main driving force for blood flow
• Is the average pressure driving blood forward into the tissues throughout the cardiac cycle
• Number is closer to diastolic BP because of how the cardiac cycle spends more time there
• Essentially MAP is same in all arteries despite differences in location/size
• Blood pressure is measured in the arteries

Arterioles: Major Resistance Vessels

• Radius is small enough to offer resistance to flow
  • Creates gradient that drives blood flow
  • Removes the pulsatile pressure so pressure/blood flow is constant in capillaries
• Can alter response by also using less elastic connective tissue
  • Vasoconstriction: narrowing of a vessel (more resistance, less flow)
  • Vasodilation: enlargement in circumference and radius of a vessel (less resistance, more flow)

Vascular Tone

• State of partial constriction of arteriolar smooth muscle
• Establishes a baseline of arteriolar resistance

Local Control of Arteriole Radius

• Determines distribution (percentage to each tissue) of cardiac output
• Fraction total of the total CO delivered to each organ depends on demands for blood
  • Alters blood flow by adjusting diameter/resistance, through chemical or physical stimuli
• Changes in flow to organs are based on vascularization differences and differences in resistance offered by arterioles
  • Can divert some blood to areas of higher demand
• Changing flow without stopping it
  • Accompanied by an increase in CO

Metabolic Influence

• Local metabolic influences on arteriolar radius help match blood flow with organ needs
• Local chemical changes that increase blood flow:
• Active hyperemia: increased blood flow due to enhanced tissue activity
• SkM contraction changes activity and need for blood
• Local metabolic changes that influence arteriolar radius:
  • Decreased O2
  • Adenosine release (from lower O2 or higher metabolic activity)
  • Increases in CO2
  • Acid
  • K+ (potassium)
  • Osmolarity

Reactive Hyperemia

• Endothelial-derived vasoactive paracrines: Endothelin causes arteriolar smooth muscle contraction
• Changes don't directly alter constrictive state
  • Sensed at endothelial level and paracrine activity alters response
• Angiogenesis: vascular endothelial growth factor (VEGF)
  • Stimulates new vessel growth from chronic changes in blood flow
    • Long term/adaptation, not acute inducer of vasodilation
• Post-occlusion increase in blood flow: Same metabolic state responding to reduced flow restore chemical composition
  • Examples: tourniquet, unclamping during surgery, removal of blockage in coronary artery

Histamine

• Local histamine release pathologically dilates arterioles
• Is released after injuries or during allergic reactions,
• Histamines acts a paracrine in the damaged region,
• Not a player with normal control of blood flow
• Myogenic response of arterioles to stretch helps tissues autoregulate their blood flow
• Local mechanisms keep tissue blood flow fairly constant despite rather wide deviations in pressure
• Vasodilation maintains blood flow despite systemic drops in BP/MAP

Vasodilating Nitric Oxide

• Arterioles release vasodilating NO in response to an increase in shear stress
  • This is the most studied and responsive paracrine
  • NO works by reducing phosphorylation of myosin

Temperature Influence

• Temperature change can alter arteriole reactions
  • Local heat dilates arterioles
  • Cold constricts arterioles

Extrinsic Control of Arteriolar Radius

• Helps with regulating blood pressure
• Affects total peripheral resistance on MAP
• Sympathetic induced vasoconstriction reduces blood flow down stream while increasing upstream MAP
  • Norepinephrine is released by nerves
• Constant constriction/tone maintains constant blood flow
• Local controls override sympathetic vasoconstriction
  • Overcomes vascular tone through metabolically induced vasodilation
• No parasympathetic innervation to arterioles
  • Accomplishes vasodilation by reducing constriction below normal level of vascular tone

Hormone Regulation

• CvCC and hormones regulate blood pressure
  • Norephinephrine produces vasoconstriction
  • Epinephrine reinforces local vasodilatory mechanisms in tissues
• Achieves this because it has a specific factor that are mostly on cardiac and skeletal muscle
  • Contrast to digestive/kidney arterioles which only have alpha 1
  • Vasopressin and angiotensin II are powerful vasoconstrictors, responsive on sudden drops
    • Vasopressin maintains water balance
    • Angiotensin II regulates salt balance

Arteriolar Radius Categories

• The adjustable arteriolar radius is the determinant of TPR
• Two factors can alter arteriolar radius:
  • Local (instrinsic) control
    • Important in matching blood flow with tissue's metabolic needs
    • Mediated by local acting factors on the arteriolar smooth muscle
  • Extrinsic control Mediated by sympathetic influence on arteriolar smooth muscle
    • Important in regulating blood pressure

Capillaries

• Capillaries are suited to serve as sites of exchange

Fick's Law of Diffusion

• Factors enhance diffusion across capillaries because diffusing molecules have a short distance to travel
• Very thin, 1/100th thickness of a hair
• Each capillary is narrow, so red blood cells squeeze through for transport
• Almost every cell is within .1mm of a capillary
• Slow velocity of flow through capillaries
  • Facilitates volume of blood per unit of time

Slow Capillary Velocity

• Slowest in the capillaries, which have the largest total cross sectional area
• Water-filled capillary pores permit passage of small, and water-soluble substances
  • Pores: narrow, water-filled clefts
• Many capillaries are not open under resting conditions
• Role of precapillary sphincters: wisps of spiraling smooth muscle cells
  • Act as stopcocks to control blood flow, which capillaries each one guards

Diffusion and Interstitial Fluid

• Interstitial fluid is a passive intermediary between blood and cells
• Cells bathed in ISF have an internal connection with cells through this fluid, with 80% consistency
• Important is solute exchange across capillary walls
• Exchange is determined by each gradient between blood and surrounding cells
  • Exercise is a good example. Use of oxygen increases thus lowering level in the cell thus permitting a larger gradient for diffusion. Same for CO2

Bulk Flow

• Important in that water and solutes are moved as a unit or in bulk
• Driven by pressure, happens through openings in the tiny vessels
• Causes ultrafiltration
• Direction depends on location within capillary network
  • Arterial side = filtration (fluid leaves capillaries enters tissue)
  • Venous side = reabsorption (fluid from cells back into tissue)
• Results in a protein-free plasma

Lymphatic System

• An way for return of interstitial fluid to the blood, because more fluid gets filtered rather than reabsorbed
• Pickup and flow of lymph, the ISF enters a lymphatic vessel
  • Lymph openings are larger, so larger materials can travel through the pores in capillaries to the lymph

Functions of Lymphatic System

• Return fluid
• Return of excess filtered fluid
• Defense
• Defense against disease (bacteria destroyed by phagocytes)
• Transport and Return
  • Transport of absorbed fat (size is too large for capillaries)
  • Return of filtered protein

Lymph System and Rate

• Lymph flows into venous system near the right ventricle
  • Lymph averages 3 liters per day, with an average of 7200 liters of blood flow per day

Edema Issues

• Happens when accumulated fluid occurs, often due to:
  • Reduced plasma protein levels
  • Increased vessel permeability
  • Higher venous pressure
  • Lymphatic blockage

Veins

• Venules have chemical connections with neighboring arterioles
  • In order to have accurate inflow and outflow
  • Almost no restriction due to their tone
• Functions as blood reservoirs, and passage back to the heart
  • Stores blood in reserve when its unneeded due to their lack of rigidity
  • Are capacitant, because they hold lots of blood
• Transit is slow, though blood still moves, so more blood is here
• Also larger CSA and stretchy

Blood Volume

• Veins contain the majority of volume at 64%, compared to:
  • Systemic arteries 13%
  • Systemic capillaries 5%
  • Systemic veins 7%
  • Pulmonary 9%

Venous Enhancement

• Venous return gets enhancement from various external influences
  • Sympathetic activation on venous return
  • Skeletal pumps doing muscle things
    • Respiratory pumps also activate
  • Cardiac chambers helping suck blood

Arteriole Pressure

• Controlling that output, resistance, and overall volume controls arteriole pressure
• Pressure reasons:
  • Good enough to make sure of adequate force
  • So high, could stress the heart
• Average arteriole pressure influences:
  • Cardiac output
  • Total peripheral resistances
• Short and longterm actions:
  • Adjustments usually made by adjustments done to cardiac output in short periods
  • Restoration and balance done long term

Reflex Blood Actions

• Baroreceptors and reflexes are primary in pressure decisions
• Vessels and vital components for pressure help in maintaining pressure and maintaining proper blood flow
• Other responses:
  • Blood and volume reception, hypo reaction and pressure sensing, ETC, help in maintaining reactions

Hypertension Concerns

• Extremely common, though unknown the underlying cause primarily
  • Types of pressure issue
    • Usually just develops from an overall condition or from a series of negative actions
  • Baroreceptors influence arterial output
    • Causes problems with the heart with loss of life and major organ complications
    • Issues with circulation can also influence other outside actions which could increase risk
      • Low activity levels can increase blood clots or stress the heart
      • Straining can influence risks to the head and torso
• Can be the result of transient sympathetic influences

Circulatory Shock

• Negative outcome that can be potentially fatal
  • Pressure lowers to the point of inadequate blood flow and tissue oxygen deficiencies
• Conditions:
  • Overbearing loss of pressure and volume
  • Weaker heart with issues pumping correctly overall
  • Improper arteriole widening
  • Faults in contracting

Exercise

• Exercising changes different cardiovascular variables
  • Heart rate increases due to decreased parasympathetic activity
  • The skeletal pump increases venous return
  • Stroke volume is influenced by venous return
  • Blood flow influenced by vasodilation, which is stronger with increased sympathetic influence
• Brain activity is unaffected
• The hypothalamus influences vasodilation and blood flow
• Issues such as organ shutdown is caused by arterial constriction
• Skeletal reduction is the primary influence because of pressure
  • Heart receives less stress by lower pressure on the system due to these circumstances
• Cardiac gets less exercise due to high pressure that is on the system
  • Exercise greatly increases the use of muscle, lowering all other processes by comparison
• Skeletal increase for blood transport comes from the vessels overall

Blood flow percentage during exercise

• Digestive tract decreases by 56%
• Kidneys by 45%
• Skin, which increases by 370%
• Remains unchanged in the brain
• Increases by 367% in the heart
• Skeletal muscle, which increase the most, by 1066%
• Bone decreases by 30%

Explore the role of reconditioning organs in homeostasis and the factors affecting blood flow, such as blood viscosity, pressure gradients, and vasoconstriction. Understand how these elements interact to maintain circulatory balance. During exercise, blood flow distribution changes to support increased metabolic demands.