Chapter 17 Local Control of Blood Flow

Local Control of Blood Flow

• Tissues can manage their local blood flow based on metabolic needs
• Blood flow is adjusted based on the need for oxygen and nutrients like glucose, amino acids, and fatty acids
• Blood flow helps remove carbon dioxide and hydrogen ions
• It adjusts ion concentrations and transports hormones to varying tissues

Organ Specific Blood Flow Requirements

• Skin blood flow regulates heat loss aiding in body temperature control
• Kidneys need lots of blood plasma for waste filtration, fluid volume regulation, and electrolyte balance

Organs Blood Flow Volumes

• Thyroid and adrenal glands require high blood flow, measured in hundreds of ml/min/100g
• The liver sees a total flow of 1350 ml/min or 95 ml/min/100g of tissue
• The kidneys need 1100 ml/min to cleanse waste and maintain fluid composition
• Inactive muscles receive low blood flow (750 ml/min compared to active muscles)

Local Tissue Blood Flow Importance

• Organs regulate blood flow to the minimum level to meet energy requirements
• Tissues prioritize Oxygen delivery
• This keeps heart workload minimal and prevents oxygen deficiencies

Mechanisms of Blood Flow Control

• Involves the phases of acute and long-term blood flow control
• Acute control: arterioles and pre-capillary sphincters cause vasodilation/vasoconstriction in seconds/minutes for tissue needs
• Long-term control: occurs over days/weeks/months; physical changes occur in blood vessels

Acute Control of Local Blood Flow

• Metabolism increase can raise blood flow about fourfold (exercising)
• Tissue blood flow rises when systemic oxygen availability declines
• Arterial oxygen saturation drops to 25% blood flow isolates to legs and increases threefold
• Oxygen decrease leads to higher blood flow, but less than systemic oxygen, making blood levels constant
• Cyanide poisoning can cause a sevenfold blood flow increase

Vasodilator Theory

• Metabolism increase/ nutrient, oxygen decrease causes tissue cells to form vasodilators
• Vasodilators that diffuse tissues to widen precapillary sphincters, metarterioles, and arterioles include adenosine, carbon dioxide, adenosine phosphate compounds, histamine, potassium ions, and hydrogen ions
• Decreased oxygen triggers adenosine, lactic acid release, which causes vasodilation

Adenosine as a Vasodilator

• Adenosine controls blood flow (minute amounts released from heart muscle if blood flow is low)
• Adenosine will cause local vasodilation to return flow to normal
• Increases in heart metabolism cause higher oxygen utilization, which degrades ATP, releasing adenosine

Oxygen Demand Theory

• Reduced oxygen can inhibits vascular muscle contraction, causing vasodilation and increased flow
• Tissues can decrease oxygen levels to cause vasodilation

Oxygen Availability Operation

• Figures show that the tissue vascular unit has one metarteriole with one sidearm capillary surrounded by muscle fibers
• The capillary origin had a precapillary sphincter
• Tissue observations indicate precapillary sphincters are normally always open or closed
• The number of open sphincters is relational to nutrition needs
• Precapillary sphincters/ metarterioles close/open in cycles that depend on tissue oxygen needs (vasomotion)

Nutrient Demand

• Tissues may trigger vasodilation when lacking glucose, amino acids, or fatty acids

Metabolic Control

• Metabolic mechanisms function in response to the metabolic needs of tissues
• Reactive hyperemia (increased flow after blockage) and active hyperemia (increased flow with activity) are metabolic

Reactive Hyperemia

• Blocking blood to tissues for seconds to hours and then unblocking it will increase blood flow four to seven times normal

Active Hyperemia

• Highly active tissues will increase blood flow
• Local metabolism increases and uses nutrients rapidly, while releasing vasodilators
• Blood flow can increase up to 20-fold in skeletal muscles

Autoregulation of Blood Flow

• Rapid blood flow increase will quickly normalize in less than one minute due to autoregulation
• Between arterial pressures of 70-175 mm Hg blood flow only increase 20-30% even though arterial pressure increases by 150%

Autoregulation Theories

• Metabolic Theory: higher pressure provides excess oxygen/nutrients that wash out released vasodilators causing blood vessels to constrict
• Myogenic Theory: sudden stretches will cause vessel walls to contract (vessel constriction)

Myogenic Response

• Most pronounced in arterioles but seen in arteries, veins and lymph vessels
• Calcium influx from extracellular fluid cause vascular smooth muscle to contract
• Myogenic mechanism cannot detect changes in tissue blood flow directly
• Metabolic factors override the myogenic mechanism when significant increases in metabolic needs occur

Special Mechanisms for Acute Blood Flow Control

• Noted mechanisms include what occurs in the kidneys, brain, and skin
• Kidney vascular control is controlled by macula densa, which detects fluid composition in distal tubule
• Distal tubule sends signals via afferent arterioles to reduce oxygen as needed

Brain

• Levels of carbon dioxide and hydrogen determine blood flow
• The Cerebral vessels will increase or decrease to maintain the required levels

Skin

• Blood flow is tied to body adjustments via sympathetic nerves
• Blood flows at 3 ml/min/100g of tissue (cool weather)
• Blood flow increases to 7-8 L/min when body is heated
• Blood flow will fall to almost zero during very cold periods

Endothelium Control

• Endothelial cells that line the blood vessels can synthesize substances that affect relaxation/contraction of vascular wall
• Most important factor is Nitric Oxide, a vasodilator

Nitric Oxide Vasodilation

• Nitric Oxide (NO) synthesizes from arginine by endothelial derived nitric oxide synthase (eNOS)
• NO causes blood vessel relaxation
• Blood flow causes shear stress (blood drag causes stress to endothelial cells)
• The NO then relaxes blood vessels
• NO will vasodilate even against vasoconstrictors unless the endothelium cells are damaged
• Damage to vessels can cause heart and kidney issues
• Release of NO is also seen with drugs like nitroglycerin, amyl nitrate
• Sildenafil is used to treat erectile dysfunction

Endothelin Vasoconstriction

• Endothelin is a vasoconstrictor present in endothelial cells unless there is damage
• Endothelin can constrict for an artery up to 5mm
• Release of endothelin can be because of hypertension

Long-term Blood Flow Regulation

• Acute mechanisms can only initially adjust three-quarters of the way to meet the additional requirements
• Local blood flow regulation helps give more complete flow control long-term
• The body has extreme effectiveness in long-term blood flow regulation
• The long-term blood flow regulation becomes important when a tissue's needs change
• Blood vessels will adjust and increase in size to to align with tissue oxygen and nutrient needs

Blood Flow Regulation by Tissue Vascularity

• Vascularity increases with metabolism (angiogenesis)
• Vascularity decreases when metabolism decreases
• Intermittent electrical stimulation converts muscle and increases capillary count

Oxygen in Long-term Regulation

• Oxygen effects are more important in long-term control
• Vascularity increases in low atmospheric areas (high altitudes)
• Oxygen excess can stop growth in premature babies and overgrowth afterwards causes retinal vessels to grow into the eye, even causing blindness (retrolental fibroplasia).

Vascular Growth Factors

• Angiogenic factors (Vascular Endothelial Growth Factor, Fibroblast Growth Factor, Platelet-Derived Growth Factor) cause faster blood vessel growth
• When tissue oxygen is deficient the body causes creation of vascular growth factors
• New vessels sprout from older vessels to make new arteries or veins
• Angiogenesis explains how metabolic factors can cause growth of new vessels
• Other substances (steroid hormones) can trigger dissolution and cause vessel disappearance
• Additional peptides can block new blood vessels from forming

Vascularity Determination

• Vascularity is determined by the blood flow requirements of tissue
• The need is to have enough angiogenic factors to increase the muscularity as needed

Development of Collateral Circulation

• A blocked artery or vein will grow a new channel and allow blood flow
• Vessels above constriction will undergo dilation allowing for partial resupply to tissues
• Small channels will occur, rarely a single large vessel
• There will be an increase in the number of blood vessels for the new blood flow
• The coronary arteries can develop thrombosis as one example

Vascular Remodeling

• Occurs during a change (blood flow or pressure); vascularity will happen on trained muscles to allow accommodation
• Changes can occur depending on pressure and resistance on blood vessels to adapt to changes
• Vasoconstriction reduces lumen diameter to reduce wall tension
• Increase in blood with tension can cause more extracellular matrix protein formation to reinforce strength against increased blood pressure, as well as hypertension
• Arteries will use vasodilation to increase increase, blood, and vice-versa for decreased function.

Humoral Control of the Circulation

• Management of how the bloodstream acts through hormones

Vasoconstrictors

• Norepinephrine and epinephrine (released when sympathetic system is stimulated during stress or exercise)
• Angiotensin II(constricting arterioles to increase peripheral resistance or decrease sodium and water excretion)
• Vasopressin (increases water reabsorption from renal tubules)

Vasodilators

• Bradykinin (causes arterial dilation and increased capillary permeability)
• Histamine (released if tissue is damaged, inflamed, or subject to reaction and cause dilation of pores); produced during allergic reactions

Vascular Control of Ions

• An increase in intracellular calcium ion concentration causes vasoconstriction
• Physiological range potassium causes vasodilation; decreased potassium inhibits contraction
• Increase in magnesium causes powerful vasodilation
• Increased hydrogen decreases pH causing arterial dilation
• Blood flow regulated according to needs to achieve specific vasodilation effects