chapter 15: blood flow and the control of blood pressure

what is pressure in the circulatory system

force exerted by blood on vessel walls

how does blood flow through the circulatory system

from areas of high pressure to low pressure

what determines the direction of blood flow 

pressure gradients between regions as well as resistance factors 

what regulates blood flow to organs and overall blood pressure 

intrinsic and extrinsic controls 

intrinsic controls

local mechanisms within tissues

extrinsic controls

system-wide mechanisms like neural and hormonal input

cardiac output (CO) equals

change in mean arterial pressure (MAP) / total peripheral resistance (TPR)

CO =

MAP/TPR

compliance

the ease with which a hollow vessel expands

low compliance

small increase in blood volume causes a large increase in pressure

high compliance

large increase in blood volume is required to produce a large increase in pressure

what acts as a pressure reservoir in the circulatory system

the arteries

why are arteries considered pressure reservoirs 

they store pressure generated during ventricular systole and release it during diastole 

what structural features allows arteries to store pressure

thick, elastic arterial walls

What is the compliance level of arteries?

low compliance

What happens to arteries during systole?

they expand as blood is ejected from the heart to the arteries

what happens to arteries during diastole

They recoil, helping maintain blood flow even when the heart is relaxed

ventricular contraction 

pushes blood into the elastic arteries, causing them to strech

Why do capillaries facilitate efficient exchange?

They lack vascular smooth muscle and elastic tissue

What is the structural makeup of capillaries?

A single layer of endothelial cells resting on a basal lamina

Why is the capillary wall so thin?

To allow rapid diffusion of gases, nutrients, and waste

What are metarterioles?

Vessels intermediate between arterioles and capillaries

What is the function of metarterioles?

Act as shunts to bypass capillary beds when needed

What are precapillary sphincters?

Rings of smooth muscle at the entrance to capillaries

What regulates precapillary sphincter activity?

Local chemical signals (e.g., oxygen, CO₂, metabolites)

What happens when precapillary sphincters contract?

Blood bypasses the capillary bed via metarterioles

What happens when precapillary sphincters relax?

Blood flows into the capillary bed for exchange

if precapillary sphincters are relaxed

blood flows through all capillaries in the bed

if precapillary sphincters constrict

blood flow bypasses capillaries completely and flow through metarterioles

what is the primary function of veins 

to return blood back to the heart

what structural features do veins have

thin walls of vascular smooth muscle

why are veins considered volume reservoirs

they can hold large volumes of blood due to their high compliance

what ensures unidirectional blood flow in veins

valves prevent backflow 

what role do arterioles play in circulation

provide greatest resistance to blood flow

what part of the vascular system are arterioles a part of

microcirculation

what do arterioles connect

arteries to capillaries or metarterioles

what structures allows arterioles to regulate resistance

rings of smooth muscle that control vessel radius

How much of total peripheral resistance (TPR) do arterioles account for?

more than 60%

where does the largest pressure drop in the vasculature occur

across the arterioles

what are the two main functions of arterioles

1. control blood flow to capillary beds AND 2. regulate mean arterial pressure (MAP)

what do intrinsic and extrinsic mechanism regulate in arterioles

contractile state of arteriolar smooth muscle (diameter)

what are intrinsic controls of arterioles

local signals like metabolites that match blood flow to tissue needs

what is the primary function of intrinsic control

to regulate blood flow to individual capillary beds based on metabolic demand 

what are extrinsic controls of arterioles 

neural (ANS) and hormonal signals that affect arteriolar tone systematically 

what is the primary function of extrinsic control

to regulate mean arterial pressure (MAP) across the whole body

which branch of the ANS primary controls arterioles tone

sympathetic nervous system

how do hormones like epinephrine affect arterioles

they can cause vasodilation or vasoconstriction depending on receptor type and tissue

what principle guides intrinsic control of organ blood flow

blood flow is regulated based on each organs metabolic need

what analogy explains intrinsic control

like a utility company providing water to all houses-each house takes what they need 

what determines how much blood an organ receives

the resistance of its arterioles, pressure gradient, and metabolic needs of the organ

how is organ flow calculated

organ blood flow = MAP/ organ resistance

how do organs and tissues sense whether their blood flow is adequate

through vascular smooth muscle in arterioles

what are the 4 main factors arteriolar smooth muscle respond to

metabolic activity, changes in blood flow, stretch of arteriolar smooth muscle, local chemical messengers

how does metabolic activity influence arterioles

increased activity —> more waste products —> vasodilation to increase blood flow

how do changes of blood flow affect arterioles

reduces flow can trigger compensatory vasodilation to restore perfusion

what does stretch of arteriolar smooth muscle indicate 

increased pressure —> triggers myogenic vasoconstriction to protect capillaries 

what role do chemical messengers play

fine tune vessel diameter based on tissue needs

what is active hyperemia

increased blood flow in response to increased metabolic activity 

what happens when metabolic activity increases

vasodilation to deliver more oxygen

what happens when metabolic activity decreases

vasoconstriction

what is reactive hyperemia

increased blood flow in response to a period of restricted blood flow

what causes a reactive hyperemia

temporary blockage of blood flow to a tissue

what happens during a blockage (reactive hyperemia)

metabolites increase and oxygen decreases

how does the body respond to a blockage (reactive hyperemia)

vasodilation occurs to prepare for restored flow

what happens when the blockage is released (reactive hyperemia)

blood flow increases due to vasodilation (low resistance)

what is the result of increased flow after release

metabolites removes and oxygen delivered

what is myogenic response 

change in vascular resistance in response to stretch of blood vessels in absence of external factors 

what causes the myogenic response 

increased pressure in an arteriole stretches the vessel wall 

how does vascular smooth muscle respond to stretch

it contracts

what effect does the smooth muscle contraction have on blood flow 

vasoconstriction reduces blood flow to maintain constant perfusion

what is the purpose of myogenic response

to autoregulate blood flow and protect capillaries from from pressure fluctuations

is the myogenic response intrinsic or extrinsic

intrinsic- originates within the vessel itself without neural or hormonal input 

What is the basic flow equation?

Flow = ΔP ÷ R (Pressure difference divided by resistance)

In the systemic circuit, what does flow represent?

Cardiac output (CO)

What does ΔP represent in systemic circulation?

mean arterial pressure (MAP)

What does R represent in systemic circulation?

Total peripheral resistance (TPR)

How is cardiac output calculated from MAP and TPR?

CO = MAP ÷ TPR

How is MAP expressed in terms of stroke volume and heart rate?

MAP = SV × HR × TPR

What does extrinsic control of arterioles regulate?

Arteriolar radius to control mean arterial pressure (MAP)

What are the two main extrinsic mechanisms?

Sympathetic nervous system activity and circulating hormones

How does sympathetic activity affect arterioles?

Causes vasoconstriction via α-adrenergic receptors, increasing TPR and MAP

What is the goal of extrinsic control?

To maintain systemic blood pressure and ensure adequate perfusion of vital organs

Is extrinsic control organ-specific or systemic?

Systemic — it affects multiple vascular beds to regulate overall MAP

What type of control regulates arteriolar radius systemically?

Extrinsic control via the sympathetic nervous system

What part of the arteriole does the sympathetic system innervate?

Smooth muscle of the arteriolar wall

Which neurotransmitter is released by sympathetic nerves?

norepinephrine

what receptors does norepinephrine bind to on arterioles 

a-adrenergic receptors 

What is the effect of norepinephrine binding to α receptors?

vasoconstriction

How does vasoconstriction affect total peripheral resistance (TPR)?

It increases TPR

How does increased TPR affect mean arterial pressure (MAP)?

It raises MAP

Where is epinephrine released from?

the adrenal medulla

What effect does epinephrine have on β₂ receptors?

Vasodilation in heart, liver, and skeletal muscle

What effect does epinephrine have on α receptors?

Reinforces vasoconstriction in other vascular beds

Where is vasopressin (ADH) secreted from?

Posterior pituitary

What does vasopressin do in the kidneys?

Increases water reabsorption and concentrates urine.

What vascular effect does vasopressin have?

Vasoconstriction

What is the effect of angiotensin II on arterioles?

vasoconstriction

How does angiotensin II affect TPR and MAP?

It increases total peripheral resistance (TPR) which raises mean arterial pressure (MAP).

What are baroreceptors?

Pressure-sensitive sensory neurons in blood vessels and the heart

What do baroreceptors respond to?

Changes in pressure via stretch of the vessel wall