Cardiovascular Regulatory Mechanisms (Exam 4)

Describe the anatomy of the arteries

Made of thick wall of smooth muscle and connective tissue and behaves like a baloon

Capillaries

-smallest of vessels

-arterioles branch within the organs into capillaries across which all exchanges are made with surrounding cells

Aorta/Arteries Roles

-Rapid-transit passageways for blood from the heart to the organs

-Act as a pressure reservoir to drive blood when the heart relaxes (Diastole)

How do arteries maintain arterial pressure during diastole?

the elastic recoil exerts pressur eon the blood in the aorta during diastole, which drives blood into the vessels downstream

how can blood flow and arterial pressure be maintained when the heart relaxes?

the elastic recoil of arteries contributes to the pressure and flow, even when the heart is at rest

why does arterial pressure not fall to 0 mmHg during diastole when ventricular pressure does?

elastic recoil of arteries and the next cardiac contraction refills the arteries before all the blood drains off

Why do arterioles provide the majority of resistance to the vascular tree?

COLLECTIVELY the capillaries do not offer as much resistance to flow as arterioles (due to branching/increased surface area)

what is the pressure fall from blood entering arterioles to entering the capillaries

93 mmHg to 38 mmHg

What 2 things does controlling the radius of the arterioles help to do?

1. Variably distribute cardiac output among target organs

2. Helps regulate arterial BP

the thick layer of smooth muscle in arteriolar walls

-innervated by sympathetic nerve fibers

-sensitive to many local chemical changes, few circulating hormones, and mechanial factors (stretch)

What innervates the smooth muscle of arterioles?

sympathetic nervous system

What is the smooth muscle in arterioles sensitive to? 

chemical changes

hormones and stretch

What is vascular tone?

A state of partial constriction

• arterioles are always somewhat contracted

Factors Responsible for Vascular Tone

1. the smooth muscle in arterioles is tonic and will always have enough ca channels open to maintain a partial contraction → myogenic self-induced contractile activity

2. Sympathetic fibers are constantly releasing norepinephrine (enhances vascular tone)

Factors that increase arteriolar tone

-Increase in O2

-Decrease in CO2

-Increase in sympathetics stimulation

-Increase in Myogenic Activity

-Increase in Endothelin

-Vasopressin; Cold

Factors that decrease arteriolar tone

-Decrease in myogenic activity

-Decrease in O2

-Increase in CO2

-Increase in Nitric Oxide

-Decrease in sympathetics stimulation

-Histamine release; Heat

How are materials exchanged across capillaries at target organs?

through Diffusion

How do capillaries maximize surface area and time available for exchange?

• Thin walled (one cell layer thick)

• Small diameter

• Branch extensively

• Numerous amounts of them

• Blood flows the slowest in these

what 3 features of capillaries make them perfect for maximizing exchange between plasma and ECF?

1. large surface area

2. small radii

3. slow blood velocity

What is a metarteriole? Important feature of them?

a channel that runs between arterioles and venules

• surrounded by smooth muscle

Describe Pre-capillary Sphincters

-Not innervated

-Have high degree of myogenic tone

• Sensitive to local metabolic changes

• Act as stopcocks to control blood flow through particular capillaries

When a muscle becomes more active, what happens to the precapillary muscles supplying the capillaries?

A greater % of them relax in response to chemical changes

• total volume and surface area increase.

• Diffusion distance between cells and capillaries decreases

What happens concurrently with the relaxation of pre-capillary muscle to aid in increased blood flow to active organs?

arteriolar vasodilation

Blood flow through tissue is regulated by what 2 major factors?

1. Degree of resistance by arterioles in the organ

2. Number of open capillaries (controlled by pre capillary sphincters)

What acts as the go between for blood and tissue cells?

interstitial fluid

Is exchange across the capillary wall between plasma and Interstitial fluid passive or active? Why?

Largely passive diffusion

• (only limited vesicular transport requires energy)

Because the capillaries are really permeable and exchange is rlly thorough → interstitial fluid is the same composition as incoming arterial blood

What two ways is exchange between blood and surrounding tissue accomplished?

1. passive diffusion (primary method)

2. bulk flow

What effects whether a capillary will be leaky or not?

1. how tightly endothelial cells are joined

2. whether fenestrations are present (varies between different organs)

What is bulk flow?

protein-free plasma filters out of the capillary, mixes with surrounding inter fluid, and is then reabsorbed

-Does NOT play an important role in exchange of solutes, but it is very important for distribution of ECF

Explain the process of ultrafiltration

a process in which the pressure inside the capillary exceeds the pressure in the ECF, and fluid is pushed out through the capillary pores

Why does bulk flow occur?

due to differences in the hydrostatic and colloid osmotic pressures between plasma and interstitial fluid

What pressures influence fluid movement into and out of the capillary?

out: capillary blood pressure and interstitial fluid colloid osmotic pressure

in: plasma-colloid osmotic pressure and interstitial fluid hydrostatic pressure

If plasma proteins pathologically leak into the interstitial fluid, how does that affect fluid movement involving the capillaries?

fluid is moved out of the capillaries and into the interstitial fluid

How does the extra fluid that is filtered out of the capillaries get reabsorbed? What does this system do?

By the Lymphatic system (one way vessel)

• Provides an accessory route for this fluid to be returned to the blood

• works like a sewer system that carries away excess rainwater so floods don’t occur

How does blood leaving the capillary beds get back to the heart?

Through the venous system

• capillaries drain into venules which converge to form veins that exit the organ

What happens ot the total cross sectional area of the venous system as blood approaches the heart?

it decreases as smaller veins converge into lesser, larger cessels (blood flow speeds up)

Why are veins called capacitance vessels?

because of their storage capacity. when CO increases and more vascular beds open, venous blood is transferred to the arterial side of the systemic circulation

What factors enhance venous return to the heart?

-Sympathetic fibers inducing vasoconstriction

-Skeletal muscle activity

-Venous valves

-Respiratory activity

-Cardiac "suction"

What is the key principle of autoregulation of blood flow?

• as BP increases, vessels constrict to reduce blood flow

• BP decreases, vessels dilate to decrease blood flow

→ this all allows organs to maintain stable perfusion

An Increase in MAP leads to what in Autoregulation at the kidneys?

• Vasoconstriction in afferent arterioles (Decreases blood flow into the glomerulus)

• Decrease in GFR

A decrease in MAP leads to what in Autoregulation at the kidneys?

• Vasodilation (increases blood flow into the glomerulus)

• Increase in GFR

What do arterioles do when arterial pressure gets around 100?

they auto-regulate by vasoconstricting to insure that renal blood flow is kept constant

when it get’s way way too high, they won’t fight back as much and renal blood flow will increase

What is the mean arterial pressure

mean pressure in the artery that is measured by baroreceptors

• Can use systolic/diastole pressures to assess MAP

How is Mean Arterial Pressure (MAP) Calculated?

MAP = P (Diastolic) + (P sys - P dias)/3

Why is MAP highly regulated?

1. Must be high enough to provide sufficient pressure for blood flow to the tissues

2. Pressure must not be too high because it could create extra work for the heart and increase risk of vascular damage

What factors mainly contribute to determining MAP?

1. Cardiac output (flow)

2. Total Peripheral Resistance

P= F x TPR

What makes short-term adjustments from deviations of MAP

made by alterations in cardiac output & total peripheral resistance

-Mediated by ANS

What makes long-term adjustments from deviations of MAP?

involves adjusting total blood volume

• Restores normal salt & water balance through regulating urine output and thirst

What are baroreceptors at the carotid sinus & the Aortic Arch?

Mechanoreceptors sensitive to changes in MAP

• Located before bifurcations to ensure adequate blood flow to the brain and the rest of the body

When MAP is increased, what will happen with the baroreceptors?

The receptor potential of these baroreceptors increases

• this increases the rate of firing in the corresponding afferent neurons

and vice versa

Where is the integrating center for the cardiovascular system for MAP located? What does it do?

Located within the medulla within the brain stem

• It receives afferent impulses about the MAP state

What system regulates the efferent pathway of the cardiovascular system for MAP?

The ANS

• Alters the ratio between sympathetic & parasympathetic activity to the heart and blood vessels

Any change in MAP triggers baroreceptors to do what in an attempt to restore blood pressure?

adjust CO and total peripheral resistance (vasodilation/vasoconstriction)

What is circulatory shock?

Inadequacy of blood flow leading to deterioration of vital tissues and organ function

MAP < 60mmHg

What is vasogenic shock and what are its effects of vasogenic shock on the cardiac system?

septic shock or anaphylaxis → widespread vasodilation → decreased peripheral resistance → lower MAP <60 (circulatory shock)