Circulatory System and Blood Flow

Questions and Answers

What is the primary requirement for the circulatory system to function properly?

The vessels must be completely rigid to maintain blood pressure.

The flow of blood returning to the heart must equal the flow of blood pumped by the left ventricle into the aorta. (correct)

The heart must pump blood at a fixed rate.

The total volume of blood must be constant at all times.

What are the two circulations in series in the cardiovascular system?

Systemic and muscular circulations

Pulmonary and systemic circulations (correct)

Pulmonary and nervous circulations

Cardiac and peripheral circulations

What is the primary function of the pulmonary circulation?

To regulate blood pressure

To remove carbon dioxide from the body

To deliver oxygenated blood to the body

To carry deoxygenated blood away from the heart to the lungs and return oxygenated blood back to the heart (correct)

What are the factors that determine the exact amount of flow around the circulatory circuit?

The number and strength of heart contractions, the total volume of blood, and the characteristics of the vessels Signup and view all the answers

What is the term for the part of the cardiovascular system that carries oxygenated blood away from the heart to the body and returns deoxygenated blood back to the heart?

Systemic circulation Signup and view all the answers

What is the primary function of the ventricles in the circulatory system?

To pump blood into the arteries Signup and view all the answers

Where does the exchange of oxygen and carbon dioxide occur in the circulatory system?

In the capillaries Signup and view all the answers

What is the function of the collecting system in the circulatory system?

To collect deoxygenated blood from the body Signup and view all the answers

Which of the following is NOT part of the distribution system?

Capillaries Signup and view all the answers

What is the term for the part of the circulatory system that includes venules, veins, and atria?

Collecting System Signup and view all the answers

What is a characteristic feature of the walls of arteries?

Extensive development of elastic tissue, smooth muscle, and connective tissue Signup and view all the answers

What type of blood flow occurs in arteries?

High-pressure flow Signup and view all the answers

What type of volume do arteries carry?

Stressed volume Signup and view all the answers

What is the primary function of arteries?

To deliver oxygen and nutrients to organs Signup and view all the answers

Which artery is the largest in the systemic circulation?

Aorta Signup and view all the answers

What is the primary function of elastic arteries?

To hold large volumes of blood Signup and view all the answers

What is the significance of compliance in the circulatory system?

It describes the volume of blood a vessel can hold at a given pressure Signup and view all the answers

What is the primary feature of muscular arteries?

More smooth muscle Signup and view all the answers

What is the formula to calculate compliance?

C = V/P Signup and view all the answers

What is the primary difference between elastic and muscular arteries?

Amount of elastin and smooth muscle Signup and view all the answers

Which of the following is a characteristic of arterioles?

They are extensively innervated by sympathetic adrenergic nerve fibers. Signup and view all the answers

What is the primary function of arterioles?

To regulate blood flow in each tissue according to its needs. Signup and view all the answers

Where is the highest resistance to blood flow encountered?

In the arterioles Signup and view all the answers

What is the purpose of vasoconstriction and vasodilation in arterioles?

To control blood flow in each tissue according to its needs Signup and view all the answers

Which receptor subtype is predominantly expressed in the arterioles of most visceral organs and couples to Gq?

Alpha 1 Signup and view all the answers

What is the primary endogenous agonist that binds to alpha 1 and alpha 2 receptors in arterioles?

Norepinephrine Signup and view all the answers

What is the effect of alpha 1 receptor activation on smooth muscle in arterioles?

Vasoconstriction Signup and view all the answers

In which location are alpha 2 receptors also present, in addition to arterioles?

Nerve terminals Signup and view all the answers

What is the primary function of alpha 1 receptor-mediated vasoconstriction in arterioles?

To increase peripheral resistance Signup and view all the answers

What type of receptor does beta 2 adrenergic receptors couple to?

Gs Signup and view all the answers

What is the primary endogenous agonist that binds to beta 2 adrenergic receptors?

Epinephrine Signup and view all the answers

Where are beta 2 adrenergic receptors primarily expressed?

Arterioles of the coronaries, skeletal muscle, and liver Signup and view all the answers

What is the effect of beta 2 adrenergic receptor activation on smooth muscle?

Vasodilation Signup and view all the answers

What is the significance of beta 2 adrenergic receptors in the body?

They help regulate fight or flight responses Signup and view all the answers

What type of receptor do M3 receptors on coronaries couple to?

Gq Signup and view all the answers

What is the primary endogenous agonist that binds to M3 receptors?

Acetylcholine Signup and view all the answers

What is the effect of M3 receptor activation on vascular smooth muscle?

Vasodilation Signup and view all the answers

Where are M3 receptors typically found?

Vascular smooth muscle Signup and view all the answers

What is the downstream effect of M3 receptor activation on eNOS?

Activation of eNOS Signup and view all the answers

What is unique about arterioles in the vasculature?

They are the site of highest resistance and can be changed by sympathetic activity. Signup and view all the answers

What can change the resistance in arterioles?

Sympathetic activity, circulating catecholamines, and other vasoactive substances. Signup and view all the answers

Where is the highest resistance to blood flow encountered?

In the arterioles. Signup and view all the answers

What type of substances can change the resistance in arterioles?

Circulating catecholamines and vasoactive substances. Signup and view all the answers

What is the significance of arterioles in the vasculature?

They are the site of highest resistance and can be changed by sympathetic activity. Signup and view all the answers

What is the primary function of capillaries in the circulatory system?

To exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid Signup and view all the answers

How do lipid-soluble substances cross the capillary wall?

By dissolving in and diffusing across the endothelial membrane Signup and view all the answers

What type of capillaries have large pores in their walls?

Fenestrated capillaries Signup and view all the answers

Why are not all capillaries always perfused with blood?

Because it depends on metabolic needs Signup and view all the answers

What is the structure that surrounds the endothelial cells in capillaries?

Basal lamina Signup and view all the answers

What is the main function of venules in the circulatory system?

To collect blood from capillaries and transport it back to the heart Signup and view all the answers

What is a characteristic of veins in the circulatory system?

They are thin-walled structures with low blood pressure Signup and view all the answers

What type of volume do veins carry?

Unstressed volume Signup and view all the answers

What is the characteristic of veins compared to arteries?

They hold larger volumes of blood at lower pressures Signup and view all the answers

What determines the volume of blood a vessel can hold?

The compliance of the vessel Signup and view all the answers

Why do arteries hold lower volumes of blood?

Because they have lower compliance Signup and view all the answers

What is the significance of compliance in the circulatory system?

It determines the volume of blood a vessel can hold Signup and view all the answers

What is the relationship between compliance and distensibility?

Compliance is directly related to distensibility Signup and view all the answers

What happens when the veins constrict?

Blood is shifted from the veins to the arteries Signup and view all the answers

What type of nerve fibers innervate the smooth muscle in the walls of veins?

Sympathetic nerve fibers Signup and view all the answers

What is the effect of alpha-1 adrenergic activation on the veins?

Decreasing the capacitance and unstressed volume Signup and view all the answers

What is the result of changes in compliance of the veins?

Redistribution of blood between the veins and arteries Signup and view all the answers

What is the significance of compliance in the veins?

Storing blood and regulating blood pressure Signup and view all the answers

What is the rate of displacement of blood per unit of time?

Velocity of blood flow Signup and view all the answers

Which blood vessels have larger cross-sectional areas?

Veins Signup and view all the answers

What is the significance of the large cross-sectional area of veins?

Increased blood storage capacity Signup and view all the answers

What is the formula to calculate the cross-sectional area of a blood vessel?

πr^2 Signup and view all the answers

What is the result of a larger cross-sectional area in blood vessels?

Decreased resistance to blood flow Signup and view all the answers

What happens to the total cross-sectional area of blood vessels as systemic arteries branch to form small arteries, arterioles, and capillaries?

It increases Signup and view all the answers

What is the relationship between the velocity of blood flow and the total cross-sectional area of blood vessels?

Velocity varies inversely with cross-sectional area Signup and view all the answers

What happens to the velocity of blood flow when it enters venules and veins?

It increases Signup and view all the answers

Why does the velocity of blood flow decrease in small arteries, arterioles, and capillaries?

Due to the increase in total cross-sectional area Signup and view all the answers

What is the primary reason for the change in velocity of blood flow in different parts of the circulatory system?

Change in total cross-sectional area Signup and view all the answers

Study Notes

The Circulatory System

The circulatory system is a closed circuit, meaning that the flow of blood returning to the heart must equal the flow of blood pumped by the left ventricle into the aorta.

Factors Affecting Blood Flow

The exact amount of flow around the circuit depends on three key factors: • The number and strength of heart contractions • The total volume of blood • The characteristics of the vessels

The Cardiovascular System

The cardiovascular system has two circulations in series:

Pulmonary Circulation

Carries deoxygenated blood away from the heart to the lungs
Returns oxygenated blood back to the heart

Systemic Circulation (Peripheral or Greater)

Carries oxygenated blood away from the heart to the body
Returns deoxygenated blood back to the heart

Circulatory System Divisions

Each circulation (systemic and pulmonary) has 3 major divisions.

Distribution System

Composed of ventricles, arteries, and arterioles.

Perfusion/Exchange System

Composed of capillaries where exchange of oxygen and nutrients with cells occurs.

Collecting System

Composed of venules, veins, and atria.
Responsible for collecting and returning blood to the heart.

Characteristics of Arteries

Aorta is the largest artery of the systemic circulation, receiving blood directly from the heart
Arteries have strong vascular walls to withstand the high velocity of blood flow
The walls of arteries are composed of extensive elastic tissue, smooth muscle, and connective tissue
Arteries transport blood under high pressure to the tissues

Function of Arteries

Deliver oxygen and other nutrients to the organs
Carry the stressed volume of blood

Artery Types

There are two main types of arteries: Elastic Arteries and Muscular Arteries
Elastic Arteries are high-compliance arteries, meaning they can hold large volumes of blood
They contain more elastin, which increases their distensibility
Examples of Elastic Arteries include the Aorta and Carotid Artery

Characteristics of Elastic Arteries

Have more elastin, allowing for increased distensibility
Capable of holding large volumes of blood
Can stretch to accommodate increased blood pressure

Characteristics of Muscular Arteries

Contain more smooth muscle, enabling vasoconstriction and dilation
Are high-resistance arteries, distributing blood according to tissue needs
Examples of Muscular Arteries include the Femoral and Mesenteric Arteries

Compliance and Capacitance

Compliance (C) is the volume of blood a vessel can hold at a given pressure, measured in mL/mm Hg
C = V/P, where V is the volume in mL and P is the pressure in mm Hg
Compliance describes the ability of a vessel to expand and accommodate changes in blood pressure

Vascular Resistance

Vascular Resistance is the opposition to blood flow in the circulatory system
Must be overcome to create blood flow and maintain blood pressure

Arterioles

Are the last smallest branches of the arterial system
Act as control conduits that regulate blood flow into capillaries
Characterized by strong muscular walls that enable vasoconstriction and vasodilation
Heavily innervated by sympathetic adrenergic nerve fibers
Allow alteration of blood flow in each tissue in response to its specific needs
Site of the highest resistance to blood flow

Receptor Distribution and Function

Alpha 1 and 2 receptors are found in the arterioles of most visceral organs.
Alpha 1 receptors are predominant in these organs and couple to Gq protein.
Alpha 2 receptors, on the other hand, couple to Gi protein.

Receptor Location and Feedback Mechanism

Alpha 2 receptors are also present in nerve terminals, where they inhibit norepinephrine (NE) release through a feedback mechanism.

Norepinephrine and Vasoconstriction

Norepinephrine is the primary endogenous agonist for these receptors.
It is released from postganglionic neurons and acts on vascular smooth muscle.
Activation of these receptors leads to smooth muscle contraction, resulting in vasoconstriction.

Beta 2 Adrenergic Receptors

Expressed in arterioles of coronaries, skeletal muscle, and liver
G protein-coupled receptor that couples to Gs, a stimulatory G protein
Activates the cAMP pathway

Agonist and Release

Epinephrine is the primary endogenous agonist for beta 2 adrenergic receptors
Released from the adrenal gland

Physiological Effects

Found in vascular smooth muscle
Cause vasodilation in areas that will need increased blood flow in "fight or flight" situations

Parasympathetic Innervation and Vasodilation

Parasympathetic innervation is minimal or absent in most blood vessels of the body

M3 Receptors and Vasodilation

M3 receptors are present on coronary vessels and respond to vagal tone
M3 receptors are G protein-coupled receptors that couple to Gq
Activation of M3 receptors stimulates phospholipase C, leading to the production of DAG and IP3
DAG and IP3 trigger calcium release, which activates eNOS
Activated eNOS produces nitric oxide (NO), a potent vasodilator
Acetylcholine is the primary endogenous agonist that activates M3 receptors
Acetylcholine is released from postganglionic neurons and acts on vascular smooth muscle
Vasodilation occurs through the activation of the cAMP pathway, leading to smooth muscle relaxation

Arterioles Function

Arterioles are the site of highest resistance in the vasculature
They are the site where resistance can be changed by: • Sympathetic activity • Circulating catecholamines • Other vasoactive substances

Capillaries

Exchange various substances, including fluids, nutrients, electrolytes, hormones, and others, between blood and interstitial fluid
Thin-walled structures with a single layer of endothelial cells surrounded by a basal lamina
Lipid-soluble substances cross the capillary wall by dissolving in and diffusing across the endothelial membrane
Water-soluble substances, such as ions, cross the capillary wall through:
- Water-filled clefts (spaces) between endothelial cells
- Large pores in the walls of some capillaries (e.g., fenestrated capillaries)

Blood Perfusion in Capillaries

Not all capillaries are always perfused with blood
Blood perfusion in capillaries depends on metabolic needs

Venules

Collect blood from capillaries and merge into larger veins
Thin-walled structures composed of endothelial layer, elastic tissue, smooth muscle, and connective tissue

Veins

Function as conduits to transport blood from venules back to the heart
Characterized by low pressure and thin walls
Carry unstressed volume, meaning they can expand or contract to meet changing blood volume needs
Serve as a major controllable reservoir for extra blood depending on the body's requirements

Vascular Compliance

Compliance of veins is higher than that of arteries
Compliance is directly related to distensibility
Higher compliance vessels can hold more volume at a given pressure
Veins can hold larger volumes of blood at lower pressures
Arteries have lower compliance compared to veins
Arteries hold lower volumes of blood, but at higher pressures

Compliance of Veins

Changes in compliance of the veins lead to redistribution of blood between the veins and arteries
Constriction of the veins results in a decrease in the volume they can hold
This causes blood to shift from the veins to the arteries

Regulation of Vein Constriction

Smooth muscle in the walls of veins is innervated by sympathetic nerve fibers
Alpha-1 adrenergic activation leads to constriction of the veins
Constriction of the veins reduces capacitance and unstressed volume

Blood Flow and Vessel Characteristics

The velocity of blood flow is defined as the rate of displacement of blood per unit of time.
Blood vessels vary in terms of diameter and cross-sectional area.
Veins have much larger cross-sectional areas compared to arteries.
The large cross-sectional area of veins explains their high blood storage capacity.
The cross-sectional area of a blood vessel is equal to the area of a circle.

Blood Flow Velocity and Cross-Sectional Area

As systemic arteries branch to form smaller vessels (arterioles and capillaries), the total cross-sectional area of these vessels increases.
This increase in cross-sectional area leads to a decrease in the forward velocity of blood flow.
The reverse happens when blood is collected into venules and veins: the total cross-sectional area decreases, and the velocity of blood flow increases again.
There is an inverse relationship between the velocity of blood flow and the total cross-sectional area of blood vessels.

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Description

This quiz covers the basics of the circulatory system, including the flow of blood, heart contractions, and the characteristics of blood vessels.