(19.2.3.4.5) Arteries, Capillaries, Veins, Anastomoses

Name the three types of Arteries

Based on size and function 

1. Elastic arteries 

2. Msucular arteries 

3. Arterioles 

Describe Structure & Function & Example of Elastic Arteries

• STRUCTURE

  • Thick-walled with large LOW RESISTANCE lumen

  • Elastin found in all three tunics, mostly tunica media 

  • Contain substantial smooth muscle

    • But INACTIVE in vasoconstriction 

• FUNCTION

  • Acts as pressure reservoirs that expand and recoil as blood is ejected from heart 

  • Allows for continuous blood flow downstream even between heartbeats

  • Also called conducting arteries because they conduct blood from heart to medium sized vessels

• EXAMPLE 

  • Aorta and its major branches

Elastic arteries give rise to what?

Muscular arteries

Describe Structure & Function & Example of Muscular Arteries 

• STRUCTURE

  • Diameters range from pinky finger size to pencil-lead size 

  • Have THICKEST tunica media, with more smooth muscle, but less elastic tissue

    • Tunica media sandwich between elastic membranes 

    • ACTIVE in vasoconstriction 

• FUNCTION

  • Also called distributing arteries because they deliver blood to body organs (Femoral, brachial, external carotid arteries) 

• EXAMPLE 

  • Account for most of named arteries

Describe Structure & Function & Example of Arterioles

• STRUCTURE

  • Smallest of ALL arteries 

  • Larger arterioles contain ALL three tunics 

  • Smaller arterioles are mostly single layer of smooth muscle surrounding endothelial cells 

• FUNCTION

  • Control flow into capillary beds via vasodilation and vasoconstriction of smooth muscle 

  • Also called resistance arteries because changing diameters change resistance to blood flow

• EXAMPLE 

  • Leads to capillary beds

Arterioles lead to what?

Capillary beds

Summary of Artery Anatomy 

Structure and Function of General Capillaries

• STRUCTURE

  • Microscopic vessels

  • Diameters so small only single RBC can pass through at a time

  • Pericytes: spider-shaped stem cells help stabilize capillary walls, control permeability, and play a role in vessel repair

  • ALL capillary endothelial cells are joined by tight junctions with gaps called intercellular clefts → allows passage of fluids and small solutes

• FUNCTION

  • Supply almost every cell, except for cartilage, epithelia, cornea, and lends of eyes

  • Exchange of gasses, nutrients, wastes, hormones, etc., between blood and interstitial fluid

Name the three types of Capillaries

1. Continuous capillaries

2. Fenestrated capillaries

3. Sinusoidal capillaries

Describe Location & Function of Continuous capillaries

• LOCATION

  • Abundant in muscle, lungs, and CNS 

• FUNCTION

  • Continuous capillaries of brain are unique → form blood brain barrier, totally enclosed with tight junctions and no intercellular clefts 

Describe Location & Function of Fenestrated capillaries

• LOCATION

  • Found in areas involved in active filtration (kidneys), absorption (intestines), or secretion (endocrine hormone)

Describe Location & Function of Sinusoid capillaries

• LOCATION

  • Found ONLY in the liver, bone marrow, spleen, and adrenal medulla 

  • Fewer tight junctions (usually fenestrated with larger inerceullar clefts

  • Incomplete basement membranes 

  • Usually have larger lumens 

• FUNCTION

  • Blood flow is sluggish → allows time for modification of large molecules and blood cells that pass between blood and tissue 

  • Contain macrophages in lining to capture and destroy foreign invaders 

Define Capillary Bed

Interwoven network of capillaries between arterioles and venules

Define Microcirculation

Flow of blood through bed

Name and Describe the Two Types of Vessels in Capillary Beds

1. Vascular shunt 

   • Channel that connects arteriole directly with venule (metarteriole-thoroughfare channel)

2. True capillaries 

   • Actual vessels involved in exchange (10 to 100 exchange vessels) 

   • Normally branch from metarteriole and return to thoroughfare channel 

Role of Precapillary sphincters

• Regulate blood flow into true capillaries

  • Blood may go into triw capillaries or to shunt

• Regulated by local chemical conditions and vasomotor nerves

Describe the Pathway Blood takes through Capillary Beds 

1. Arteriole

   • Blood enters the capillary bed from a terminal arteriole. 

2. Metarteriole

   • This is a short vessel that links the arteriole to the capillary network. 

3. Precapillary Sphincters

   • Ring-like muscles at the entrance to the true capillaries that act as valves. 

4. Capillary Bed

   • A network of interconnected capillaries where exchange with tissues occurs. 

5. Vascular Shunt

   • A vessel that connects the metarteriole directly to the venule, allowing blood to bypass the capillary network. 

6. Venule: Blood exits the capillary bed and flows into a venule, which merges to form larger veins. 

Summary of Capillary Anatomy 

Structure and Function of Veins

• STRUCTURE

  • Formation begins when capillary beds unit in post-capillary venules and merge into larger and larger veins

• FUNCTION

  • Carry blood TOWARD the heart

Role of Venous Valves

• Folds of the tunica intima of certain veins that prevent blood from flowing backward

• Most abundant in the veins of the limbs.

Mechanism of Venous Blood Flow

1. Veins carry blood from the capillary beds toward the heart

2. Along the route, the diameter of successive venous vessels increases, and their walls gradually thicken as they progress from venules to larger and larger veins

Name the Two types of Veins

1. Venules 

2. Veins

Describe Structure & Function of Venules

• STRUCTURE

  • Capillaries unite to form post-capillary venules 

    • Consists of endothelium and few pericytes

  • Larger venules have one or two layers of smooth muscle cells

• FUNCTION

  • Very porous → allow fluids and WBCs into tissues

Describe Structure & Function of Veins

• STRUCTURE

  • Formed when venules coverge 

  • Have all tunics, but thinner walls with large lumens compared with corresponding arteries 

  • Tunica media is thin, but tunica externa is thick 

    • Contain collagen fibers and elastic networks

• FUNCTION

  • Large lumen and thin walls make veins good storage vessels → called capacitance vessels (blood reserviors) because they contain up 65% of blood supply

Relative proportion of blood volume throughout the cardiovascular system.

Summary of Veins

T/F: BP lower in arteries

→ TRUE

• BP lower in arteries so adaptations ensure return of blood to heart

  • Large-diameter lumens offer little resistance

Explain Effect and Causes Varicose Veins

• EFFECT

  • Dilated and painful veins due to incompetent (leaky) vavles

• CAUSE

  • Factors that contribute include heredity and conditions that hinder venous return

  • Prolonged standing, obestity, pregancy

  • Elevated venous pressure

  • Straining to deliver a baby or have movement raises intra-abdominal pressure → resulting varicosities in anal veins called hemorrhoids

SUMMARY of Blood Vessel Anatomy 

ARTERIES → Directs blood AWAY from the heart

1. Elastic arteries

   • Can absorb the most pressure

   • Responsible for feeling of pulse

   • Most suited to expanding and recoiling in response to the ejection of blood from the

     heart

2. Muscular arteries

   • With a thick tunica media and abundant smooth muscle

   • Responsible for feeling of pulse

   • Vasoconstriction

3. Arterioles

   • Directly controls blood flow to capillaries

   • Vasoconstriction

CAPILLARIES → Regulated by sphicters

1. Continuous capillaries

2. Fenestrated capillaries

3. Sinusoid capillaries

   • Most permeable

VEINS

1. Venules

   • Directly drains blood from capillaries

2. Veins

   • Directs blood one way TOWARD the heart using valve

Name and describe the Two Types of Anastomoses

1. Venous Anastomoses

   • Special interconnections of blood vessels that provide alternative pathways for blood to reach a given body region

2. Arterial Anastomoses

   • Two or more arteries that connect together in order to provide alternative pathways for supplying blood to a given body region

Role of Anastomoses

These anastomoses provide alternate pathways, called collateral channels, for blood to reach a given body region

Location of the Two Types of Anastomoses 

1. Venous Anastomoses

   • Abundant

   • Skin dorsum of hand

2. Arterial Anastomoses

   • Around joints where active movement may hinder blood flow

   • Abdominal organs

   • Heart

   • Brain

   • Arteries that supply the retina, kidneys, and spleen either DO NOT have anastomose or have a poorly developed collateral circulation

Metarteriole–thoroughfare channel shunts are example of what? 

Arteriovenous anastomoses