General Anatomy of Blood Vessels

Tunics

• walls of vessels composed of three layers

Tunica Interna Histology

• simple squamous epithelium

• sparse loose connective tissue

Tunica Media Histology

• smooth muscle

• collagen

• some cases, elastic tissue

Tunica Media Functions

• strengthens blood vessel and prevents pressure from rupturing them 

• regulates blood vessel diameter 

Tunica Externa Histology

• loose connective tissue merging into blood vessels, nerves, or other organs

Tunica Externa Function 

• anchors vessel to surrounding tissue 

• provides passage for small nerves, lymph vessels,

• allows smaller blood vessels to enter large ones 

What is endothelium and were is it found

• found in tunica interna

• simple squamous epithelium 

What are the different functions of the endothelium 

• selectively permeable barrier

• secrete chemicals for dilation/constriction 

• produce cell adhesion molecules that snare leukocytes 

• normally repel RBC

• adhere to platelets when damaged 

Where is the vasa vasorum found

• located in vessels, but most conspicuous in the tunica externa 

Why is the vasa vasorum important?

• it provides a passageway of smaller vessels into larger ones so that they have adequate nutrition, oxygenation, and waste removal services

Arteries

• called resistance vessels because they are strong and musclular 

Conducting Arteries Function

• expand as they receive blood and recoil during diastole 

• relieve pressure of smaller arteries downstream 

Structure of Conducting Arteries

• internal elastic lamina 

• tunica media

  • 40-70 layers of elastic sheets with smooth muscle, collagen and elastic fibers 

• external elastic lamina

• sparse tunica externa 

Distributing Arteries Examples

• brachial, femoral, renal, splenic 

Conducting Arteries Example

• aorta, common carotid, subclavian, pulmonary trunk 

Distributing Arteries Function

• medium size

• distribute blood to specific organs

Distributing Arteries Structure

• 40 layers of smooth muscle

• thick internal and external elastic laminae

Resistance Arteries Structure

• 25 layers of smooth muscle and little elastic tissue

• thicker tunica media compare to large arteries 

Arterioles

• only 3 layers of smooth muscle

• major point of control over how much blood an organ or tissue receive

Metarterioles

• thoroughfare channels that link arterioles directly to venules and provide shortcuts that bypass capilaries

What is the function of metarterioles in capillary beds? 

• capillary beds that are supplied by metarterioles have a smooth muscle cell that wraps around the opening to the capillary and acts as a precapillary sphincter 

What does it mean when the precapillary sphincter is relaxed?

• capilaries are well perfused 

What does it mean when the precapillary sphincter is constricted?

• capillary beds are not perfused

• blood bypasses capilaries and goes directly through metarterioles 

Location of Carotid Sinus

• wall of internal carotid artery above branch point 

Structure of Carotid Sinus

• abundance of glossopharyngeal nerve fibers in the tunica externa

Function of Carotid Sinus

• baroreceptors that adjust blood pressure

Location of Carotid Bodies

• near branch point of common carotid arteries 

Structure of Carotid Bodies

• innervated by glossopharyngeal nerves 

• small oval receptors 

Function of Carotid Bodies

• chemoreceptors that sense and monitor changes in blood chemistry 

• adjust breathing to stabilize pH, CO2, and O2 levels 

Location of Aortic Bodies

• aortic arch

Structure of Aortic Bodies

• similar to carotid bodies

Function of Aortic Bodies

• similar to carotid bodies but transmit signals via the vagus nerve 

Capilaries

• consist of only endothelium and basal lamina 

• have very thin walls 

Continuous Capilaries Function 

• occur in most organs 

• least permeable 

Structure of Continuous Capilaries

• endothelial cells held by tight junctions

• thin protein carb. layer

• basal lamina surrounds endothelium/ separates it from adjacent tissue

• have pericytes and  intercellular clefts 

Structure of Fenestrated Capilaries

• endothelial cells with filtration pores

Function of Fenestrated Capilaries

• rapid passage of small molecules but retain most proteins and large particles in the bloodstream 

• important for rapid absorption/ filtration 

Sinusoids Structure

• twisted tortoise passageways 

• endothelial cells separated by wide gaps 

• no basal lamina

• have large fenestrations

Sinusoids Function

• allow for proteins and blood cells to pass through them

How is perfusion of a capillary bed regulated?

• dilation or constriction of arterioles upstream of the capillary bed 

Why are veins called capacitance vessels

• they are able to expand to accommodate an increase in blood volume 

• this is why they don’t need a thick, pressure resistant wall

Structure of Postcapillary Veins

• thin tunica interna with no muscle

• surrounded by pericytes 

Function of of Postcapillary Veins

• receive blood from capillaries or distal end of metarterioles

• allow fluid exchange with surrounding tissue

Muscular Veins Structure

• tunica media with 1-2 layers of smooth muscle

Function of Muscular Veins

• receive blood from postcapillary veins 

Medium Vein Examples

• radial and ulnar

• great saphenous veins

Structure of Medium Veins

• tunica interna with endothelium, basement membrane, and loose connective tissue

• thin internal elastic lamina

• thick tunica media with smooth muscle bundles

  • has collagenous, reticular, and elastic tissue

• thick tunica externa

Function of Medium Veins

• has venous valves that keep blood flowing in one direction

• skeletal muscle pump is used as mechanism for blood flow

Large Veins Structure

• smooth muscle in all 3 tunics

• thin tunica media 

• thick tunica externa with longitudinal bundles of smooth muscle 

Function of Large Veins

• not capable of vasoconstriction 

• have venous sinuses that allow blood flow 

Venous Valves Location

• medium veins 

Venous Valves Structure

• infoldings of the tunica interna that meet in the middle of the lumen 

Venous Valves Function

• keep blood from dropping down when muscles relax 

Why do arteries not have venous valves?

• arteries have high pressure as well as movement due to contraction and relaxation of heart 

What is the function of the Portal System

• blood flows through two consecutive capillary networks before returning to the heart 

• connect hypothalamus to anterior pituitary 

• connect intestines to the liver

• present in kidneys 

Anastomosis

• point of convergence between two vessels other than capillaries 

Arteriovenous Anastomosis

• blood flows from artery directly into vein 

• bypasses capilaries 

Venous Anastomosis

• one vein empties directly into another 

• provides alternative routes of drainage for organs

Arterial Anastomosis

• two arteries merge and provide collateral routes of blood supply to tissue