General Anatomy of Blood Vessels

Arteries

• efferent vessels

• carry blood AWAY from the heart

Veins

• afferent vessels

• carry blood BACK to the heart

Capillaries

• microscopic thin walled vessels that connect the smallest arteries to the smallest veins

• engages in fluid exchange with the surrounding tissue

Tunics

• three layers that line the walls of arteries and veins 

Tunica Interna

• lines the inside of vessel and contacts the blood

Tunica Interna Histology

• simple squamous epithelium 

• sparse layer of loose connective tissue

Endothelium

• simple squamous epithelium that lines lumen of blood vessels, heart, and lymph vessels

Endothelium Functions

• selectively permeable barrier 

• secretes chemicals for dilation/constriction 

• repels blood cells and platelets so they flow freely 

• produce cell adhesion molecules 

Tunica Media

• middle layer, usually the thickest 

• strengthens vessel and prevents blood pressure from rupturing them 

• regulates diameter of vessel 

Tunica Media Histology

• smooth muscle

• collagen 

• some cases there are elastic tissue 

Tunica Externa

• outermost layer 

• anchors vessel to adjacent tissues 

• provides passage for small nerves, lymph vessels, and smaller blood vessels

Tunica Externa Histology

• connective tissue 

• merges with neighboring vessels, nerves, or other organs 

What is the Function of Vasa Vasorum

• supply blood to at least half of vessel wall 

• tissues of inner half are nourished enough by diffusion from blood in lumen 

Where is the vasa vasorum found

• most conspicuous in tunica externa

Why are the vasa vasorum important

• blood flows to rapidly through medium and large vessels that have thick walls 

• this causes inadequate exchange of chemicals between blood and tissue fluid

What are vasa vasorum

• small vessels that penetrate into the external surface of large ones and branch into capillaries that supply the deeper tissues of the larger vessel 

• (network of small capillaries into larger ones)

Why are arteries called resistance vessels?

• they have a strong, resilient tissue structure

• build to withstand pressure surges and are more muscular than veins

Conducting (Large) Elastic Arteries Examples

• aorta

• common carotid

• subclavian

• pulmonary trunk 

• common iliac

Internal elastic lamina of conducting arteries

• elastic tissue at the border between tunica interna and media 

External elastic lamina of conducting arteries

• at border between media and externa 

• sparse in largest arteries 

Conducting Arteries

• expand as they receive blood during systole and recoil during diastole 

• this relieves smaller arteries downstream from pressure surges 

How do the tunics differ between arteries and veins?

• arteries have an internal and external elastic lamina

• veins do not have this

Distributing (muscular or medium) arteries

• smaller branches that distribute blood to specific organs 

Distributing arteries examples

• brachial

• femoral

• renal

• splenic

Disrupting arteries form

• 40 layers of smooth muscle that contribute to wall thickness

Resistance (small) arteries

• to variable in number and location to have specific names 

• smallest of them are arterioles

Arterioles

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

Resistance arreries Form

• have a very thick tunica media in proportion to the lumen compared to large arteries

Metarterioles

• short vessels that link arterioles directly to venules and provide shortcuts that bypass capillaries 

Arterial Sense Organs

• sensory structures in walls that monitor blood pressure and composition 

• transmit information to brainstem to regulate heartbeat, blood vessel diameter, and respiration 

Carotid Sinuses Location

• located in the wall of the internal carotid artery above the branch point 

Baroreceptors

• triggers autonomic reflexes in response to fluctuations in blood pressure

• located in carotid sinuses, heart, and aortic arch 

Carotid Sinuses Form

• have abundance of glossopharyngeal nerve fibers in the tunica externa

Carotid Bodies Location 

• located near branch of common carotid arteries and are innervated by glossopharyngeal neveres

Chenoreceptors

• sensors that monitor changes in blood chemistry (pH, CO2, O2)

• transmit signals to brainstem respiratory centers 

Aortic Bodies

• located near the aortic arch

• transmit signals to brainstem via vagus nerve

• similar structure and function as carotid bodies

What are the three places that nutrients, wastes, hormones, and leukocytes pass between blood and tissue fluid?

• capillaries

• some venules

• sinusoids 

Microvasculature

• microcirculation 

  • arterioles, capillaries, venules 

Capilaries form

• constant onl of an endothelium and basal lamina 

• thin walls 

• have a wider diameter at the distal end

Continuous Capillaries location

• occur in most tissues and organs

Continuous Capilaries Form

• have a basal lamina

• intercellular clefts

• pericytes 

• endothelial cells held together by tight junctions 

Basal lamina

• thin protein carbohydrate layer

• in continuous capilaries they surround the endothelium and separate it from adjacent connective tissue 

Intercellular Clefts

• found in continuous capilaries 

• narrow gaps that separate endothelial cells 

• allows small solutes to pass through 

  • plasma proteins and large molecules like platelets and blood are held back

Pericytes

• found in some continuous capilaries

• external to the endothelium 

• have elongated tendrils that wrap around capillary 

Pericytes Function

• contain contractile proteins that contract and regulate blood flow of the capillary 

• can differentiate into endothelial and smooth muscle cells 

  • contribute to vessel growth and repair 

Fenestrated Capilaries Importance

• important in organs that engage in rapid absorption/filtration 

  • kidneys, endocrine glands, small intestines

Fenestrated Capilaries Function

• allow for rapid passage of small molecules

• still retain most proteins and larger particles in the bloodstream 

Fenestrated Capilaries Form

• endothelial cells

• have patches of filtration pores 

Filtration pores

• spanned by a glycoprotein membrane thinner than the cells plasma membrane 

Sinusoids

• irregular blood filled spaces in the liver bone marrow, spleen, and other organs 

• have a twisted passageway that conform the shape of surrounding tissue 

Sinusoids Form

• endothelial cells separated via wide gaps 

• no basal lamina 

• hae large fenestrations that allow passageway of proteins and blood cells 

Importance of Sinusoids

• large molecules such as albumin, clotting factors, and proteins can be passed through 

• how newly formed blood cells enter circulation from bone marrow 

Capillary Beds

• capillaries are organized into webs

Distal End of Capillaries

• transition to venules

• adding a thin tunica media 

How often do capilaries function?

• about ¾ of capilaries are always shut down because there isnt enough blood to supply all of them at once 

What regulates capillary flow?

• dilation or constriction of arterioles upstream the capillary beds 

Precapillary Sphincter 

• found in capillary beds supplied with metarterioles 

• single smooth muscle cell that wraps around opening of each capillary 

• regulates blood flow

What happens when the Precapillary Sphincter  relaxes? 

• capillaries are well perfused 

What happens when the Precapillary Sphincter constricts?

• blood bypasses the capilaries and leaves them less perfused or even bloodless 

• blood takes a shortcut through the metarterioles into nearby venule 

Why are veins called the capitance vessels

• they are relatively thin walled and flaccid

• expand to accommodate increased blood volume

• have a GREATER capacity for blood containment than arteries 

Where does most blood sit when at rest?

• systemic veins

  • 64%

• only 15% is in the systemic arteries 

Why are veins so thin walled?

• they are distant from heart ventricles and have low blood pressure 

• causes blood flow in veins to be steady so they do not need thick pressure resilient walls 

Compare splitting in arteries vs veins?

• arteries split into smaller and smaller branches

• veins converge to form larger and larger ones as they approach the heart 

Postcapillary venules

• smallest 

• receive blood from capilaries directly or by distal ends 

Postcapillary venules Form

• tunica interna with few fibroblast

• have no muscle 

• often surrounded by pericytes 

Postcapillary venules Function 

• more porous than capillaries 

• exchange fluid with surrounding tissues 

Muscular veins

• receive blood from postcapillary venules

• have tunica media with smooth muscle

• thin tunica externa

Medium Veins examples

• radial and ulnar in forearm 

• great saphenous veins in leg 

Medium Veins Form

• thin tunica interna with endothelium 

• basement membrane

• loose connective tissue 

• thin internal elastic lamina 

• thin tunica media with smooth muscle bundles 

• have collagenous, reticular, and elastic tissue

• thick externa media 

Venous Valves

• found in medium veins 

• infoldings of the tunica interna 

• keep blood flowing in one direction 

Large Veins Form

• smooth muscle in all thee tunics

• thin tunica media

• thick tunica externa 

Large Veins Examples:

• vena cavae

• pulmonary vein

• internal jugular veins

• renal vein

Venous Sinuses

• modified veins with thin walls, large lumens and no smooth muscle 

• not capable of vasoconstriction 

Common Route of blood flow

heart → arteries → capilaries → veins → heart 

Portal System

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

• occur in kidneys 

• connect hypothalamus to pituitary 

• connect intestines to liver 

Anastomosis

• point of convergence between two blood vessels other than capilaries 

Arteriovenous Anastomosis

• blood flows from artery directly into vein 

• bypasses capilaries 

• fingers, palms, toes, and ears 

Venous Anastomoses

• most common

• one vein empties directly into another 

Arterial Anastomoses

• two arteries merge

• provide collateral routes of blood supply to a tissue