Chapter 22 Study Guide (Blood Vessels)

Blood Vessels

Any tube in the body that transports blood.

• Tunica Interna

• Tunica Media

• Tunica Externa

Basic Structure of a Blood Vessel

Tunica Interna

• The innermost layer of a blood vessel. It is in direct contact with blood itself, continuous with the endocardium of the heart. Surrounds lumen.

• It is made of endothelium (simple squamous epithelium). There is also an underlying layer of connective tissue with elastic fibers.

• Contains a basement membrane that is underneath the endothelium to anchor it to tissues. It is acellular, made of proteins, and has two layers (basal and reticular lamina).

• Only in arteries: There is an internal elastic lamina on the outer margin. Which is a sheet of thick elastic fibers and its continuous with the basement membrane.

Tunica Media

• The middle layer of a blood vessel. It is innervated with neurons from the automatic nervous system to stimulate contractions.

• It is made of smooth muscle tissues that encircle the lumen. It runs circularly along the circumference of blood vessels. It con contract and cause the lumen to shrink down. Supported by a framework of loose connective tissue. 

• Only in Arteries: There is an external elastic lamina. It is more superficial than the internal elastic lamina. It is a thin layer of elastic fibers located between the media and externa. 

Tunica Externa

• The outer layer of a blood vessel that stabilizes and anchors the vessel.

• It is made of a variety of connective tissues: could be areolar or dense irregular CT. Has a lot of collagen and elastic fibers.

• There are a bunch of nerves running through the layer.

• Around large arteries, they are supplied with their own tiny arterioles and capillary beds to help feed tissues of large arteries from the outside.

• Tiny capillaries = Vasa Vasorum, feed large arteries, always on the outside of vessels.

• In veins, this layer is thicker than the media. 

Lumen

Any tube/space inside the tube. The inner opening through which blood is actually flowing. It is a cavity in the center of a blood vessel. 

Arteries:

• Vessel Wall: Thicker than vein. Media contains more smooth muscle and elastic fibers than veins.

• Vessel Lumen: Smaller, retain a circular shape.

• Vessel Lining: Endothelium folds, have pleated appearance.

• Valves: Don’t have valves.

Veins:

• Vessel Wall: Thinner, less smooth muscle in media (thin). 

• Vessel Lumen: Look flattened or distorted.

• Vessel Lining: Lack folds.

• Valves: Contain valves, prevent backflow of blood toward capillaries.

Arteries vs. Veins

Arteries

• Blood vessels that transport blood away from the heart.

• They are very strong and elastic to withstand high blood pressure. They branch into smaller and smaller ones as they travel away from the heart.

• Tunics: Media and Externa are thick. Anchor them in place so they don’t move around within your body. 

• Have two types: Elastic and Muscular.

Elastic Arteries (conducting arteries) 

• A type of artery that are the largest in the body. They are able to stretch and recoil to transport large valves of blood away from the heart. Prevents blood flow from slowing down. 

• The interna is thick: endothelial lining and underlying basal lamina of collagen and elastic CT fibers. Internal elastic membrane separates inerna from media. The smooth muscles in media of this artery do not contract.

• They have thin walls for their size but they are extremely resilient. Contain elastic lamellae, which are layers of collagen fibers between the smooth muscles in the tunica media (not in veins). 

• Larger diameter with thinner walls.

• Examples: Aorta and Pulmonary Trunk

Muscular Arteries (distributing arteries)

• A type of artery that are medium-sized in the body. They are not as stretchy. Have a lot of muscles that allow them to control the size of their lumen. Allows them to control where blood is flowing in the body. 

• They transport blood to the body’s skeletal muscles and internal organs. 

• They have thicker walls for their size. More smooth muscles making the media thicker. There are fewer collagen fibers. Contain elastic lamellae but they are less elastic. Tunica externa is even thicker than tunica media in most. There is also a prominent internal elastic membrane that separates intima from media. 

• Narrow diameter with thicker walls.

• Examples: Femoral and brachial

Arterioles

The smallest arteries. They are microscopic and help to regulate blood flow into the capillary beds. They have a thin externa. Media contains one to two layers of smooth muscle cells. Control blood flow between arteries and capillaries. Tunics only one to two cells thick.

Blood from the heart goes down to arterioles, goes into capillary beds, where it drops off its nutrients, picks up waste, and then return to the heart through a vein. 

Arterioles to Capillary Bed

Metarteriole

A junction between an arteriole and a capillary bed that contains a pre-capillary sphincter. Sphincters are all open to allow blood to go into capillary beds so the tissues in the area can be provided with nutrients and waste pickup. 

Precapillary Sphincters

• Band of smooth muscle that is a part of the metarteriole. It constricts a tube to help regulate the movement through the lumen of the tube.

• Relaxed: Allow arterioles to control how much blood is going through the arteriole and into the capillary bed.

• Contracted: Constricted. Halt blood flow from entering into the capillary bed and instead the blood would be shunted through the thoroughfare channel and go directly to the vein and return to the heart. It is a way for the body to turn off blood flow to tissues. The body may do this to regulate where the blood is going.

Capillaries

• The smallest blood vessels in the body. Connects arterioles to the venules. They are extensions of the tunica interna. They are the only vessels where the walls allow for the exchange of nutrients and blood and the interstitial fluid (walls are thin).

• They are the site of nutrient, waste, oxygen, carbon dioxide, and fluid exchange. How the body tissues get nutrients and oxygen, while getting their waste picked up.

• They are so thin that RBCs have to travel in a single file line. Might have a fold or bend in half to squeeze through the vessel. They are typically 1-3 endothelial cells sitting on the delicate basal lamina. 

• Made of only endothelium and basement membrane, no media or externa.

• Contain intercellular clefts, which are gaps/slits between epithelial cells that allow passage of fluids and small solutes.

• There are 3 types: Continuous, Fenestrated, and Sinusoid

Continuous Capillary

• The most common type of capillary. They are the least leaky, meaning the only thing that can leave the blood from the capillary is the blood plasma. All cells and large proteins stay behind and can’t leave. Plasma squeezes out through the slits between endothelial cells. There are no holes between the cells.

• They are found in all tissues except epithelial and cartilage. Endothelium forms complete lining. Endothelial cells connected by tight junctions and desmosome. 

Fenestrated Capillary

A type of capillary that has holes in the endothelial cells. There are pores in their walls, due to incomplete or perforated endothelial lining. Has a “swiss cheese” look. Permits fast exchange of fluids and solutes. Allows larger substances to leave the blood but formed elements still can not go through the holes. They are found in kidneys, endocrine glands, pancreas, and intestines.

Sinusoid (capillary)

A types of capillary that is flattened and irregularly shaped with large holes in the endothelial cells. The gaps between adjacent endothelial cells, and basal lamina are thinner or absent. Permits exchange of water and solutes. Very large substances, including blood cells, may leave. They are found in bone marrow, liver, spleen, and adrenal glands. 

Veinules

The smallest veins that collect blood from capillaries and transport them to veins.

Post-capillary Veinules

The smallest veinules. They resemble expanded capillaries. Some lack media, only isolated smooth muscle cells. They receive blood directly from capillaries. very permeable, site of WBC migration into tissues. 

Muscular Veinules

A type of veinule that has thicker walls and few layers of smooth muscle. They exhibit a clear tunica media. The intima is made of endothelium on top of a basal lamina, media is made of smooth muscle tissues, externa is thicker than the media with elastic and collagen fibers. They also contain valves. 

Veins

• A type of blood vessel that collects blood from all tissues and transports it back towards the heart. They start out small and merge into larger and larger ones as they return towards the heart. 

• They cannot withstand high blood pressures since they are not as stretchy and do not have as much muscle. 

• They follow similar pathways as arteries but they are much more numerous. They also have large lumens.

• These vessels that are inferior to the heart have valves. Valves help the blood to travel up against gravity back to the heart. Vessels that are superior to the heart do not have valves, they rely on gravity to bring blood back towards the heart. Valves are folds of the tunica intima, ensures blood can only go one way, prevents backflow. 

Skeletal Muscle Pump

• The concept that there are a lot of skeletal muscles that surround veins. And every time you contract those skeletal muscles, it squeezes the veins and pumps blood up the one-way valves toward the heart. 

• Sedentary lives or sitting for too long: Blood can get stuck in the bottom of the foot and pool there, potentially forming a blood clot.

Respiratory Pump

The process of breathing in your torso, sucking the blood up back towards the heart. 

Anastomosis

When two arteries merge, or when two veins diverge. This provides redundancy. 

Arterial Anastomosis (Circle of Willis)

A type of anastomosis where an artery has diverged into the circle and then merges back together again. It forms a round circle or roundabout at the base of your brain. Ensures that if yo have a blockage, there wouldn’t be a stoppage of blood flow to the brain. Two arteries merge when they usually diverge as they go away from the heart. It is an evolutionary adaptation. 

Venous Anastomosis (Dorsal Venous Arch)

A type of anastomosis where veins diverge as they go towards the heart (they usually merge). This ensures that you will always have blood flow going up your leg.

Arteriovenosus Anastomosis (Shunt)

A type of anastomosis that is more common. It is when you have an alternative pathway to a capillary bed. Allows blood to take a bypass and go straight from arteriole to venule and back to the heart (usually goes from arteriole > capillary > venule > heart). It bypasses the capillary bed but does not stop blood flow to the area.

Vascular Sinus

A specialized type of vein that contains only tunica interna and dense connective tissue. It is a very specific hole, not really a vessel, used for alternative pathways. An example is the coronary sinus, which is a special back door. Coronary circulation can return blood into the heart by going straight into the right atrium instead of going through vena cavae. 

Portal Vein

A specialized type of vein that connects two capillary beds together. Blood goes from artery into capillary bed and then into a 2nd capillary bed after. Artery>Capillary bed>portal vein>2nd capillary bed>final vein>heart. 

Varicose Veins

• A vein whose valve has weakened and blood flows backwards against gravity (pools). It usually occurs in the legs as blood pools lower in the body.

• It is a result of the one-way valves and veins weakening over time (blood cannot travel upwards).

• Symptoms: Inflammation in veins and surrounding tissues, pain, discomfort

• Prevention: Avoid mechanical stress in the veins (standing to long).

• Causes: Pregnancy (volume of blood increases), blood pressure increase, age, prolonged standing. 

Hemorrhoids

When a varicose vein occurs in the anal canal.