CBIO 2210 Exam 2

Blood vessels / vasculature

Long tubes that transport blood to and from the heart.

Arteries

Blood vessels that carry blood away from the heart.

Veins

Blood vessels that carry blood back to the heart.

Capillaries

Blood vessels that connect the smallest arteries (arterioles) to the smallest veins (venules) and allow gas and nutrient exchange.

Most common circulatory route

Heart → arteries → arterioles → capillaries → venules → veins.

The three layers of a blood vessel wall

tunica interna/intima, tunica media, and tunica externa

Tunica interna / tunica intima

Innermost blood vessel layer that lines the lumen; made mostly of endothelium.

Endothelium

Simple squamous epithelium of the blood vessels

Endothelium properties

selectively permeable

secretes chemicals that stimulate dilation or constriction

Tunica media properties

• Middle muscular layer made of smooth muscle, collagen, and elastic tissue

• strengthens the vessel to prevent rupture from pressure

• changes vessel diameter by contracting or dilating.

Tunica externa properties

• Outermost loose connective tissue layer

• merges with surrounding connective tissue

• anchors the vessel

Arteries vs. veins: wall thickness

• Arteries are thick-walled

• veins are thin-walled and collapse when empty.

Arteries are also called

• resistance vessels

• Arteries have a resilient, rigid structure that resists damage from sudden BP changes.

Veins are also called

• capacitance vessels

• Veins expand easily and contain about 64% of the blood.

Artery classes by size

Conducting/elastic arteries, distributing/muscular arteries, resistance arteries, and metarterioles

Conducting / elastic arteries

• aka Large arteries

• the largest arteries in the body.

Examples of conducting / elastic arteries

Aorta, common carotid, subclavian, common iliac, and pulmonary trunk

Elastic laminae in conducting arteries

Two elastic laminae made of elastic connective tissue: internal elastic lamina and external elastic lamina.

Internal elastic lamina of conducting arteries

Elastic layer between the tunica interna and tunica media.

External elastic lamina of conducting arteries

Elastic layer between the tunica media and tunica externa.

Elastic arteries during systole and diastole

They expand during systole and recoil during diastole.

Why do elastic arteries expand during systole

They expand because of sudden blood volume increase and help prevent downstream pressure from increasing too much.

Why do elastic arteries recoil during diastole

Recoil maintains BP and keeps blood flowing.

Distributing / muscular arteries are

Medium arteries that distribute blood to specific organs.

Examples of distributing / muscular arteries

Brachial, femoral, renal, and splenic arteries.

distributing / muscular arteries are made mostly of

tunica media. 3/4ths of wall

Resistance arteries properties

• aka small arteries.

• thick tunica media and very little tunica externa

Arterioles

The smallest resistance arteries that lead directly to capillaries.

Main function of arterioles

They control the amount of blood going to capillary beds and organs.

Arteriole diameter and wall structure

200 μm in diameter; 1–3 layers of smooth muscle and tunica interna. no tunica externa.

Metarterioles

• aka thoroughfare channels

• Short vessels that link arterioles directly to venules in some places.

Function of metarterioles

allow blood to bypass capillary beds.

Capillaries as exchange vessels

They are where gases, nutrients, wastes, and hormones pass between blood and tissue fluid.

True or false: capillaries are microscopic

true

Capillary wall composition

Endothelium and basal lamina only, no tunica media or tunica externa.

Basal lamina is

A thin layer of extracellular matrix under the capillary endothelium.

Where are capillaries are located

Nearly everywhere in the body except tendons, ligaments, epithelial tissue, cornea, and lens of the eye.

Types of capillaries

Continuous capillaries, fenestrated capillaries, and sinusoids.

Continuous capillaries are

Least permeable and most common capillaries.

Continuous capillaries are made of

• Endothelial cells joined by tight junctions

• form a continuous tube with only small intercellular clefts.

What can pass through continuous capillaries?

Solutes like glucose and gases

Pericytes wrap around what

capillaries

What are pericytes

contractile cells containing actin and myosin

Function of pericytes

They can contract to restrict blood flow to a capillary.

Fenestrated capillaries are more permeable than what

continuous capillaries

Why are fenestrated capillaries permeable

• the endothelial layer has many holes

Where fenestrated capillaries are found

in organs needing rapid absorption to/from the blood or filtration of blood, such as the kidneys and the small intestine.

Fenestrations are also called

filtration pores

Fenestration/filtration pore function

allow small molecules to pass through quickly

What cannot pass through fenestrated capillaries?

Large particles like proteins.

Sinusoids are

the most permeable capillaries

Where are sinusoid capillaries found?

in the liver, bone marrow, and spleen

What are sinusoid capillaries made of?

Very irregular endothelium with many large fenestrations.

What can pass through sinusoid capillaries?

Proteins and whole blood cells

Sinusoids allow ____ ____ to enter circulation through ___ ______

blood cells; bone marrow

What are the 5 types of veins

Postcapillary venules, muscular venules, medium veins, large veins, and venous sinuses.

Postcapillary venules properties

• Smallest veins (10–20 nm diameter)

• mostly tunica interna with only a few fibroblasts around it.

Main function/site of postcapillary venules

Major site where leukocytes/white blood cells exit the bloodstream.

How big are muscular venules

up to 1 mm diameter

Where do muscular venules receive blood from

postcapillary venules

The muscular venule is made up of

• All three tunics

• tunica media has 1–2 muscle layers

• tunica externa is thin.

How big are medium veins?

up to 10 mm diameter

What are venous valves

valves formed by the tunica interna in medium veins that help prevent blood from pooling in the lower extremities due to gravity.

How big are large veins

Veins larger than 10 mm diameter with relatively thin tunica media and very thick tunica externa.

What makes up large veins

relatively thin tunica media and very thick tunica externa

What are venous sinuses?

• Very thin-walled veins with very large lumens

• no smooth muscle (tunica media)

• can’t vasoconstrict

Function of venous sinuses

They hold a lot of blood.

Examples of venous sinuses

Coronary sinus and superior sagittal sinus.

What is capillary exchange

Two-way movement of fluid across capillary walls.

Examples of substances moved in capillary exchange

Water, oxygen, CO2, glucose, amino acids, lipids, minerals, hormones, ammonia

Where does exchange occur between blood and tissues

Capillaries

What three routes do chemicals pass through the capillary wall

• Endothelial cell cytoplasm

• intercellular clefts between endothelial cells

• filtration pores/fenestrations

What are the two mechanisms of capillary exchange

Diffusion and transcytosis

Diffusion is

the most common capillary exchange mechanism

Diffusion definition

movement of molecules from high concentration to low concentration.

Examples of diffusion out of blood

Glucose and oxygen diffuse out of the blood.

Examples of diffusion into blood

CO2 and wastes diffuse into the blood.

Two situations that allow capillary exchange diffusion

• The cell membrane is permeable to the solute

• there are passages through the capillary wall large enough for the solute to pass through

Solutes that diffuse through endothelial cell membranes

Lipid-soluble solutes and small, uncharged gases.

What are the types of capillary wall passages for diffusion

Fenestrations and intercellular clefts

Substances that pass through capillary wall passages

Glucose and ions

Why can’t proteins usually diffuse through capillaries

Proteins are too large, except in sinusoids

Capillary exchange mechanisms: Transcytosis

Vesicle-mediated transport that moves materials across the endothelium.

Steps of transcytosis

1. Endothelial cell forms a vesicle around material entering the cell

2. vesicles move material across the cell

3. material is released by exocytosis on the other side.

How much capillary exchange uses transcytosis?

a very small fraction

Substances transported by transcytosis

Fatty acids, albumin, and peptide hormones including insulin. speeds up process

Filtration definition

Fluid exiting the capillary into surrounding tissue. inside to outside

Reabsorption definition

Fluid entering the capillary from surrounding tissue. outside to inside

Where does filtration occur

At the arterial end of the capillary.

Where does reabsorption occur

At the venous end of the capillary.

What are the driving forces of filtration and reabsorption?

Hydrostatic pressure and colloid osmotic pressure (COP)

Hydrostatic pressure is a

Physical force exerted against a surface by a liquid.

Blood hydrostatic pressure

• Blood pressure in any vessel

• high at the arterial end and low at the venous end

• pushes fluid out of the capillary

• filtration

Interstitial hydrostatic pressure

• Force exerted by fluids outside the capillary

• consistent at both ends

• pushes fluid into many cells and draws fluid out of the capillary

• filtration

Colloid osmotic pressure / COP

Force that causes osmosis; mainly due to protein concentration.

Blood COP

Force causing osmosis into the capillary (blood); quite strong.

Tissue fluid / interstitial COP

Force causing osmosis out of the capillary (blood); quite weak.

Protein basis of COP

• There are more plasma proteins in blood than proteins in extracellular fluid

• albumin is the main cause of COP in blood

COP at arterial vs. venous end

COP stays consistent at both the arterial and venous ends.

Oncotic pressure

• Net COP = blood COP − tissue COP

• causes reabsorption