Blood Vessels and Circulation

Tunica intima:

endothelium (simple squamous epithelium) plus connective tissue

Some arteries have

an internal elastic lamina

Tunica media:

concentric smooth muscle and connective tissue (elastic fibers

thickest layer in arteries

Tunica externa:

collagen and elastic fibers, a tough covering that anchors the vessel to surrounding tissue; areolar and adipose connective tissue

thickest layer in veins

Vasa vasorum ('vessels of vessels'):

small arteries and veins in the wall of large vessels that supply the vessel itself

Vaso vasorum support the

vessel tissue with its own blood supply

Companion vessels

lie next to each other (an artery and its vein)

Systemic arteries carry

highly oxygenated blood at high pressure

Systemic arteries carry highly oxygenated blood at high pressure, which

pushes blood against the vessel wall (this is why arteries need a thick, elastic media; a damaged artery leaks fast)

Tunica media is thicker in

arteries

Tunica externa is bigger in

veins

In larger veins the tunica intima

folds over to form valves

In larger veins the tunica intima folds over to form valves. The pressure gradient is low, so

valves prevent backflow

A capillary bed has shed the

tunica externa and smooth muscle tissue (only endothelium plus basement membrane remain)

Artery muscle is

thick and tends to hold its round shape; veins often collapse

The artery's inner surface has a

scalloped appearance

The internal diameter is very different: the vein lumen is

much larger

Artery lumen is more:

narrow

Vein lumen is

wider; often looks collapsed in cross section

Artery walls are more:

Thicker than companion vein

Vein walls are more:

Thinner than companion artery

Artery shape:

Keeps circular shape

Vein shape:

Flattens (collapses) when empty

Artery thickest tunic:

Media

Vein thickest tunic:

Externa

Artery elastic and collagen fibers:

More

Vein elastic and collagen fibers:

Fewer

Artery valves:

None

Vein valves:

Present in most veins

Artery blood pressure:

Higher (larger arteries typically above 90 mm Hg)

Vein blood pressure:

Lower (about 2 mm Hg)

Artery blood flow:

Away from heart

Vein blood flow:

Toward heart

Artery blood oxygen:

Systemic arteries high in O₂;

pulmonary arteries low in O₂

Vein blood oxygen:

Systemic veins low in O₂;

pulmonary veins high in O₂

Elastic (conducting) arteries:

largest (aorta, pulmonary trunk, carotids)

The elastic lamina runs through

the wall

Elastic fibers stretch with

systole and recoil, smoothing pressure

Muscular (distributing) arteries:

medium; internal and external elastic laminae; more smooth muscle for vasoconstriction and vasodilation

Arterioles:

smallest arteries; major control of resistance and blood flow into capillaries

Venules:

no media, sometimes no externa

Small to medium veins have

valves

Large veins have

a thick externa

Thin media and valves formed from the

intima

Venous return is helped by the

skeletal muscle pump and the respiratory pump

Skeletal muscle contraction makes blood flow in one direction because of

the one-way valves (the valve opens above the contracting muscle and closes below it)

Distribution of blood:

30-35% is in the heart, arteries and capillaries

60-65% is in the venous system (veins are the reservoir)

Capillary:

an endothelial tube on a basement membrane

Capillaries lack a

tunica media or externa

Capillaries have a diameter about that of

a red blood cell (about 8 micrometers)

Cells run single file through

the tube, and the wall is size selective

Capillary bed (capillary plexus):

connects an arteriole to a venule; fed by a metarteriole

Precapillary sphincters (smooth muscle rings):

open or close flow to true capillaries

Vasomotion:

pulses of constantly rerouted flow

Collaterals:

multiple arteries that feed one bed

An anastomosis (such as an arteriovenous anastomosis):

lets blood bypass the capillaries

Continuous capillary:

Complete basement membrane; the common type; restricts RBCs and plasma proteins (brain has extra tight junctions)

Continuous capillary example:

Muscle, skin, brain

Fenestrated capillary:

Pores (fenestrations) but a continuous basement membrane; rapid movement across

Fenestrated capillary example:

Kidneys, intestines, endocrine glands

Sinusoid capillary:

Incomplete or no basement membrane, large gaps between cells; proteins and RBCs can cross

Sinusoid capillary example:

Liver, spleen, bone marrow

What is capillary hydrostatic pressure (CHP)?

Pressure that pushes fluid out of the capillary

What is blood colloid osmotic pressure (BCOP)?

Pressure created by plasma proteins that pulls fluid into the blood

What determines net filtration pressure (NFP)?

CHP − BCOP

What is CHP at the arterial end?

35 mm Hg

What is BCOP throughout the capillary?

25 mm Hg

What is NFP at the arterial end?

+10 mm Hg (35 − 25)

What happens at the arterial end of a capillary?

Filtration: fluid moves out into surrounding tissues (~24 L/day)

What is CHP at the capillary midpoint?

25 mm Hg

What is NFP at the capillary midpoint?

0 mm Hg (25 − 25)

What happens at the capillary midpoint?

No net fluid movement

What is CHP at the venous end?

18 mm Hg

What is NFP at the venous end?

−7 mm Hg (18 − 25)

What happens at the venous end of a capillary?

Reabsorption: fluid moves back into the blood (~20.4 L/day)

What causes filtration out of capillaries?

Hydrostatic pressure pushes fluid against and through the capillary wall

What causes reabsorption into capillaries?

Blood colloid osmotic pressure pulls fluid back into the bloodstream

What happens to fluid that is not reabsorbed?

It accumulates in tissues and can cause edema (swelling)

What does the lymphatic system do with excess tissue fluid?

Collects excess fluid, filters it, and supports immune responses

How does increased blood pressure affect filtration?

Increases filtration, causing more fluid to leave capillaries

What does size-selective filtration mean?

The capillary wall allows some substances through while restricting others

What are the three main types of capillaries?

Continuous, fenestrated, and sinusoid

Fluid leaks out of the capillary into the surrounding tissue because

hydrostatic pressure pushes against the capillary wall

Filtration filters the

blood and is size selective

The venule end has

lower pressure, so the net filtration pressure turns negative and fluid moves back in

Osmotic pressure is created by

the blood proteins in plasma; it pulls fluid back into the blood

Fluid that is not recovered causes

swelling (edema)

The lymphatic system picks up the lost fluid,

identifies it and filters it

The lymphatic system is tied to

the immune system: lymph vessels begin with the fluid and the immune response follows

Higher blood pressure means

more filtration, so more fluid is lost into the tissue

Filtration beats reabsorption by about

3.6 L/day

The lymphatic system returns that fluid to

the blood

Placental blood supply:

a pair of umbilical arteries (carry blood to the placenta) and one umbilical vein (brings blood to the fetus)

Umbilical arteries and an umbilical vein drains into the…

ductus venosus and then the inferior vena cava, bypassing the liver

Fetal pulmonary bypass:

the foramen ovale is a right-to-left atrial shunt, and the ductus arteriosus connects the pulmonary trunk to the aorta

Both the foramen ovale and ductus arteriosus let blood…

skip the fetal lungs

The foramen ovale closes at

birth and leaves the fossa ovalis

Low-pressure blood is mixed with

high-pressure oxygenated blood

The ductus arteriosus becomes

tissue (it closes off and shuts down after birth; in the textbook this remnant is the ligamentum arteriosum)

The umbilical cord tears from

the placenta at birth