Lecture 9 (10/14)

Ateries

large vessels, carry blood away from heart.

Arterioles

control distribution to capillary beds by adjusting state of constriction or dilation.

Capillaries

exchange between blood and interstitial fluid.

Venules

returns blood to veins.

Veins

return blood to heart.

All vessels are lined with?

Endothelium, which is the surface that blood touches.

What is endothelium made up of?

Endothelial cells, which are a type of epithelial cell.

The endothelium makes a barrier between the blood and the surrounding interstitial fluid in where?

In arteries, arterioles, venules, and veins.

In capillaries, the endothelium functions to?

Control the flow of substances between the blood and the interstitial fluid.

Vessel Adaptations: Arteries

• Endothelial cells line inside the vessel, providing a smooth surface with little resistance to flow of blood.

• In arteries and arterioles, the endothelial cells are tightly bound together to prevent leakage.

• Walls have extracellular collagen and elastin fibers to give the vessel strength and elasticity.

• Elastic recoil during diastole (heart relaxation) pushes blood forward.

Vessel Adaptations: Veins

• Pressure is very low

  • Pressure alone is not enough to send blood back to the heart.

  • Blood accumulates in veins (below heart bc of gravity)

Two strategies:

1. 1-way valve helps direct blood unidirectionally.

2. Skeletal muscle contraction helps with blood flow.

Smooth muscle in arterioles controls what?

Blood flow to various tissues.

Smooth muscle cuffs

“Precapillary sphincters” regulate blood flow to specific capillary beds.

Autoregulation of arteriole diameter depends on what?

Local environment

All of these cause local dilation

• high CO2

• low O2

• Byproduct of cellular metabolism

  • lactic acid, low pH

• Locally produced factors (histamine and nitric oxide) cause local vasodilation.

Capillaries

vessel wall made up of a single layer of endothelial cells.

• some substances can diffuse across endothelial cells: O2, CO2

• gaps between cells (pores/clefts) provide pathways between cells for water-soluble substances.

Clefts between endothelial cells allow for?

exchange between interstitial fluid and blood.

• Clefts are small enough that blood cells and large proteins are retained in the capillary.

• some large proteins are selectively endocytosed and exocytosed by endothelial cells.

Two forces to consider concerning movement of fluid (water) between capillary lumen and the interstitial fluid:

• blood pressure, created by the beating of the heart

• osmotic pressure. theres a net movement of water from lower to higher solute.

When both forces are taken into consideration, theres a net movement of water out of the capillaries.

Is it true that water moving out of the capillaries into the interstitial fluid needs to get back into the circulatory system?

Yes.

This water is taken up by what?

The lymphatic system.

• Its low pressure vascular system that moves the fluid back to the circulatory system.

• Contraction of the vessel wall will create pressure and valves to ensure the one-directional flow of the lymph.

Review of blood pressure (again)

• pressure created by squeezing and ejection of the blood by ventricles in heart.

• We see a surge of pressure each time the heart beats in the arteries and arterioles (your pulse)

• when heart is between beats (diastolic), the pressure does not drop to 0. The elastic recoil of the arteries ensure that your blood continues to flow.

Arterial pressure is the most important because?

It’s driving blood into the arterioles and capillaries.

• Pressure is close to 0 at the end of the venous system.

Proper blood pressure is essential because?

It ensures that blood continues to flow through the system (normal is 120-129/80-84 - that is systolic over diastolic)

What happens when your blood pressure is too low? (less than 90/60)

Tissues don’t get enough blood flow (causes such as heart damage, blood loss, and dehydration)

What happens when your blood pressure is too high? (above 140/90)

Can lead to coronary artery disease, stroke, heart failure, atrial fibrillation, vision loss, etc.

• causes: obesity, genetic factors, drug use.

We are constantly doing things that could disrupt our blood pressure, such as?

• drinking

• standing up/sitting down

• sweating

• stress

• exercising

  • heart rate goes up

  • blood flow to muscles goes up

  • mean arterial blood pressure (times avg between diastolic and systolic) goes up a little

Total peripheral resistance

Sum of all vascular resistances within systemic circulation.

• Arteries supply tissues and organs in parallel circuits

  • changes in resistance determine relative blood flow.

Whats the problem with exercising when it comes to muscle blood flow and blood pressure?

• If we increase blood flow to muscles, there would be a drop in blood pressure throughout the circulatory system.

• Vasoconstriction to counter the muscle vasodilation helps maintain blood pressure.

• increased heart beat can also help maintain blood pressure.

Redistribution of blood flow during exercise

During exercise, the percentage of blood going to digestion and the kidneys goes way down.