Capillaries and Venules

Question 1

General characteristics of Capillaries

Answer 1

• Greatest Cross-sectional area
• Walls are one cell thick
• Small diffusion barrier
• Blood flows slowly through these vessels enhancing exchanges

Question 2

What are the two types of capillaries?

Answer 2

• Continuous capillaries
• Fenestrated capillaries

Question 3

Continuous capillaries

Answer 3

• Most numerous in the body
• Endothelial cells are bound together by tight junctions that prevent leakage
• Highly permeable to small water-soluble molecules that can pass through the narrow junctions called intercellular clefts between the endothelial cells
• Lipid soluble molecules that can pass through the membrane of cells are also permeable
• Low permeability to large macromolecules and proteins that are too large to pass through intercellular clefts and are not lipid-soluble

Question 4

Fenestrated Capillaries

Answer 4

• Located where rapid diffusion across capillary walls needs to occur (Intestines, Endocrine glands, and Kidneys).
• Pores or fenestrations are in the endothelial cell walls can be large enough to allow proteins to pass
• Allows rapid diffusion of small water-soluble molecules

Question 5

Discontinuous Capillaries or Sinusoids

Answer 5

• Found in the spleen, bone marrow, and liver areas where proteins and immune cells
• Large blood-filled space where gas exchange and exchange of substances between the capillaries and tissues.
• No basement membrane
• Large blood filled space lined by fenestrated endothelial cells

Question 6

What two structures regulate blood flow into certain capillaries?

Answer 6

• Metarterioles
• Precapillary sphincters

Question 7

What are metarterioles?

Answer 7

Intermediate of arterioles and capillaries as they possess some isolated smooth muscle rings at specific points instead of arterioles continuous layer

Question 8

What are the function of metarterioles and precapillary sphincters?

Answer 8

Avoid capillary bed by clamping down on the precapillary sphincters and directing it through the metarteriole which connects the arteriole and venule

They act as a means of pathway bypassing the capillary bed by shunting and cut off the blood flow to a specific capillary bed by contracting the sphincter and directing the blood from the arteriole to the venule

Question 9

How do the metarterioles act as a bypass mechanism?

Answer 9

When the precapillary sphincter is contracted that increases resistance of blood flow to the capillary bed shunting most of the blood to the metarterioles and negating to the capillary bed

When the precapillary sphincter is relaxed there is increased blood flow to the capillary bed as there is less resistance to flow.

Question 10

What determines whether the smooth muscle is contracted or relaxed?

Answer 10

Local metabolic activity

⇑⇑CO₂ will cause the sphincters to relax increasing blood flow to the capillary beds to allow gas exchange

⇓⇓⇓CO₂ will cause sphincters to constrict to decreasing blood flow as increased gas exchange is not as crucial

Question 11

What is the process of vasodilation and vasoconstriction called?

Answer 11

Vasomotion

Question 12

What is hydrostatic pressure?

Answer 12

This is the pressure that the fluid within the capillaries push on the walls of the vessel forcing gases and fluids out into the surrounding tissue.

Question 13

What are the two types of pressure that affect exchange across capillaries?

Answer 13

Hydrostatic pressure

Osmotic or Oncotic pressure

Question 14

What is osmotic pressure?

Answer 14

This is the drawing force that proteins and salts within the capillaries have on the water within the interstitial fluid.

Water always moves from an area of high concentration to one of low concentration so if the [H2O] is lower in the capillaries than in the interstitial fluid water will flow into the vessel (reabsorption)

Question 15

What creates hydrostatic pressure?

Answer 15

THE HEART

Question 16

Why is hydrostatic pressure highest in the side of the capillary bed closest to the arterioles?

Answer 16

This is because the arterioles are closest to the heart so there is higher pressure on that end allowing for higher hydrostatic pressure to push substances out of the capillary and into the interstitial fluid than the venule side.

Question 17

Which end of the capillary bed does filtration occur?

Answer 17

Arteriole side as high hydrostatic pressure pushes fluid out of the blood plasma

Question 18

Which end of the capillary bed does absorption occur?

Answer 18

Venule side as there is a low pressure within the capillaries and because water left the arteriole the concentration of proteins and salts within the vessel is increased causing water to move from an area of high concentration to low concentration.

Question 19

What is oncotic pressure?

Answer 19

Oncotic force of proteins (the pull of non-permeable proteins)

Question 20

Interstitial hydrostatic pressure is 0-1 mmHg why does this favor reabsorption?

Answer 20

High osmotic pressure within the capillaries allows more water to be draw into the vessel than pushed out of the vessel.

Question 21

What is the formula for Net pressure?

Answer 21

Net filtration pressure = filtration pressure – absorption pressure

Question 22

What is transcytosis?

Answer 22

This is the facilitated movement of proteins across the cell membrane

Question 23

If the net filtration is positive what is occurring within the capillaries?

Answer 23

Filtration is occurring

Question 24

If the net filtration is negative what is occurring within the capillaries?

Answer 24

Reabsorption is occuring