Section 4

Question 1

What is the purpose of the extensive network of capillaries throughout all tissues?

Answer 1

The extensive network of capillaries ensures that all cells of the body are in close proximity to them.

Question 2

What is the structural composition of capillary walls?

Answer 2

Capillary walls are made up of a single layer of endothelial cells. There are no smooth muscle cells.

Question 3

How does exchange across capillaries occur in different parts of the body?

Answer 3

In all parts of the body, except the brain, exchange across capillaries occurs by diffusion.

Question 4

How does exchange occur across capillaries within the brain?

Answer 4

Within the brain, exchange across capillaries occurs by carrier-mediated transport due to the presence of the blood-brain barrier.

Question 5

What is the primary role of capillaries in the body?

Answer 5

Capillaries are considered the primary site in the body for the exchange of materials between blood and cells.

Question 6

What is the thickness of capillary walls, and how does it contribute to the diffusion of nutrients and wastes?

Answer 6

Capillary walls are around 1 micrometer in thickness, consisting of a single, thin layer of endothelial cells. This short distance of diffusion facilitates the efficient exchange of nutrients and wastes.

Question 7

Why is the narrow lumen diameter of capillaries significant for diffusion?

Answer 7

Capillaries have a narrow lumen diameter of 7 micrometers, requiring even red blood cells (8 micrometer diameter) to squeeze through. This constriction enhances the efficiency of nutrient and waste exchange.

Question 8

ow close are surrounding cells to a capillary, and why is this distance important for diffusion?

Answer 8

Surrounding cells are no farther than 0.01 centimeter away from a capillary. This short distance is crucial for efficient diffusion of nutrients and wastes.

Question 9

What contributes to the large surface area of capillaries, and why is it important for exchange?

Answer 9

Capillaries are numerous, and if you compare the total cross-sectional area for all vessels in the body, capillaries have the greatest. This large surface area provides many sites for gas and nutrient exchange.

Question 10

How does the slow velocity of blood in capillaries contribute to nutrient and waste exchange?

Answer 10

The vascular system is a closed loop, and the same amount of cardiac output moves through all vessels. Increasing the cross-sectional area in capillaries decreases the velocity of blood, allowing more time for efficient nutrient and waste exchange.

Question 11

What is the purpose of capillary beds?

Answer 11

Capillary beds form branching networks to reach as many cells as possible, facilitating the exchange of materials between blood and cells.

Question 12

From where do capillaries generally branch, and what is the role of metarterioles?

Answer 12

Capillaries generally branch directly from an arteriole or from structures called metarterioles. Metarterioles provide a thoroughfare or bypass between arterioles and venules.

Question 13

What is the function of precapillary sphincters?

Answer 13

Precapillary sphincters, found where capillaries branch off from metarterioles, are composed of smooth muscle cells. They act as regulators, controlling blood flow into capillary beds.

Question 14

How are metarterioles structurally different from arterioles and capillaries?

Answer 14

Metarterioles are vessels structurally somewhere between arterioles and capillaries. They have some smooth muscle cells.

Question 15

What is a metarteriole’s role in the circulatory system?

Answer 15

The role of metarterioles is to provide a thoroughfare or bypass between arterioles and venules, facilitating blood flow between these larger vessels and the capillaries.

Question 16

Where are smooth muscle cells in metarterioles generally found?

Answer 16

Smooth muscle cells in metarterioles are generally found where capillaries branch off, and they form precapillary sphincters to regulate blood flow into capillary beds.

Question 17

What is the role of precapillary sphincters in controlling blood flow?

Answer 17

Precapillary sphincters control the flow of blood through a capillary bed by contracting and relaxing based on metabolic activity, regulating the appropriate amount of perfusion.

Question 18

Describe the effect of metarteriole relaxation on blood flow through a capillary bed.

Answer 18

When metarterioles are relaxed, blood flows through both the metarteriole and the entire capillary bed.

Question 19

What happens to blood flow through a capillary bed when precapillary sphincters are contracted?

Answer 19

When precapillary sphincters are contracted, blood flows only through the metarteriole, bypassing the capillary bed.

Question 20

How does metarteriole contraction influence blood flow through a capillary bed?

Answer 20

When metarterioles contract, blood flows only through the metarteriole, bypassing the capillary bed.

Question 21

What factors determine whether precapillary sphincters contract or relax?

Answer 21

Precapillary sphincters contract or relax based on metabolic activity, allowing regulation of blood flow to ensure appropriate perfusion.

Question 22

How do endothelial cells forming capillaries fit together?

Answer 22

The endothelial cells forming capillaries fit together in a jigsaw puzzle-like fashion, but there is variability in how tightly the pieces fit together.

Question 23

What are capillary pores, and how do they vary in size?

Answer 23

Capillaries contain pores, which are narrow, water-filled spaces where two endothelial cells come together. These pores vary in size throughout the body.

Question 24

Why does the brain have no pores in capillaries?

Answer 24

The brain has no pores in capillaries due to the existence of the blood-brain barrier, which restricts the passage of certain substances.

Question 25

How do liver capillaries differ in terms of pore size compared to the brain?

Answer 25

Liver capillaries have very large spaces between endothelial cells, large enough even for some protein exchange to occur, in contrast to the brain which has no pores.

Question 26

What is the purpose of capillary pores?

Answer 26

Capillary pores are narrow, water-filled spaces where two endothelial cells come together, allowing the passage of water and small water-soluble molecules. They facilitate the exchange of substances between blood and surrounding tissues.

Question 27

What types of particles can pass right through endothelial cells in capillary walls?

Answer 27

Lipid-soluble particles, including gases, can pass right through endothelial cells in capillary walls.

Question 28

Can plasma proteins, for the most part, cross endothelial walls in capillaries?

Answer 28

For the most part, plasma proteins cannot cross endothelial walls in capillaries.

Question 29

In organs where the crossing of plasma proteins is possible, how do most of the exchangeable proteins accomplish this?

Answer 29

In organs where crossing of plasma proteins is possible, most of the exchangeable proteins do so by vesicular transport.

Question 30

Why does the velocity of blood decrease as it enters the capillaries, and how does this impact nutrient exchange?

Answer 30

The velocity of blood decreases as it enters the capillaries due to the increased cross-sectional area. This slowing down allows more time for nutrient exchange.

Question 31

What is the primary mechanism for the exchange of plasma proteins in organs where crossing endothelial walls is possible?

Answer 31

In organs where crossing endothelial walls is possible, most of the exchangeable proteins do so by vesicular transport.

Question 32

In which organ would you expect capillaries to have very large spaces between endothelial cells, allowing for some protein exchange to occur?

Answer 32

Liver capillaries have very large spaces between endothelial cells, allowing for some protein exchange to occur.

Question 33

What is bulk flow in the context of capillary exchange, and what does it primarily involve?

Answer 33

Bulk flow in capillary exchange refers to the movement of protein-free plasma from the capillary lumen, through the pores, into the interstitial space. This flow of fluid also transports small water-soluble molecules.

Question 34

In the analogy of a capillary being like a sieve, what conditions lead to fluid being forced from the capillaries, and what is this process called?

Answer 34

When outward driving forces are stronger than inward driving forces, fluid is forced from the capillaries, and this process is called filtration.

Question 35

What is reabsorption in the context of capillary exchange, and what conditions lead to fluid being forced into the capillaries?

Answer 35

Reabsorption in capillary exchange occurs when inward driving forces are stronger than outward driving forces, leading to the movement of fluid into the capillaries.

Question 36

How is bulk flow defined in the context of capillary exchange, and what does it represent?

Answer 36

Bulk flow is the summation of filtration and reabsorption in capillary exchange. It represents the overall movement of protein-free plasma and small water-soluble molecules between the capillary lumen and the interstitial space.

Question 37

Define protein-free plasma, and what components are removed to obtain this portion of blood?

Answer 37

Protein-free plasma is the portion of blood that remains after red blood cells, white blood cells, platelets, and other cellular components are removed.

Question 38

What is the average capillary blood pressure when blood enters the capillaries, and how does it change by the end of the capillaries?

Answer 38

On average, capillary blood pressure (Pc) is around 37 mmHg when blood enters the capillaries. However, by the end of the capillaries, the hydrostatic pressure decreases to around 17 mmHg.

Question 39

What is the force represented by interstitial fluid colloid osmotic pressure, and what is its usual value under normal conditions?

Answer 39

Interstitial fluid colloid osmotic pressure (πIF) is the osmotic pressure created by proteins in the interstitial fluid. Under normal conditions, this pressure is considered to be 0 mmHg as any small amount of protein leaving the capillaries is taken up by the lymphatic system.

Question 40

What is interstitial fluid hydrostatic pressure, and why is there controversy over its contribution?

Answer 40

Interstitial fluid hydrostatic pressure (PIF) is the hydrostatic pressure exerted on the outside of the capillary by fluids in the interstitial space. There is controversy over its contribution as it is challenging to measure, and its value is considered marginal, around 1 mmHg.

Question 41

What is another term for plasma-colloid osmotic pressure, and why does it remain relatively constant at around 25 mmHg?

Answer 41

Plasma-colloid osmotic pressure (πp), also called oncotic pressure, is due to the presence of protein in the plasma. It remains constant at around 25 mmHg because proteins are generally not exchanged across capillary walls.

Question 42

How does the presence of protein in the plasma contribute to bulk flow, and why is its osmotic pressure significant?

Answer 42

The presence of protein in the plasma creates an osmotic pressure known as plasma-colloid osmotic pressure (πp), which tries to bring fluid into the capillary lumen. This osmotic pressure remains significant as proteins are generally not exchanged across capillary walls.

Question 43

What is capillary blood pressure?

Answer 43

Capillary blood pressure (Pc) is the hydrostatic pressure exerted by the blood on the walls of the capillaries. It represents the force that tends to push fluid out of the capillaries and into the surrounding tissues. Capillary blood pressure is highest at the arteriolar end of the capillaries when blood first enters them, and it decreases as blood moves through the capillary network, reaching its lowest value at the venular end.

On average, when blood enters the capillaries, the capillary blood pressure is around 37 mmHg. However, by the end of the capillaries, the hydrostatic pressure decreases to around 17 mmHg. This pressure gradient is essential for facilitating the exchange of nutrients, gases, and waste products between the blood and the surrounding tissues across the capillary walls.

Question 44

what is interstitial fluid colloid osmotic pressure?

Answer 44

Interstitial fluid colloid osmotic pressure (πIF) is the osmotic pressure created by the presence of proteins in the interstitial fluid, the fluid that surrounds and bathes the cells in the tissues outside the blood vessels. This force is involved in the movement of fluid between the capillaries and the interstitial space.

Under normal conditions, there is typically no free protein in the interstitial fluid. Any small amount of protein that might leave the capillaries is usually taken up by the lymphatic system. Therefore, for practical purposes, interstitial fluid colloid osmotic pressure is often considered to be approximately 0 mmHg. This means that, under normal circumstances, proteins do not exert a significant osmotic pressure in the interstitial fluid to drive fluid movement.

Question 45

What is interstitial fluid hydrostatic pressure?

Answer 45

Interstitial fluid hydrostatic pressure (PIF) is the hydrostatic pressure exerted on the outside of the capillary walls by the fluids in the interstitial space, the fluid that surrounds the cells in the tissues outside the blood vessels. This pressure represents the force that tends to move fluid into the capillary from the interstitial space.

There is some controversy over the contribution of interstitial fluid hydrostatic pressure to the overall bulk flow across capillary walls, primarily because it is challenging to measure accurately. In many discussions, its contribution is considered marginal, with an estimated value of around 1 mmHg. This suggests that, compared to other forces like capillary blood pressure and plasma-colloid osmotic pressure, interstitial fluid hydrostatic pressure may have a limited role in driving the movement of fluids across capillary walls.

Question 46

What is the formula for calculating net exchange pressure across capillary walls?

Answer 46

Net exchange pressure = (Outward Pressure - Inward Pressure) = (Pc + πIF) - (πp + PIF)

Question 47

Define Pc, and what does it represent in the context of capillary exchange?

Answer 47

Pc is capillary blood pressure. It represents the hydrostatic pressure exerted by the blood on the capillary walls, trying to force fluid out of the capillaries.

Question 48

Define πIF and what force does it represent in capillary exchange?

Answer 48

πIF is interstitial fluid colloid osmotic pressure. It represents the osmotic pressure created by the presence of proteins in the interstitial fluid, wanting to move fluid out of the capillary.

Question 49

Explain the physiological significance of a negative net exchange pressure at the end of a capillary.

Answer 49

A negative net exchange pressure at the end of a capillary (e.g. -9 mmHg) indicates reabsorption, with fluid moving from the interstitial space into the plasma. This is physiologically significant as it helps prevent excessive fluid loss from the bloodstream into the surrounding tissues.

Question 50

What is the primary purpose of bulk flow in the context of capillary exchange?

Answer 50

The primary purpose of bulk flow is to facilitate the delivery of nutrients and the removal of wastes from the tissues.

Question 51

Why is it important to understand that the balance of filtration and reabsorption within a single capillary is variable?

Answer 51

The balance of filtration and reabsorption within a single capillary is variable, and it can change over time based on circumstances. Understanding this variability is crucial for comprehending capillary exchange dynamics.

Question 52

Explain a scenario under which filtration could occur along the entire length of a capillary.

Answer 52

Filtration could occur along the entire length of a capillary when capillary blood pressure (Pc) is high enough under specific circumstances.

Question 53

In what situation could reabsorption occur along the entire length of a capillary?

Answer 53

Reabsorption could occur along the entire length of a capillary when capillary blood pressure (Pc) is low enough, meeting certain conditions.

Question 54

Discuss the importance of understanding the dynamic nature of the balance between filtration and reabsorption.

Answer 54

Understanding the dynamic nature of the balance between filtration and reabsorption is crucial as it allows for a nuanced comprehension of capillary exchange, where the forces involved can shift under different physiological conditions.