March 3 - Capillaries and Veins

Question 1

What are capillaries?

Answer 1

They are tubes of endothelial cells that branch from arterioles. They are the sites of oxygen, carbon dioxide and nutrient exchange between the blood and cells

Question 2

How does exchange occur?

Answer 2

Via diffusion, enhanced by large surface area and increased time for diffusion (low blood flow velocity)

Question 3

Describe the capillary network

Answer 3

It has a large surface area. And relative to other segments of circulation, the capillary cross-sectional area is high

Question 4

What is the resistance to flow like in the capillaries?

Answer 4

Compared to arterioles, the resistance to flow (R) offered by capillaries is low due to the large cross-sectional area (600 square meters)

Question 5

How much blood is found in the capillaries?

Answer 5

Only 250 ml of the 5000 ml total blood volume is found in the capillary beds

Question 6

How does velocity and blood flow rate change?

Answer 6

Velocity of blood flow changes in different portions of the vasculature (it’s lowest in the capillaries). Total blood flow rate does not change.

Question 7

Describe capillary pores

Answer 7

Capillary pores are water-filled clefts in the capillary walls. Pores enable the passage of K, Na, and amino acids. Lipid-soluble substances (oxygen, carbon dioxide) diffuse through the bilayer. Pore are present at cellular junctions between endothelial cells

Question 8

How does capillary pore size vary?

Answer 8

Capillary pores size differs with tissue type; liver capillary pores are large, whereas brain capillary pores are non-existent. Proteins are generally excluded except in liver capillaries

Question 9

How does histamine affect capillary pores?

Answer 9

Histamine activates actin/myosin contraction and thus opens capillary pores (protein leakage may occur)

Question 10

What are metarterioles?

Answer 10

A metarteriole is a throughfare channel from an arteriole to a venule

Question 11

Under resting conditions, many capillaries don’t move much blood. Explain

Answer 11

Precapillary sphincters surround capillaries. When smooth muscle contraction reduces blood flowing into the capillary bed, myogenic tone of smooth muscle is high. Smooth muscle is sensitive to local metabolic changes (e.g., high carbon dioxide, low oxygen) provokes vasodilation and increased numbers of open capillaries (increased number of open precapillary sphincters). Ultimately, this leads to increased oxygen delivery and enhanced carbon dioxide removal.

Question 12

What is the importance of interstitial fluid (IF)?

Answer 12

The IF is an intermediary between the blood and tissue cells. Exchange between RBCs and traget cells occurs through the IF

Question 13

20% of all extracellular fluid is blood plasma while 80% is IF. The passage of plasma proteins is very limited between plasma and IF. However, solute exchange may occur in two modes. Explain

Answer 13

1. Passive exchange (of glucose, oxygen and carbon dioxide) between the IF and plasma membranes of cells (e.g., diffusion down concentration gradients)
2. Bulk flow. Bulk flow = movement of water and a number of solutes cross the capillary wall (via pores - Na, K, glucose and amino acids). Constituents in a fluid move through in bulk (no discrete movement of single components). Determines the distribution of ECF volume between vasculature and IF compartments

Question 14

What is ultrafiltration?

Answer 14

Fluid (ultrafiltrate) moves from the capillary lumen (site of higher hydrostatic pressure = p) to the interstitial fluid outside the capillary. Plasma proteins remain in the capillary by this process, unable to pass through the pores in the capillary wall

Question 15

What is reabsorption?

Answer 15

Reverse bulk flow (IF to capillary lumen): inward-driving pressure exceeds an outward opposing pressure across the capillary wall

Question 16

What is the driving force for bulk flow?

Answer 16

Capillary blood (hydrostatic fluid) pressure
Colloid osmotic pressure (oncotic pressure)

Question 17

What is Pc?

Answer 17

Capillary blood pressure is the hydrostatic pressure exerted on the inside of the capillary walls by the blood

Question 18

What is plasma colloid osmotic pressure?

Answer 18

Represented by pi p. Also known as the oncotic pressure. It is the pressure exerted by dispersed plasma proteins. It encourages fluid movement into the capillary lumen (inward pressure). More protein in plasma compared to IF. Plasma proteins attract water.

Question 19

What is Pif?

Answer 19

Interstital fluid hydrostatic pressure (1 mm Hg)

Question 20

What is interstital oncotic pressure?

Answer 20

Represented by pi if. It is the pressure due to proteins in the IF - this is usually negligible

Question 21

What is the ultrafiltration pressure?

Answer 21

The net outward pressure of 11 mmHg caused by the forces at the arteriolar end of capillaries. It is the different between Pc + interstitial oncotic pressure (outward pressure; 37 mm Hg) and Pif + plasma colloid osmotic pressure (inward pressure; 26 mmHg)

Question 22

What is the reabsorption pressure?

Answer 22

The net inward pressure of 9 mm Hg caused by the forces at the venular end of hte capillaries. It is the different between Pc + interstitial oncotic pressure (outward pressure; 17 mmHg) and Pif + plasma colloid osmotic pressure (inward pressure; 26 mmHg)

Question 23

What is the significance of bulk flow?

Answer 23

It regulates the distribution of ECF between plasma and IF. It is for the maintenance of pressure, i.e., hemmorrhage (loss of pressure in the capillaries in the luman will favour reabsorption). Both ultrafiltration and reabsorption occur by bulk flow. Fluid moves by a passive process

Question 24

The difference between the ultrafiltration pressure and the reabsorption pressure is 2 mmHg (11 mmHg - 9 mmHg). What is the result?

Answer 24

This creates extra fluid that needs to be removed by the lymphatics

Question 25

What is lymph?

Answer 25

Lymph is interstitial fluid that enters a lymphatic vessel. This fluid contains escaped plasma proteins and bacteria that are not reclaimed by the blood plasma.

Question 26

How does lymph get into the lymphatic vessels?

Answer 26

Skeletal muscle are smooth muscle propels lymph into larger lymphatic vessels. The lymph is eventually combined with venous blood near the heart

Question 27

What are the functions of the lymphatic system?

Answer 27

Defense against disease by the lymph nodes (defense against the dark arts)
Transport of absorbed fat from digestive tract

Question 28

What is oedema?

Answer 28

It occurs when there is excessive interstitial fluid in tissues

Question 29

What does oedema cause?

Answer 29

Reduced concentration of plasma proteins (starvation)
Increased permeability of the capillary walls. It allows more plasma proteins to pass from the blood fluid into the interstitial fluid
Increased venous pressure (blood dams up in the venous side, increases the venular capillary hydrostatic pressure = less reabsorption)
Blockage of lymph vessels. Filiarisis = elephantiasis; parasites that invades the lymph

Question 30

What are veins?

Answer 30

Blood flows from the capillaries to the veins. Veins collect blood for its return to the heart. They have large radii and offer low resistance to flow of blood. Compared to arterires, veins are thin-walled - thus they can be distended. Veins are capacitance vessels, serving as a large blood reservoir (they hold approx 60% of the blood volume)

Question 31

What is venous return?

Answer 31

Venous return is the volume of blood entering each atrium per minute.

Question 32

What is the mean pressure of the venous circulation?

Answer 32

17 mmHg

Question 33

What are five major factors for enhancement of venous return?

Answer 33

1. Increased sympathetic stimulation of the veins; contraction of the smooth muscle of the veins = venoconstriction. Stored blood is mobilized to the heart. Alpha-adrenoceptors mediate smooth muscle cell contraction
2. Increased skeletal muscle contraction - this compresses veins and increases venous pressure/venous return. In exercise and to counter the effect of gravity. Gravity contributes a great deal to absolute pressure (17 mmHg) + (0.8 x 100 cm) = 97 mmHg
3. Valves inside veins prevent return of blood to point of constriction by muscle pump - blood does not flow backward
4. Cardiac suction. Heart creates suction by expansion of the ventricular chamber due to the presence of “internal resistor” within the cardiac extracellular matrix
5. Respiration (breating) creates subatmospheric pressure during inspiration in the chest cavity (5 mmHg less), drawing blood toward the heart