6. Microcirculation, venous blood flow and venous return

Question 1

What is the interstitium and what does it contain?

Answer 1

Fluid filled space between a structural barrier and internal structures.

Contains interstitial fluid

Question 2

Describe interstitial fluid

Answer 2

Fluid trapped amongst filaments; “tissue gel”

Question 3

Crystalloids

Answer 3

Low molecular water solutes

Question 4

Colloids

Answer 4

Plasma proteins

Question 5

Diffusion

Answer 5

Net movement of nutrients, oxygen and metabolic end products

Question 6

Bulk Flow

Answer 6

Distribution of extracellular fluid

Question 7

What is oncotic pressure?

Answer 7

Movement of water with pressure generated by proteins (attractive force so has a suction effect)

Question 8

What is oncotic pressure generated by?

Answer 8

By albumin and globulin (to a lesser effect)

Question 9

Function of plasma oncotic pressure

Answer 9

Draws fluid into capillaries

Question 10

What determines the movement of colloids?

Answer 10

Permeability of capillary membrane

Question 11

How does capillary hydrostatic pressure work?

Answer 11

Forces fluid out of the capillaries into the intersitium (high hydrostatic pressure)

Question 12

How does capillry hydrostatic pressure change from the arterial end to the venous end of the capillaries?

Answer 12

Drops from 30-40mmHg in the arterial to 10-15mmHg in the venous

Question 13

How does interstitial hydrostatic pressure work? And what is its significance

Answer 13

• Positive forces fluid into the capillaries
• Negative draws fluid into the interstitum
• Negligible effect in most tissue

Question 14

Explain Starling’s Forces (diagram)

Answer 14

• Hydrostatic = OUT
• Oncotic = IN

Question 15

How does oncotic pressure vary along the capillary?

Answer 15

Remains constant

Question 16

What is the overall net movement of fluid in the capillaries?

Answer 16

Overall net movement = OUT (3-4mmHg)

Question 17

How is fluid drained back into the venous system?

Answer 17

Lymphatics

Question 18

Describe the lymphatic system

Answer 18

Large fenestrated walls of capillaries drain via lymphatic vessels and pass through lymph nodes

Question 19

What is the lymphatic system important for controlling?

Answer 19

• Concentration of proteins in interstitial fluid
• volume of interstitial fluid
• Interstitial fluid pressure
• immune response

Question 20

Systemic venous circulation can be described as…

Answer 20

A low pressure and high volume system

Question 21

What is a major determinant of cardiac output?

Answer 21

Venous return to the heart (FSM)

Question 22

What is responsible for influencing venous return?

Answer 22

• Sympathetic innervation
• Muscle pumps
• Inspiratory movements
• blood volume

Question 23

How do inspiratory movements affect venous return?

Answer 23

• Diaphragm descends increasing abdominal pressure, which is passively transmitted to intraabdominal veins
• Decreased pressure in the thorax causes the pressure in intrathoracic veins and RA to decrease
• Pressure difference between veins and heart = easier for blood to enter the heart

Question 24

How does sympathetic innervation influence venous return?

Answer 24

Increase venous return to the heart (increased CO)

Question 25

What is the function of vein valves?

Answer 25

Move blood back from peripheral regions and prevent back flow of blood

Question 26

Explain orthostatic (postural) hypotension

Answer 26

Supine —> upright causes blood to pool in the legs

Decreased venous return, CO and MABP