Microcirculation, venous blood flow and venous return

Question 1

What are the three types of capillaries as defined by the type of membrane and method of inter membranous transport?

Answer 1

Sinusiod or discontinuous capillary - large molecules may pass through (rbc etc)

Fenestrated capillary - Small lipophobic molecules may pass through

Continuous capillary - Diffusion of various molecules at different speeds (large lipophobic molecules pass through fused invaginations v slow, small lipophobic molecules pass through intercellular cleft slowly, gases pass through membrane v quick, etc

Question 2

What makes up the interstitium?

Answer 2

it is the gap between cells, the non fluid parts are made up of mostly collagen and proteoglycan filaments

Question 3

What makes up the interstitial fluid?

Answer 3

• Fluid trapped amongst filaments
• “Tissue gel”
• ~ 1% of water “free”
• Diffusion occurs in gel ~95-99% as rapidly in free fluid

Question 4

Make a card abt the transfer slides

Answer 4

.

Question 5

What is oncotic pressure?

Answer 5

also known as colloid osmotic pressure, a type of osmotic pressure from proteins (albumin being the largest example) in the plasma

Question 6

Discuss oncotic pressure

Answer 6

Capillary wall is (generally) a barrier to proteins
– Readily permeable to water and most solutes – Not a perfect filter
• Permeability for albumin is 1/1000th that of water
Oncotic pressure generated by plasma proteins
– ~28mmHg
– Predominately generated by albumin, lesser extent by
globulins
Plasma oncotic pressure draws fluid in to capillaries
– Interstitial oncotic pressure is much lower (~5-8mmHg)

Question 7

Discuss hydrostatic pressure

Answer 7

Capillary hydrostatic pressure
– Forces fluid out of the capillaries and in to the interstitium
– Drops from arterial end to venous end
• Pressure at arterial end ~30-40mmHg
• Pressure at venous end ~10-15mmHg
Interstitial hydrostatic pressure
– Forces fluid in to the capillary when positive
– Draws fluid in to the interstitium when negative
Remember: flow to capillaries fluctuates
– Changes in hydrostatic pressure follows, affects fluid flow
– When averaged over time and capillaries, general observations hold true

Question 8

What are Starling forces?

Answer 8

Not the way power lines sag with a flock of starlings

Forces to do with the Starling equation which relates to the flow of fluids across a semipermeable membrane

4 forces:

Capillary pressure (Pc)(out of capillary)
Plasma colloid osmotic pressure (IIp)(into capillary)
Interstitial fluid pressure (Pif)(into capillary)
Interstitial fluid colloid osmotic pressure (IIif)(out of capillary)

Question 9

What impact do starling forces have on absorption/filtration along a capillary and how do they change?

Answer 9

All will remain the same other than capillary pressure, which will drop.

This imbalance means that while filtration (out of capillary) will be favoured initially, by aruond 1mm from the arteriole absorption will be favoured by the capillary

Question 10

Discuss the lymphatic system in relation to the microcirculation

Answer 10

Capillaries lose more water than they gain
• Approx. 2-3L per day
Lymphatic system
• Large, fenestrated walls of capillaries
• Drain via lymphatic vessels
• Pass through lymph nodes
Important in controlling:
• Concentration of proteins in interstitial fluids
• Volume of interstitial fluid
• Interstitial fluid pressure
• Also in immune response

Question 11

Discuss the basic features of the systemic venous circulation

Answer 11

Low pressure system
– Between 3-18mmHg 
High volume system
– Holds ~60% of total blood volume
Venous return to the heart is a major determinant of cardiac output
– Frank-Starling mechanism

Question 12

What is the Frank-Starling law?

Answer 12

The Frank-Starling Law is the description of cardiac hemodynamics as it relates to myocyte stretch and contractility. The Frank-Starling Law states that the stroke volume of the left ventricle will increase as the left ventricular volume increases due to the myocyte stretch causing a more forceful systolic contraction.

Question 13

Describe the mechanisms by which venous return occurs

Answer 13

Sympathetic innervation

Muscle pumps

Inspiratory movements
– Diaphragm descends
• ↑ abdominal pressure
• Transmitted passively to intra abdominal veins
– ↓ Pressure in thorax
• ↓pressure in intrathoracic veins and right atrium
– Therefore ↑pressure difference between peripheral veins and heart

Blood volume
– E.g. Hemorrhage, fluid challenge

Question 14

Describe the action of sympathetic innervation of veins

Answer 14

Sympathetic innervation of veins increases venous return to the heart therefore increases cardiac output
– Important in exercise, blood loss etc

Question 15

Describe the postural effects on the venous system

Answer 15

Standing completely still
– Pressure ↑ by 1mmHg for each 13.6mm below the surface
• By feet +90mmHg
– Mean arterial pressure at
level of heart ~100mmHg
– So, in feet ~190mmHg
– Leg oedema
• 10-20% of blood volume within 15-30min
But... venous valves and ‘venous pump’

Question 16

What is meant by the term muscle pumps in the veins?

Answer 16

When veins pass by a muscle, it may contract to press on the walls of the vein and so increase venous pressure to push blood up through the valves

Question 17

Describe the impact of postural changes in hydrostatic pressure

Answer 17

Orthostatic (postural) hypotension
– Immediate effect in going from supine to upright
• Around 500 ml of blood from the upper body to legs
• ↓ venous return
– Therefore ↓ cardiac output
– Therefore ↓ blood pressure

– Reflex vasoconstriction in legs and lower abdomen
• Takes a few seconds to kick in

Question 18

Main points

Answer 18

Fluid forces favour small amounts of loss into tissue space, reclaimed as lymph
The venous system is high-volume, low pressure system
Compliance of veins can be adjusted by sympathetic innervation
Venous return limits cardiac output

Question 19

Learning outcomes

Answer 19

To identify the major routes across capillary membranes of fluids, solutes and larger molecules/proteins.
To explain how Starling’s forces contribute to fluid homeostasis and the net transcapillary movement of water across capillary beds, including the importance of the lymphatic system.
To describe the factors which affect venous return and consequently determine cardiac output and blood pressure.