Microcirculation, Venous Blood Flow and Return

Question 1

What would you find in the interstitium?

Answer 1

• collagen

- proteoglycan filaments

Question 2

What makes up the interstitial fluid?

Answer 2

• fluid trapped among filaments
• tissue gel
• 1% of free water

Question 3

What things are transferred across capillaries?

Answer 3

• crystalloids: low molecular weight solutes such as Na+, Cl- and K+
• colloids: plasma proteins

Question 4

What is bulk flow?

Answer 4

distribution of extracellular fluid

Question 5

Explain the generation of oncotic pressure in capillaries

Answer 5

• capillary is permeable to most things including water but not colloids
• because of this colloids cannot diffuse out
• high concentration in the capillary compared to outside
• oncotic pressure generated (mainly by albumin)
• results in the drawing of fluid into the capillaries which can lead to dehydration of the interstitial environment if occuring independently

Question 6

Explain the generation of hydrostatic pressure in capillaries

Answer 6

• hydrostatic pressure at arterial end is usually higher than interstitium which forces fluid out of capillaries
• the amount pushed out decreases towards the venous end as the pressure change decreases
• the interstitium generally has negligible hydrostatic pressure

Question 7

Explain Starling forces

Answer 7

• the ‘balancing’ of the hydrostatic and oncotic pressures in capillaries
• on the arterial end, hydrostatic pressure outweighs oncotic pressure so there is a net movement of fluid out
• on the venous end, hydrostatic pressure does not outweigh oncotic pressure so there is a net movement of fluid in
• the balance isn’t perfect and the excess fluid that remains is drained through the lymphatic system to be returned to the vascular system

Question 8

What is the lymphatic system?

Answer 8

large fenestrated walls of capillaries

Question 9

What does the lymphatic system control?

Answer 9

• concentration of proteins in interstitial fluids
• volume of interstitial fluid
• interstitial fluid pressure
• immune response

Question 10

Systemic venous circulation

Answer 10

• low pressure
• high volume
• venous return to the heart is the major determinant of cardiac output

Question 11

What are the factors that increase venous return to the heart when increased?

Answer 11

• sympathetic innvervation
• muscle pumps
• inspiration
• blood volume

Question 12

Explain how inspiration affects venous return

Answer 12

• diaphragm descends which increases abdominal pressure that is transmitted to the veins
• this decreases the pressure in the thorax
• decreases pressure in intrathoracic veins and right atrium
• so increases the pressure difference between the peripheral veins and heart leading to increased blood volume squeezed out for return

Question 13

Explain the effect standing still has on venous pressure

Answer 13

• pressure increases as venous blood starts to flow downwards in the body due to gravity
• feet having the highest pressure
• can result in leg oedema if standing for an extended period of time

Question 14

Explain orthostatic hypotension

Answer 14

• effect of standing quickly after lying down
• blood going from upper body to legs
• decreased venous return
• decreased cardiac output
• decreases blood pressure
• results in reflex vasoconstriction in the legs and lower abdomen