Microcirculation

Question 1

What forms the intersitium?

Answer 1

Collagen and proteoglycan filaments

Question 2

What are the different types of capillaries and what diffuses through them?

Answer 2

Discontinuous - Large proteins and RBC.
Fenestrated - Small lipophobic molecules.
Continuous - Fast movement of gases and lipophilic molecules and slow diffusion of lipophobic molecules

Question 3

What are crystalloids?

Answer 3

Low molecular weight solutes such as sodium, calcium and potassium

Question 4

What are colloids?

Answer 4

Plasma proteins like albumin

Question 5

Define diffusion and bulk flow

Answer 5

D - Net movement of nutrients, oxygen and metabolic end products.
B - Distribution of extracellular fluid

Question 6

What is oncotic pressure?

Answer 6

Pressure generated by crystalloids and colloids

Question 7

Describe how oncotic pressure arises

Answer 7

The capillary wall generally is a barrier to proteins so there is a high concentration of colloids within the capillary which generates the oncotic pressure which draws fluid into capillaries as the interstitial oncotic pressure is much lower.

Question 8

Describe the capillary hydrostatic pressure

Answer 8

It forces fluid out of the capillaries into the intersitium. The pressure at the arterial end is much higher than at the venous end.

Question 9

Describe the interstital hydrostatic pressure

Answer 9

Normally it is negligible but if its positive then it forces fluid into the capillary and when it is negative it draws fluid into the interstitium.

Question 10

Describe how starlings forces generate a net loss of fluid from capillaries.

Answer 10

Oncotic pressure in interstitum means that roughly the same amount of fluid is pulled out of the capillary from arterial to venous end but more fluid would be drawn into the capillary due to capillary oncotic pressure. However as the hydrostatic pressure is greater at the arterial end than venous end to more fluid is pushed out at the arterial end. Even though more fluid is absorbed at venous end, there is still a net loss of fluid.

Question 11

What happens to the net loss of fluid from capillaries?

Answer 11

It is drained via the lymphatic system and eventually returned to the circulatory system

Question 12

Roughly describe the lymphatic system and what it is important in controlling

Answer 12

LS - Made of large fenestrated walled capillaries which drain via lymphatic vessels and pass through LNs. It is important in controlling proteins in intersitital fluids, volume of intersitital fluids , intersitital fluid pressure and in the immune response.

Question 13

What are some basic features of the systemic venous system that are important to remember

Answer 13

Low pressure system (3-18mmHg), High volume system (holds 60% of blood volume), and the venous return to the heart is a major determinant of cardiac output.

Question 14

Describe what effects the venous return to the heart.

Answer 14

Sympathetic innervation (increases venous return to heart which therefore increases cardiac output), Muscle pumps, Inspiratory movements and blood volume (hemorrhage or fluid challange)

Question 15

Describe the two ways in which inspiratory movements impact venous return

Answer 15

• As the diaphragm descends it increases abdominal pressure which pushed blood from organs back to the heart.
• As the pressure in the thorax decreases there is a decrease in the pressure of intrathoracic veins and right atrium and therefore pressure difference between peripheral veins and heart

Question 16

Describe the effects of postural changes in hydrostatic pressure

Answer 16

It can cause orthostatic hypotension. This occurs when going from supine to upright. around 500ml of blood goes to legs which decreases venous return and therefore decreases cardiac output and blood pressure. There is a vasoconstriction reflex in the legs and lower abdomen that takes a few seconds to kick in, in elderly patients this can take longer to kick in and can be a reason for falls.