6. Microcirculation, venous blood flow and venous return Flashcards
(26 cards)
What is the interstitium and what does it contain?
Fluid filled space between a structural barrier and internal structures.
Contains interstitial fluid
Describe interstitial fluid
Fluid trapped amongst filaments; “tissue gel”
Crystalloids
Low molecular water solutes
Colloids
Plasma proteins
Diffusion
Net movement of nutrients, oxygen and metabolic end products
Bulk Flow
Distribution of extracellular fluid
What is oncotic pressure?
Movement of water with pressure generated by proteins (attractive force so has a suction effect)
What is oncotic pressure generated by?
By albumin and globulin (to a lesser effect)
Function of plasma oncotic pressure
Draws fluid into capillaries
What determines the movement of colloids?
Permeability of capillary membrane
How does capillary hydrostatic pressure work?
Forces fluid out of the capillaries into the intersitium (high hydrostatic pressure)
How does capillry hydrostatic pressure change from the arterial end to the venous end of the capillaries?
Drops from 30-40mmHg in the arterial to 10-15mmHg in the venous
How does interstitial hydrostatic pressure work? And what is its significance
- Positive forces fluid into the capillaries
- Negative draws fluid into the interstitum
- Negligible effect in most tissue
Explain Starling’s Forces (diagram)
- Hydrostatic = OUT
- Oncotic = IN
How does oncotic pressure vary along the capillary?
Remains constant
What is the overall net movement of fluid in the capillaries?
Overall net movement = OUT (3-4mmHg)
How is fluid drained back into the venous system?
Lymphatics
Describe the lymphatic system
Large fenestrated walls of capillaries drain via lymphatic vessels and pass through lymph nodes
What is the lymphatic system important for controlling?
- Concentration of proteins in interstitial fluid
- volume of interstitial fluid
- Interstitial fluid pressure
- immune response
Systemic venous circulation can be described as…
A low pressure and high volume system
What is a major determinant of cardiac output?
Venous return to the heart (FSM)
What is responsible for influencing venous return?
- Sympathetic innervation
- Muscle pumps
- Inspiratory movements
- blood volume
How do inspiratory movements affect venous return?
- 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
How does sympathetic innervation influence venous return?
Increase venous return to the heart (increased CO)
