Venous Circulation Flashcards
Explain the functions of veins
- Forward function, change their capacitance to maintain cardiac filling & cardiac output
- Backward function, drain the capillariesat lowest pressure as their resustance is minimal.
Explain the functional characteristics of veins
- Veins are large capauciatnce vessels act as blood reservoir, accomodate 60% of blood
- Veins can accomodate inc in volume with minimal inc in pressure
- More filling of veins causes stretch & marked inc in pressure and they become as stiff as arteries
- Veins can tolerate high pressure without blow
- Sympathetic innervation is less to veins than arteries
GR: Veins can inc in volume without inc in pressure, describe the shape progression
Because they have reserve in their shape
They are normally flattened except at periphery, then become elliptical then oval then circular with inc filling
GR; The behaviour of veins in becoming stiff as art when subjected to marked filling is protective
Mobilizqtion of blood towards heart
Inc capillary hydrostatic pressure and inc filtration from capillaries to minimize circulatiry overload
GR: Veins can tolerate high pressure
Mention a surgical app
Because collagen, the principal component in their walls is extremely strong
They are used as arterial grafts
Describe the role of sympathetic on veins
Venoconstriction:
A. Shift of the pressure/volume relation to the left
B. Decreased capacitance of veins with more mobilization of blood towards heart
Mention the components of venous pressure
Mean circulatory pressure
The added pressure due to activity of the heart
Define CVP
It is the pressure in right atrium & big veins in the thorax = zero.
Define MSFP
It reoresents the degree of filling of the heart and all vessels with blood, it depends on the volume of blood in relation to CV capacitance, can be measured in any vessel when heart is stopped, noemally abou 7 mmHg.
Mention the values of pressure in differenet components of venous system
- Venules, 10-12 mmHg
- Small veins, 8-10 mmHg
- Veins, 6-8 mmHg
- Vena cava, 0-2 mmHg
The speed of blood in venae cavae is …..
25 cm/sec
The reference point is a circular line at level of .,,,.
4th sternocostal joint
What happens when pressure at the reference point is increased?
Inc vent filling & force of contraction, more blood is pumped out, which leads to dec pressure at this point.
GR: The big veins of thorax are not collapsed while neck veins are collapsed
Because the intrathoracic pressure is -ve while the pressure in neck veins is zero so they are collapsed by the atmospheric pressure.
GR: although venous sinuses of skull have -ve pressure they are not collapsed
Because the skull doesn’t transmit atmospheric pressure to them and their walls are made of tough fibrous tissue which keep them patent.