PHYSIOLOGY

2. By
Dr Ahmed El_Sayed Said Nour
Eden
Lecturer of Physiology Department
Faculty of Medicine Al Azhar University(Assiut)
PHSIOLOGY OF THE
CARDIOVASCULAR
SYSTEM (CVS)

3. VENOUS
CIRCULATION

5. Functions of veins
1) Drainage of blood from all parts of the body to the heart.
2) They act as blood reservoirs→ contain about 3 liters of
blood.
3) Venous pump → aids in propelling the blood forward (towards
the heart) and helps to regulate the COP.

6. Venous Pressure
It is the pressure inside veins ) 10-11 mmHg(
a) Hydrostatic indifferent level or point (HIP):
• It lies 5-7 cm below the diaphragm, at which the venous pressure (VP) is kept
constant, independent of the body posture.
• VP below this point ↑es and above this point ↓es.
b) Peripheral venous pressure (PVP): It is VP in peripheral veins.
• It depends upon the site of vein and the gravity.
• Above right atrium→ VP is subatmospehric but intrathoracic veins are not collapsed
due to the -veintrathoracic pressure. and vice versa.
c) Central venous pressure (CVP)
• It is VP in big veins at right atrium (intrathoracic portions of vena cavae).
• It averages 4-6 mmHg in recumbancy and 2 mmHg on standing.
• It is 2 mmHg during inspiration and 6 mmHg during expiration

7. Measurement of VP
• A) Direct method:
• By a cardiac catheter introduced into the thoracic
large veins→ connected to a saline manometer.
• Patient must be recumbent, and under anaesthesia

8. Measurement of VP
B). Indirect method:
•Patient lies in a semisitting position with the head raised 45° and
the degree of filling of the external jugular vein is noted.
•Normally these veins are only distended at their lower 1/3
indicated by the sternal angle.
•The level of vein distension is ↓ed when the CVP is ↓ed e.g.
haemorrhage.
•The level of vein distension is ↑ed when the CVP is ↑ed e.g. CHF

10. Determinants of venous pressure
• Contraction of venous smooth muscle
– Sympathetic neurons
– Hormonal and paracrine vasodilators and
vasoconstrictors
• Skeletal muscle pump
• Respiratory pump

11. Factors affecting CVP
1) Gravity:
- In recumbent position → Gravity has no effect on circulation.
- On standing,
 PVP is ↑ed in lower limbs to 90 mmHg in feet veins
 CVP is ↓ed from 4-6 mmHg to 2 mmHg→↓ VR and the cardiac
filling.
 Also the COP and ABP are ↓ed (postural hypotension) in
prolonged standing.

12. Factors affecting CVP
1) Gravity:
This effect is compensated by;
a) Vasoconstriction → Corrects the fall of VP above the heart and helps VR
to prevent stagnation of blood in veins of the lower limbs.
b)Skeletal ms tone → Helps the VR (peripheral hearts) and ↓es PVP.
c) Capillary tone → Helps the VR and ↓es PVP
d)Respiratory movement → ↑es pressure gradient and helps the VR by
↑ing the +ve intraabdominal pressure and the –ve intrathoracic pressure→ ↓
PVP.
e) Arterial pulsation → Helps the VR.
 Presence of valves in veins of the lower limbs → prevents over distension of big
veins and regurgitation of blood in veins

13. If the PVP ↑ due to gravity is not corrected, it leads
to:
A) Filtration of fluid from the blood to the tissues → Edema in
lower limbs.
B) Distended veins accommodate a large portion of blood
volume →↓ The VR, COP and ABP (postural hypotension).
C) Varicose veins occur when the valves are damaged and unable to
divide blood column into segments., It is common in the veins of
legs and scrotum.

14. Factors affecting CVP
2) Rate of blood inflow into veins:
-VD of arterioles without capillary dilatation →↑inflow
into veins →↑es CVP and PVP.
-If the capillaries are dilated→↓ the blood inflow to
veins→ ↓ CVP and PVP.
3) Rate of blood outflow from veins:
 If the outflow is less than the inflow, the VR is ↓ed and
consequently the CVP and PVP are ↑ed e.g.
• Rt side heart failure or local venous obstruction.
• ↑ed +ve intrathoracic pressure (straining).

15. Factors affecting CVP
4) Venomotor tone:
• Tonic contraction in veins, maintains normal VP.
• Venous tone is ↑ed reflexly in haemorrhage to maintain
VP.
• ↑ Venous tone by symp. stimulation or noradrenaline→↑
CVP and PVP
• ↓ venous tone by α- adrenergic blockers→↓ CVP and PVP.

16. Factors affecting CVP
5) Blood volume:
• CVP and PVP are ↑ed in hypervolemia (blood
transfusion).
• CVP and PVP are ↓ed in hypovolaemia (haemorrhage)
6) Muscular exercise:
 - Contracting skeletal muscles press on veins, ↑ing the VP.
 - Arteriolar VD of active muscles, ↑ing the VP.

17. Factors affecting CVP
7) Respiratory movement:
- Inspiration:
 The diaphragm descends, the VP in abdominal veins is
↑ed and the VP in intrathoracic veins is ↓ed →↑ venous
return → ↓VP to the normal level.
- Expiration:
 The opposite effect occurs.

18. Factors affecting CVP
8) Acceleration force:
• Constant speed has no effect on the circulation.
• Anteroposterior acceleration has no effect on the circulation.
• Acceleration in the direction of longitudinal axis of the body → blacking out or reddening out.
a) Blacking out:
 When the head is upward, the blood is drained to the lower limbs→ no blood goes to the head and
temporary blindness (blackout) occurs i.e. blood is drained from the retinal vessels of the eyes and loss
of consciousness (fainting) occurs due to cerebral ischemia.
 -It occurs in:
1. Jet plane pilots (during ascent).
2. Astronauts.
b) Reddening out:
 When the head is downward, the blood is shifted from the lower limbs to the head → marked
congestion of the eyes occurs and the fields of vision become suddenly bright red with temporary loss
of vision (reddening out).
 -It occurs in Pilots during descent.

19. Venous Pulsations
• The pulse of a vein usually palpated over the internal or external
jugular veins in the neck.
• Jugular veins pulsation→ transmitted from the Rt atrium to the
jugular vein
• Veins show no pulsations→ no pulse pressure (no pulse
waves).
• However veins show pulsations in marked arteriolar VD with
conduction of pulse pressure into the corresponding veins.
• Jugular venous pressure is a clinical measure of central venous
pressure.

20. Venous Pulsations
The jugular venous pulse
1. Has two waves for every single carotid artery pulsation; make
this comparison by gently palpating the carotid pulse on the
opposite side of the neck;
2. Falls with inspiration and rises with expiration
3. Can be obliterated by pressing firmly but gently with the back of
the index finger placed horizontally just above the clavicle at the
root of the neck;
4. May be visible only when the patient is lying flat (in cases of
hypovolaemia) or when the patient is sitting upright at 90 (for
example, in severe congestive cardiac failure);
5. Is usually impalpable.

21. Elevated jugular venous pressure:
1. -Right ventricular failure (eg, cardiomyopathy)
2. -Restriction of right atrial and right ventricular filling (eg,
corpulmonale, pulmonary hypertension, constrictive
pericarditis)
3. -Fluid overload due to renal disease (eg, poststreptococcal
glomerulonephritis)
4. -Tricuspid valve incompetence
5. -Functional and organic obstruction of the tricuspid valve
6. -Superior vena cava obstruction.
7. -Bilateral elevation of the mean jugular venous pressure in
the absence of venous pulsation should raise suspicion of
superior vena cava obstruction

22. Kussmaul's sign
• Lack of a decrease or an increase in jugular venous pressure
during inspiration, called Kussmaul's sign, is abnormal and is
observed in a number of conditions:
• the most common cause is severe heart failure
• The mechanism of Kussmaul's sign in these conditions is not
entirely clear. Increased resistance to right atrial filling during
inspiration appears to be a contributory factor.
Venous hum
• Venous hum is a functional murmur produced by turbulent
flow in the internal jugular vein. It is continuous (albeit louder
in diastole) and at times strong enough to be associated with a
palpable thrill.