Venous blood pressure and capillary blood pressure
Venous Blood Pressure and Capillary Blood Pressure
Venous Blood Pressure • Factors Regulating Venous Blood Pressure • Effect of Respiration • Valsalva Maneuver • Müller Maneuver • Kussmaul’s Sign • Applied Physiology • Capillary Blood Pressure
Venous Blood Pressure
Definition
Venous blood pressure is the pressure exerted by contained blood in veins.
Pressure in vena cava and right atrium is called central venous pressure.
Pressure in peripheral veins is called peripheral venous pressure.
Pressure is not same in all the veins. It varies in different veins in extremities of the body and varies from central veins to peripheral veins.
Normal Values
Venous Blood Pressure in Extremities of the Body
Venous pressure is less in parts of the body above the level of heart and it is more in parts below the level of heart.
| Site | Venous Blood Pressure |
|---|---|
| Dorsal venous arch of foot | 13.2 mm Hg (17.9 cm H2O) |
| Jugular vein | 5.1 mm Hg (6.9 cm H2O) |
| Antecubital vein | 7.1 mm Hg (9.6 cm H2O) |
| Superior vena cava | 4.6 mm Hg (6.2 cm H2O) |
1 mm Hg pressure = 1.359 cm H2O pressure.
Venous Blood Pressure in Central and Peripheral Veins
Pressure is greater in peripheral veins than in central veins.
Factors Regulating Venous Blood Pressure
Left ventricular contraction is also called vis a tergo, meaning force from behind.
It forces the blood through arteries, arterioles, capillaries and veins to right atrium.
Venous blood pressure is directly proportional to left ventricular pressure.
By the time blood passes through capillaries and venules, pressure becomes less than 8 mm Hg when it reaches right atrium.
The pressure may be less than 1 mm Hg.
Right atrial pressure is also called vis a fronte or force from front.
It determines the venous return. It is also called central venous pressure. It regulates the peripheral venous pressure.
Normal right atrial pressure is 0 mm Hg.
Resistance offered to blood flow through the veins is also called vis a latere or force from side.
Venous pressure is directly proportional to resistance which is due to venous tone and extravascular factors.
Because of thin-walled nature, veins and venules are compressed by following extravascular factors:
- Compression of arm veins while passing over first rib.
- Compression of neck veins in erect posture due to fall in pressure and by atmospheric pressure.
- Compression of abdominal veins by increased intra-abdominal pressure.
- Compression of veins while passing in between the muscles.
Venous blood pressure is directly proportional to volume of blood in the venous system.
Venous blood pressure is inversely proportional to peripheral resistance.
When peripheral resistance is more, arterioles constrict and veins are filled with less blood. Hence, the pressure decreases.
When peripheral resistance is less, veins are filled with more blood and venous pressure increases.
Pressure is more in the veins below the level of heart and pressure is less in veins above the level of heart.
Weight of the column of blood in veins influences venous pressure. During prolonged standing, pressure in lower extremities is more (90 cm H2O).
It is because of pooling of blood in the lower extremities due to gravity. It increases the weight of the column of blood, leading to an increase in pressure.
During the movement, venous pressure in the foot decreases.
In head region, venous pressure is −10 cm H2O because of gravity. There is always a negative hydrostatic suction below the skull.
Effect of Respiration on Venous Blood Pressure
Effect of respiration on venous blood pressure is demonstrated by two procedures:
Valsalva Maneuver or Valsalva Experiment
Definition / Performance
Valsalva maneuver is the forced expiratory effort with closed glottis.
It is performed by attempting to exhale forcibly while closing the mouth and nose.
Effects of Valsalva Maneuver
During this maneuver, intrathoracic pressure increases greatly and becomes positive. It may reach +50 mm Hg.
Effects of High Intrathoracic Pressure
- Compression of central vein in thorax.
- Decrease in venous return to right atrium.
- Decrease in central venous pressure.
- Increase in peripheral venous blood pressure to about 30 cm H2O, due to accumulation of blood in peripheral veins such as veins of neck, face and limbs.
Uses of Valsalva Maneuver
- Valsalva maneuver is used as a diagnostic tool to evaluate the cardiovascular disorders.
- It is used to relieve chest pain.
- It is used to correct abnormal heart rhythms.
30 Seconds Endurance Test
The subject is asked to blow against sphygmomanometer, in which pressure is maintained at 40 mm Hg for 30 seconds.
Then the changes in heart rate, blood pressure or murmurs are observed to evaluate cardiovascular disorders.
Valsalva Maneuver vs Müller Maneuver
| Features | Valsalva Maneuver | Müller Maneuver |
|---|---|---|
| 1. Performance | Forced expiratory effort with closed glottis | Forced inspiratory effort with closed glottis |
| 2. Intrathoracic pressure | Increases up to +50 mm Hg | Decreases up to −70 mm Hg |
| 3. Central veins in thorax | Compressed | Dilated and blood rushes |
| 4. Venous return to right atrium | Decreases | Increases |
| 5. Peripheral venous blood pressure | Increases to 30 cm H2O | Decreases to 3 cm H2O |
| 6. Central venous blood pressure | Decreases | Increases |
| 7. Uses |
To evaluate cardiovascular disorders To relieve chest pain To correct abnormal heart rhythms |
To evaluate upper respiratory problems To evaluate sleep apnea syndrome |
Müller Maneuver or Müller Experiment
Definition / Performance
Müller maneuver or reverse Valsalva maneuver is the forced inspiratory effort with closed glottis.
It is performed by attempting to inhale forcibly, while closing the mouth and nose.
Effects of Müller Maneuver
During this maneuver, intrathoracic pressure decreases greatly and becomes more negative.
It is about −70 mm Hg.
Effects of More Negative Intrathoracic Pressure
- Dilation of right atrium and central vein because of increase in negative intrathoracic pressure.
- Rapid emptying of blood from peripheral veins into the central veins and increase in venous return to right atrium.
- Decrease in peripheral venous blood pressure to less than 3 to 4 cm H2O.
- Increase in central venous blood pressure.
Uses of Müller Maneuver
Müller maneuver is used to evaluate the following:
- Upper respiratory tract problems.
- Sleep apnea syndrome (temporary stoppage of breathing repeatedly during sleep).
Kussmaul’s Sign
Kussmaul’s sign is the increase in jugular venous pressure during inspiration.
Normally central venous pressure decreases during inspiration.
Conditions When Kussmaul’s Sign Occurs
- Cardiac tamponade.
- Constrictive pericarditis.
- Restrictive cardiomyopathy.
- Right ventricular infarction.
Applied Physiology
Pathological Conditions When Venous Blood Pressure Increases
- Low cardiac output.
- Congestive heart failure.
- Venous obstruction.
- Failure of valves in veins.
- Paralysis of muscles.
- Immobilization of parts of body.
- Renal failure.
- Kussmaul’s sign.
Pathological Conditions When Venous Blood Pressure Decreases
- Severe hemorrhage.
- Surgical shock.
Capillary Blood Pressure
Definition
Capillary blood pressure is the pressure exerted by blood contained in capillary.
It is also called hydrostatic pressure.
Significance
Capillary blood pressure is responsible for the exchange of various substances between blood and interstitial fluid through capillary wall.
Normal Values
| Part of Capillary | Normal Pressure |
|---|---|
| Arterial end of capillary | 30 to 32 mm Hg |
| Venous end of capillary | 15 mm Hg |
Capillary pressure varies depending upon the function of the organ or region of the body.
Capillary Blood Pressure in Lungs
In lungs, pulmonary capillaries form low pressure bed.
Capillary pressure of about 7 mm Hg favors exchange of gases between blood and alveoli.
Regional Variations of Capillary Blood Pressure
Capillary blood pressure varies in different organs, particularly in kidneys and lungs.
Regional variation in capillary pressure is in relation to the physiological activities of a particular region. So, it has some functional significance.
Capillary Blood Pressure in Kidney
Glomerular Capillaries
In kidney, glomerular capillaries form high pressure bed with a pressure of 60 to 70 mm Hg.
This high pressure is responsible for glomerular filtration.
Peritubular Capillaries
Peritubular capillaries form low pressure bed with a pressure of 8 to 10 mm Hg.
This low pressure helps reabsorption.
Regulation of Capillary Blood Pressure
Arterioles play an important role in regulating the pressure and pressure in capillaries is considered as a function of arteriolar resistance.
Arteriolar Constriction
When the arterioles constrict, resistance increases in arterioles, which raises arterial blood pressure.
At the same time, volume of blood flowing into capillaries decreases, leading to fall in capillary pressure.
Arteriolar Dilation
During dilation of arterioles, resistance decreases and arterial blood pressure decreases.
But the capillary pressure increases because of increase in volume of blood flowing into capillaries.
Capillary Oncotic Pressure
Capillary oncotic pressure is the pressure exerted by plasma protein which stay within capillaries because of impermeability of capillary membrane to plasma proteins.
Normal oncotic pressure is about 25 mm Hg.
Among the plasma proteins, albumin exerts 70% of oncotic pressure.
Oncotic pressure plays an important role in regulating filtration across capillary membrane, particularly in renal glomerular capillaries.
Quick Revision
Venous Blood Pressure
- Pressure exerted by contained blood in veins.
- Vena cava and right atrium → central venous pressure.
- Peripheral veins → peripheral venous pressure.
- Pressure is greater in peripheral veins than in central veins.
Factors Regulating Venous BP
- Left ventricular contraction or vis a tergo.
- Right atrial pressure or vis a fronte.
- Resistance or vis a latere.
- Volume of venous blood.
- Peripheral resistance.
- Gravity and posture.
Respiratory Maneuvers
- Valsalva maneuver.
- Müller maneuver.
- Kussmaul’s sign.
Capillary Blood Pressure
- Also called hydrostatic pressure.
- Arterial end → 30 to 32 mm Hg.
- Venous end → 15 mm Hg.
- Lung capillaries → about 7 mm Hg.
- Glomerular capillaries → 60 to 70 mm Hg.
- Peritubular capillaries → 8 to 10 mm Hg.
Capillary Oncotic Pressure
- Normal → about 25 mm Hg.
- Albumin → 70% of oncotic pressure.
- Important in regulation of filtration across capillary membrane.
Applied Physiology
- Venous BP increases → low cardiac output, congestive heart failure, venous obstruction, failure of venous valves, paralysis of muscles, immobilization, renal failure and Kussmaul’s sign.
- Venous BP decreases → severe hemorrhage and surgical shock.