Capillary Pressure Physiology Notes

Capillary Pressure Introduction

Capillary Pressure Definition

Capillary pressure is the pressure exerted by the blood contained in the capillary. It is also called capillary hydrostatic pressure.

Capillary PressureSignificance

Capillary pressure is responsible for the exchange of various substances between the blood and interstitial fluid through the capillary wall.

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Capillary Pressure Normal Values

The capillary pressure varies depending upon the function of the organ or the region of the body.

Generally, the pressure in the arterial end of the capillary is about 30-32 mm Hg, and in the venous end, it is 15 mm Hg.

Regional Variations

The capillary pressure varies in different organs, particularly in the kidneys and lungs. The regional variation in capillary pressure is in relation to the physiological activities of the particular region.

So it has some functional significance.

Capillary Pressure in the Kidney

In the kidney, the glomerular capillary pressure is high. It is about 60 mm Hg. This high capillary pressure is responsible for filtration.

Capillary Pressure in the Lung

In the lungs, the pulmonary capillary pressure is low. It is about 7 men Hg. It favors the exchange of gases between blood and alveoli.

Direct Method

• Capillary pressure was first measured by EM. Landis when he was a medical student.
• The minute vessels in the web of the foot in the frog were cannulated by using a micropipette with a diameter of 5 p at the tip with the aid of a microscope.
• The cannula was connected to a manometer.
• This method was later followed to measure capillary pressure in other organs.

Indirect Method

• This is based upon the principle of exerting an external pressure necessary to obstruct the flow of blood in capillaries.
• The capillaries are observed under the microscope.

Regulation

• Arterioles play an important role in regulating capillary pressure. The pressure in capillaries is considered a function of arteriolar resistance.
• When the arterioles constrict, the resistance increases in the arterioles which raises the arterial blood pressure.
• At the same time, the volume of blood flowing into capillaries decreases leading to a fall in capillary pressure.
• On the other hand, during dilatation of arterioles, the resistance decreases and arterial blood pressure decreases.
• However, the capillary pressure increases because of the increase in the volume of blood flowing into capillaries.

Capillary Oncotic Pressure

• The capillary membrane is permeable to all substances except plasma proteins.
• The plasma proteins exert some pressure within capillaries. This pressure is referred to as oncotic pressure or colloidal osmotic pressure because it is exerted by colloids. The normal oncotic pressure is about 25 mm Hg.
• Albumin exerts 70% of oncotic pressure.
• The oncotic pressure plays an important role in filtration across capillary membranes, particularly in renal glomerular capillaries.