In this article we will discuss about:- 1. Introduction to Capillary Circulation 2. Methods of Studying Capillary Circulation 3. Peculiarities 4. Control 5. Interchange.
Introduction to Capillary Circulation:
The average length of a capillary is 0.5 -1 mm, average diameter, 8 µ or often less than a red cell, average velocity of flow, 0.5 -1 mm per second. Capillaries are lined by a layer of the flat endothelial cells bound together by an intracellular cement substance. A basement membrane surrounds capillary endothelial cells. A pericapillary sheath forms the outer wall of the capillary.
In the pericapillary sheath there are branched Rouget cells or pericytes representing modified muscle cells. Zweifach has shown various types of arrangements of the capillaries in the muscle and nail bed (Figs 7.10 & 7.108). The arterioles are connected with metarterioles.
Metarterioles leads into thoroughfare (preferential) channels which resemble a large capillary and are connected with venules. The true capillaries are shorter in length and led from the metarterioles and arterioles into venules. At the root of each capillary there is a pre-capillary sphincter which adjusts the flow of blood whereas in the thoroughfare (preferential) channel there is no such sphincter and the blood flow cannot be adjusted through it.
Methods of Studying Capillary Circulation:
Capillary circulation can be studied as follows:
It may be watched in the web of frog’s foot or omentum, etc., under a microscope with suitable light arrangement. Projection of the some on a screen with the help of an Epidiascope is very helpful for demonstration.
Vasomotor Supply:
Sympathetic constrictor fibres pass to the Rouget cells. Antidromic dilator fibres are also present.
Peculiarities of Capillary Circulation:
i. Direction of Flow:
There is no fixed direction of capillary flow. Blood may be found to pass in opposite directions through two capillaries running side by side.
ii. Nature of the Stream:
In the arterioles, the blood stream can be divided into two parts:
(a) Axial stream consisting of red cells mainly, and
(b) Peripheral stream consisting of plasma and stray leucocytes.
But in the capillaries, due to narrow lumen, only a single file of cells can pass through. Often a red cell may be found to be folded upon it and squeezing somehow through the capillaries.
iii. Capillary Tone:
In the resting condition the majority of the capillaries remain collapsed. During activity they open up and increase the vascularity of the part. In the muscles, the blood supply may increase by 750 times during activity.
iv. Independent Contractility of the Capillaries:
The capillaries can adjust their diameters independent of arterioles and venules. Raised venular and arteriolar pressure cannot distend the capillaries if the tone is high. Ordinarily, however, the capillary lumen changes in the same direction as the arterioles and venules.
v. Capillary Pressure — Method of Recording Intracapillary Pressure:
Landis method of recording intra-capillary pressure is the most satisfactory one. A fine micropipette is introduced into the capillary loops of the skin of the nail bed and observed through a binocular microscope. It is connected by fine tubes containing citrate solution to a mercury manometer by which the pressure may be recorded.
In man, the average pressure at the arterial end is about 32 mm of Hg, at the venous end 12 mm of Hg, and at the summit 20 mm of Hg. There is a difference of 5-10 mm of Hg between systolic and diastolic pressure, at the arterial end. Hence, the flow is pulsatile there. But at the venous end it is continuous.
Capillary pressure is affected by the following factors:
i. Heart Level:
When lowered below the heart level, the capillary pressure rises proportionately. But if the part is raised above the level of heart, there is a slight fall in the arterial side of capillary pressure but none on the venous side.
ii. Venous Pressure:
Venous obstruction raises capillary pressure.
iii. Condition of Arterioles:
Dilatation of arterioles (without fall of general blood pressure) increases both arterial and venous capillary pressure (from 40-60 and 10-40 mm of Hg respectively). Under such conditions, capillary pulsation is seen. This is due to the fact that, systemic blood pressure is more directly transmitted to the capillary area.
General vasodilatation, with marked fall of blood pressure, will have opposite effects. Constriction of the arterioles beyond the optimum limit may raise systemic blood pressure but reduces capillary pressure and flow.
iv. Visceral Characteristics:
In the viscera, capillary pressure differs according to the nature of local mechanism. For instance, in the renal glomeruli, it is about 75 mm of Hg, in the intestinal capillaries about. 10 – 20 mm of Hg, in the liver about 2 mm of Hg, etc.
Control of Capillary Circulation:
The extent of capillary flow is directly proportional to the activity of the locality. The adjustment of vascular supply to local needs can be carried out by adjusting the general blood pressure and arteriolar tone on the one’ hand and by number of the patent capillaries and their diameters on the other. Apart from their independent contractility, the capillaries respond to chemical, hormonal, physical and nervous stimuli.
They are as follows:
i. Chemical Stimuli:
CO2 excess, increased H-ion concentration, O2 lack, non-acid dilators (metabolites) histamine, adenylic acid and certain other products of tissue activity dilate the capillaries.
ii. Hormonal Stimuli:
Adrenaline, noradrenaline and pituitrin not only constrict the arterioles but also the capillaries.
iii. Physical Stimuli:
Application of heat or strong light (including ultraviolet and infrared rays) dilates the vessels and increases the vascularity of the part due to direct action of the rise of temperature capillaries and probably also due to histamine liberation. Cooling constricts the arterioles and capillaries. Freezing produces damage of the tissues and liberation of H-substances which causes redness, flare and wheal.
iv. Nervous Stimuli:
Nervous controls of capillaries are carried out in two ways:
a. Through the vasoconstrictor nerves which reduce the capillary lumen by causing contraction of the Rouget cells. Their inhibition will naturally produce dilatation.
b. Axon reflex (Antidromic vasodilators)—Irritation of the skin produces vasodilatation in the locality. This response persists even when the posterior root is severed either proximal or distal to the posterior root ganglion, but not when the peripheral nerves are allowed to degenerate. This shows that, the dilatation is not due to the direct action of any substance on the blood vessels but through the intact peripheral sensory nerves (probably nocifensor system).
Since no nerve cell is involved in the process, the reflex arc is completed by the two branches of the same sensory fibre. Through one branch, the impulse is received and through the other, it is transmitted back to the peripheral vessels, causing vasodilatation. The impulse is called antidromic impulse and the reflex is known as the axon reflex (Fig. 7.89).
Interchange in the Capillary Circulation (Fig. 7.109):
At the arterial end of the capillary loop, blood pressure is about 32 mm of Hg and colloidal osmotic pressure about 25 mm of Hg. Hence, filtration takes place and water, nutrition, salts, etc., pass out into the tissue fluid. At the venous end, blood pressure is only 12 mm of Hg and colloidal osmotic pressure is raised due to concentration of proteins. Hence, water and crystalloids are again re-absorbed. In this way capillary circulation and interchange go on.
