Capillary Circulation Physiology Notes

Capillary Circulation Introduction

Microcirculation

Microcirculation refers to the flow of blood through the minute blood vessels such as arterioles, capillaries, and venules. Capillary circulation forms the major part of microcirculation.

Features Of Capillaries

• Capillaries arise from arterioles and form the area for the actual function of the circulatory system, i.e. exchange of materials between blood and tissues.
• Structurally, capillaries are very narrow and short.
• However, quantitatively, these vessels outnumber the other blood vessels. About ten billion capillaries are present in the body.
• Each capillary lies in very close proximity to the cells of the tissues at a distance of about 20-30 mm.
• This enables easy and rapid exchange of substances between the blood and the tissues through interstitial fluid.

Read And Learn More: Medical Physiology Notes

Study of Capillary Circulation

• Blood flow through capillaries is studied by focusing the capillaries under a dissecting microscope.
• The frog’s web, mesentery of mammals, and fingernail bed of humans can be observed by using a microscope.

Dimensions Of Capillaries

• Total number of capillaries: About 10 million
• The surface area of all capillaries: 500-700 sqm
• Average length: 0.5-1 mm
• Average diameter: 8 μ
• Pressure at the arterial end: 30-32 mm Hg
• Pressure at the venous end: 15 mm Hg

Velocity And Volume Of Blood Flow

The average velocity of blood flow through capillaries is 0.5-1 mm/second. About 5% of total blood is present in capillaries.

Structure Of Capillaries

The capillaries are formed by a single layer of endothelial cells which are wrapped around by pericytes.

Structure Of Capillaries Endothelial Cells

• The endothelial cells of the capillaries are thin, flattened, nucleated polygonal cells joined together by a cement substance.
• The capillaries do not have muscular coats. Yet, these blood vessels actively modify their own diameter in response to nervous, hormonal, chemical, and physical stimuli.
• The endothelial cells themselves alter the diameter of capillaries by swelling or shrinking.
• In most of the capillaries, the adjacent endothelial cells leave a cleft called fenestra through which several substances may traverse the endothelium by means of transcytosis.
• However, in cerebral capillaries, the fenestra is absent because the endothelial cells fuse to each other by tight junctions.

Structure Of Capillaries Pericytes

Pericyte is a perivascular mesenchymal-like cell associated with walls of small blood vessels such as capillaries and postcapillary vessels. It is similar to renal mesangial cells.

It is also known as the mural cell or Rouget cell (named after the discoverer Charles Rouget).

• Pericytes extend long cytoplasmic processes that wrap around the endothelial cells.
• Pericytes play an important role in the remodeling and maintenance of the capillary system.
• These cells are contractile in nature and secrete several vasoactive agents, growth factors, extracellular matrix, and components of the basement membrane.
• Pericytes are also involved in the regulation of blood flow through endothelial junctions, particularly in conditions such as inflammation.

Structure Of Capillaries Pattern Of Capillary System

Capillaries are disposed between arterioles and venules. From the arterioles, the meta arterioles take origin.

From meta arterioles, two types of capillaries arise:

1. Preferential channels
2. True capillaries.

1. Structure Of Capillaries Preferential Channels

• The preferential channels or continuous capillaries have the same diameter as meta-arterioles.
• After arising from the meta-arterioles, the preferential channels form a network and finally join the venules.

2. Structure Of Capillaries True Capillaries

• The diameter of the true capillaries is less than that of the meta-arterioles.
• Arising from meta-arterioles, the true capillaries also form a network and join the venules.
• At the beginning of true capillaries, there is an encircling of smooth muscle fibers.
• It functions as a sphincter; so it is known as a precapillary sphincter. It controls the blood flow through true capillaries.

Anatomical And Physiological Shunts

Structure Of Capillaries Anatomical Shunt

• The anatomical shunt is the direct link between arterioles and venules. It is also called arteriovenous shunt.
• The flow of blood through the capillaries where the exchange of nutrients, gases, and other substances takes place is called nutritional flow.
• The flow of blood through the anatomical shunt is called non-nutritional flow.
• Non- Non-nutritional blood flow occurs in many tissues of the body, particularly during resting conditions when metabolic activities are low.

Structure Of Capillaries Physiological Shunt

The physiological shunt is the link between the arterial and venous side of circulation provided by meta-arteriole. Many tissues of the body such as muscles do not have anatomical shunts.

However, the meta-arteriole in these tissues acts as the physiological shunt between arterial and venous sides of the circulation.

The non-nutritional blood flow occurs through a physiological shunt under resting conditions.

Shunt in Capillaries vs Shunt in Heart

The physiological shunt in capillaries is different from the physiological shunt in the heart.

In capillaries, the oxygenated blood flows towards deoxygenated blood. But in the heart, the deoxygenated blood flows towards the oxygenated blood.

Peculiarities Of Capillary Blood Flow

1. The blood does not pass through the capillary system continuously. It is because of the alternate constriction and dilatation of meta arterioles and alternate opening and closure of precapillary sphincters.
2. The direction of blood flow through capillaries is not fixed as in the case of other blood vessels. The blood may flow in opposite directions in two adjacent capillaries.
3. In capillaries, blood flows as a single pile or single row of blood cells. In other blood vessels, the blood flows in either the axial stream containing mainly blood cells, or the peripheral stream containing plasma.
4. Under resting conditions, most of the capillaries lie in a collapsed state. Only during activity, do all the capillaries open up and increase the vascularity.
5. The amount of blood flowing through the capillary system throughout the body is very low. It is only about 150 mL/minute.
6. The velocity of blood flow is the least in capillaries. It is only about 0.5-1 mm/second. It facilitates the exchange of substances between capillaries and tissues.

Functions of capillaries

• The most important function of capillaries is the exchange of substances between blood and tissues.
• Oxygen and other essential substances enter the tissues from capillary blood; carbon dioxide, metabolites, and other unwanted substances are removed from the tissues by capillary blood.
• The exchange of materials across the capillary endothelium occurs primarily by diffusion.
• It also occurs by means of filtration and pinocytosis.

Functions of capillaries Diffusion

• Diffusion is the main process for the exchange of gases, water, glucose, sodium, urea, and many other substances.
• These substances diffuse through the intercellular clefts present in the endothelial wall of the capillaries.
• Diffusion occurs because of the concentration gradient across the capillary wall.

Functions of capillaries Filtration

• The site of filtration of substances through the capillary membrane varies in different organs.
• In skeletal muscles, cardiac muscles, kidneys, and intestine filtration occurs through the slit pores present in capillary endothelium.
• Capillaries in other organs have discontinued endothelium, through which filtration occurs.
• The filtration of substances through capillary endothelium depends upon the net filtration pressure.
• Net filtration pressure is the balance between the driving pressures and the opposing pressures. It is well explained by Starling’s hypothesis.

Functions of capillaries Pinocytosis

• Larger molecules are transported across the capillary endothelium in the form of vesicles.
• Large molecules are packed as vesicles in the capillary endothelial cells.
• These vesicles are transported across the endothelial membrane by a process called pinocytosis.

Factors Controlling Capillary Circulation

The capillary blood flow is controlled by the nervous and chemical factors.

Controlling Capillary Circulation Nervous Factors

Capillaries are mainly supplied by sympathetic vasoconstrictor fibers.

Controlling Capillary Circulation Chemical Factors

Dilatation of capillaries occurs due to excess carbon dioxide, increased hydrogen ion concentration, lack of oxygen, histamine, and metabolites like lactic acid. Serotonin causes constriction of capillaries.