the circulation

The Peripheral Circulation

B.Pimentel, M.D.

University of Makati – College of Nursing

PERIPHERAL CIRCULATION

1. Capillaries

2. Arteries

3. Veins

PERIPHERAL CIRCULATIONCapillaries

Structure - the capillary wall consists primarily of endothelial cells on a basement membrane. A layer of loose connective tissue merges with the connective tissue surrounding the capillary. 7 to 9 micrometers in diameter.


Function - exchange of gases, nutrients, hormones... between blood and the interstitial fluid.

Substances cross capillary walls by diffusing through the endothelial cells, through fenestrae, or between the endothelial cells. Lipid soluble substances such as oxygen and carbon dioxide readily diffuse through the plasma membrane of the endothelial cells.


Types of Capillaries

1. Continuous capillaries - walls exhibit no gaps between endothelial cells, less permeable. Located in muscle and nervous tissue.

2. Fenestrated capillaries - endothelial cells have numerous fenestrae. Located in intestinal villi, ciliary process of the eye, choroid plexus of the CNS, and glomeruli of the kidneys.

3. Sinusoidal capillaries - larger diameter capillary with larger fenestrae. Located in endocrine glands.

PERIPHERAL CIRCULATIONContinuous Capillaries

Capillaries which lack pores = fenestrations and in which adjacent endothelial cells are held together by tight junctions to limit fluid exchange between the plasma and the tissue fluid by controlling fluid transfer between endothelial cells.

PERIPHERAL CIRCULATION Fenestrated Capillaries

Primarily seen in the glomerular capillaries in the nephrons of the kidney, which have pores = fenestrations

Permitting regulated fluid exchange.

PERIPHERAL CIRCULATION Sinusoidal Capillaries

Primarily the capillaries in the liver, bone marrow, some endocrine glands and in lymphoid tissue, which lack pores = fenestrations


Capillary Network

Arterial capillaries - ends of the capillaries closest to arterioles.

Venous capillaries - ends closest to venules.

Blood flows from an arteriole to a metarteriole, which have smooth muscle cells along their walls. Blood then flows into a thoroughfare channel, which extends in a relatively direct fashion from a metarteriole to a venule


Precapillary sphincters - are smooth muscle cells located at the origin of the branches from the thoroughfare channel, they function in

regulating flow in the capillaries

PERIPHERAL CIRCULATIONArteries And Veins

Structure - consists of three layers or tunics1. Tunica Intima - consists of endothelium (continuous with the heart

and all blood vessels), connective tissue basement membrane, lamina propria (thin layer of connective tissue), and internal elastic membrane (fenestrated layer of elastic fibers.

2. Tunica Media - smooth muscle cells arranged circularly around the vessel. arteries have a thicker tunica media than veins.

3. Tunica Adventitia - outer tunica composed of connective tissue. Contain blood vessels called vaso vasorum which supplies the artery. The tunica media and adventitia can not obtain nutrients from the lumen. Also contains nerves.

PERIPHERAL CIRCULATIONArteries And Veins

PERIPHERAL CIRCULATIONArteries

Conduct blood away from the heart, to the lungs and body.

Types of arteries1. Large elastic arteries - largest diameter, a greater amount of elastic

tissue and a smaller amount of smooth muscle in their walls.

2. Muscular arteries - walls are thick compared to their diameter, tunica media has the most amount of muscle than any other vessel. Partially regulate blood supply to different regions of the body by constricting or dilating.

3. Arterioles - transport blood from arteries to capillaries. Capable of vasodilation and vasoconstriction

PERIPHERAL CIRCULATIONElastic Arteries

PERIPHERAL CIRCULATIONMuscular Arteries

PERIPHERAL CIRCULATIONArterioles

PERIPHERAL CIRCULATIONVeins

Conducts blood from the body and lungs to the heart.

Types of veins1. Venules and small veins - venules conduct blood from the

capillaries to small veins, they are structurally similar to capillaries. Small veins have a thin layer of smooth muscle in a continuous layer.

2. Medium veins - collect blood from small veins and deliver to large veins.

3. Large veins - transport blood from medium veins to the heart.

PERIPHERAL CIRCULATIONVeins

Valves - veins with a diameter greater than 2mm. contain valves. Prevents blood flow away from the heart

Arteriovenous anastomoses - allow blood to flow from arterioles to venules without passing through the capillaries

PERIPHERAL CIRCULATION Factors Affecting Blood Flow

1. Resistance to flow of the walls

2. Blood pressure

3. Viscosity

4. Vascular compliance


1. Resistance to flow of vessel walls

• Laminar flow - streamlined flow of blood through smooth walled vessels.

• Turbulent flow - when blood passes a constriction, sharp bend, or a rough vessel wall, will interrupt laminar flow.


2. Blood Pressure

A measure of the force blood exerts against blood vessel walls.

• Measured in millimeters of mercury (mmHg).

• Systolic pressure - the highest pressure of blood that stretches the aorta to its maximum, as blood is ejected out of the left ventricle.

• Diastolic pressure - lowest level of aortic pressure as the aorta and elastic arteries recoil maintaining pressure on the reducing blood volume.

• Pulse pressure - the difference between systolic and diastolic pressures. Two major factors influence pulse pressure; stroke volume and vascular compliance.


3. Viscosity

The measure of the resistance of a liquid to flow.

• The viscosity of blood is influenced largely by Hematocrit - the percentage of the total blood volume composed of RBC's.

• Increased hematocrit will increase blood viscosity, and the converse is true.


4. Vascular compliance

• The tendency for blood vessel volume to increase as the blood pressure increases.

• The more easily the vessel wall stretches, the greater its compliance.

PERIPHERAL CIRCULATION Capillary Exchange and Regulation of Interstitial Fluid Volume

• Capillary exchange - the movement of substances into and out of capillaries. The most important means by which capillary exchange occurs is diffusion.

• Net filtration pressure - the force responsible for moving fluid across capillary walls.

• Interstitial fluid pressure - the pressure of interstitial fluid within tissue spaces. It si -3 mm hg. because of the suction effect produced by the lymphatic vesels as they pump fluid from the tissues.

PERIPHERAL CIRCULATIONLocal Control of Blood Flow by the Tissues

1. Functional characteristics of the capillary bed. Local factors regulate these structures primarily. As the rate of metabolism increases in a tissue, blood flow through its capillaries increases. The precapillary sphincters relax, allowing blood to flow into the local capillary bed. Vasodilator substances are produced as the rate of metabolism increases. The substances then diffuse from the tissue supplied by the capillary to the area of the precapillary sphincter, the metarterioles and arterioles, to cause vasodilation.


• Chemicals that cause vasodilation: 1. Carbon dioxide, 2. Lactic acid, 3. Adenosine, 4. Adenosine monophosphate, 5. Adenosine diphosphate, 6. Endothelium derived relaxation factor, 7. Potassium ions, and 8. Hydrogen ions.

• Lack of nutrients can also regulate local blood flow; oxygen and other nutrients.

• Blood flow through capillaries is cyclic. The cyclic fluctuations is the result of periodic contraction and relaxation of the precapillary sphincters called vasomotion.


2. Autoregulation of blood flow. The maintenance of blood flow by tissues is called autoregulation. The mechanisms responsible for autoregulation are the same for vasomotion.

3. Long term local blood flow. Through tissues is matched closely to the metabolic requirements of the tissue if elevated or decreased for a long period of time


4. Nervous and hormonal regulation. Nervous control of arterial blood pressure is important in minute to minute regulation of local circulation. The blood pressure must be adequate to cause blood flow through capillaries in response to; rest, during exercise, or in circulatory collapse (shock). Nervous regulation also provides a means of shunting blood from one region to another.

Vasomoter center - pons and medulla oblongota is tonically active. Alow frequency of action potentials is continually transmitted. This accounts for slight constiction of all blood vessels, a condition called vasomotor tone.

REGULATION OF MEAN ARTERIAL PRESSURE

The mechanisms that operate to maintain arterial blood pressure within a normal range.

Mean arterial pressure - is slightly less than the average of systolic and diastolic pressures.

Short term regulation1. Baroreceptors 2. Adrenal medullary 3. Chemoreceptor reflexes 4. CNS ischemic response - in response to emergency situations in

which blood flow to the brain is severely restricted or when blood pressure falls below approximately 50 mmHg.


Long term regulation

1. Renin-Angiotensin-Aldosterone mechanism - regulates kidney function, and can also influence peripheral circulation causing vasoconstriction. The control of urine output continues to operate until the blood pressure is precisely within normal limits.

2. Vasopressin (ADH) mechanism works in harmony with the renin-angiotensin-aldosterone mechanism.

3. Atrial natriuretic mechanism - is released from the cells of the atria. A major stimulus for its release is increased venous return.


4. Fluid shift mechanism - important role when dehydration occurs. The interstitial volume acts as a reservoir, shift of this fluid back into circulation is a powerful mechanism in which blood pressure is maintained.

5. Stress relaxation response - smooth muscle cells. Action of the smooth muscle of vessels is directly related to blood volume and blood pressure. A decline in blood volume causes a decline in blood pressure, the smooth muscles of vessels will contract causing vasoconstriction, this in turn reduces the volume of blood vessels thereby increasing the blood pressure. The converse is true.

Thank You….

the circulation

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