Cardiovascular Physiology: Types of Capillaries

Types of Capillaries

• Continuous capillaries are found in muscles, lungs, adipose tissue, and the central nervous system, where adjacent endothelial cells are closely joined together with no intercellular channels.
• The lack of intercellular channels in continuous capillaries contributes to the blood-brain barrier.
• An intercellular cleft is the space between adjacent cells, and a tight junction is where the lipid portions of the membranes of adjacent cells are bound together.
• Fenestrated capillaries are found in the kidneys, endocrine glands, and intestines.
• Discontinuous (sinusoidal) capillaries are found in the bone marrow, liver, and spleen, with a large distance between endothelial cells.

Physical Laws Describing Blood Flow

• Blood flow is directly proportional to the pressure difference (ΔP) between the two ends of the tube and inversely proportional to the frictional resistance to blood flow through the vessels.
• The pressure gradient can be determined mathematically by taking the difference in pressure between two locations and dividing it by the distance between the two locations.
• The resistance to blood flow through a vessel is directly proportional to the length of the vessel and to the viscosity of the blood.
• According to Poiseuille's law, a vessel with half the radius of another will have 16 times the resistance, resulting in 16 times less blood flow.
• The changes in diameter of veins do not contribute to resistance to blood flow.

Blood Vessel Regulation and Blood Pressure

• Dilation and constriction of blood vessels are controlled by the sympathetic nervous system.
• Decreasing the diameter of blood vessels, increasing the length of blood vessels, or increasing the viscosity of blood will increase peripheral resistance.
• Peripheral resistance is greater than pulmonary resistance, resulting in lower blood pressure in the pulmonary arteries than in the aorta.
• The right ventricle does not need to produce as high a pressure as the left ventricle to move blood through the pulmonary circuit.

Cardiac Output and Blood Pressure

• Cardiac output = heart rate X stroke volume = volume flow rate (ml/minute).
• Blood pressure is affected by total peripheral resistance, cardiac output, and blood volume.
• Mean arterial blood pressure = cardiac output X total peripheral resistance.

Pascal's Principle

• Pascal's principle states that a change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and to the walls of its container.
• Example: the pressure generated by the left ventricle is transmitted to the brachial artery, causing pulsations.