Vascular System

Vascular functions and characteristics

Blood flows through a circuit of vessels consisting of, in sequence, arteries, arterioles, capillaries, venules, and veins. Each of these vascular segments has different attributes and functions within the circulation.

Large arteries (e.g., the aorta) have a lot of elastic fibers. They are high pressure reservoirs that store energy in their elastic walls during ventricular ejection and release it during diastole to maintain blood flow.

Pulse wave. Pulse waves generated by the heart in systole move the arterial walls. Forward movement of blood  extends arteries and this one is enough to create a palpable pressure wave.

Small muscular arteries and arterioles have a thick smooth muskce layer. They are the site of greatest vascular resistance in the circulation. This accounts for the large decrease in blood pressure that occurs across this segment of the circulation. Neurohumoral regulation of arteriolar tone controls vascular resistance and has a major effect on arterial blood pressure. 

Capillaries are thin-walled vessels that provide a large surface area for the exchange of gases, nutrients, and wastes. Imbalances of hydrostatic and osmotic pressure gradients across the capillary walls are the most common causes of edema (swelling due to excess interstitial fluid volume).

Venules and veins are very extensible. They present little resistance to flow. The venous system is a high capacitance, low pressure reservoir. The largest blood volume in the cardiovascular system is in the systemic veins. 

Blood flow around the circulation is driven by a difference in pressure between the arteries and the veins. 
Blood is an incompressible fluid, and its volume cannot decrease when the ventricles contract. Instead, blood is pressurized, creating the potential energy for blood flow.

Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow.  Systemic vascular resistance is mainly determined by changes in the diameter of the arterioles. These vessels exhibit partial constriction under normal physiologic conditions, called vascular tone, and are the sites of active regulation of blood flow in the circulation.

• Blood volume: 60-70% of MBV (minute blood volume) in veins, 15-20% in arteries, 10-15% in capillaries и 5% in the heart.
• Rate of blood flow: minimum in capillaries. 
• Total lumen: maximum in capillaries, less in veins and minimum in arteries.
• Hydrostatic pressure - maximum in the aorta, minimum - in the veins (may be less than zero).

Arterial blood flow

There are determinants of arterial blood pressure: 

Venous blood flow

Venous blood flow needs a special mechanisms because generally blood is moving opposite to gravity. There are :

• Valves that prevent backflow.

• Musculovenous pump. Rhythmical contraction of limb muscles as occurs during normal locomotory activity (walking, running) promotes venous return.

• Respiratory pump. During inspiration, the intrathoracic pressure is negative (expending of the chest), and abdominal pressure is positive (compression of abdominal organs by diaphragm). This makes a pressure gradient between the infra- and supradiaphragmatic parts of v. cava inferior, "pulling" the blood towards the right atrium.
• Decreased venous capacitance. Sympathetic activation of veins increases venomotor tone, decreases venous compliance, increases central venous pressure and promotes venous return.

The capillary blood flow

The exchange of materials between blood and interstitial fluid occurs across capillaries in the microcirculation.
Capillaries have the largest collective surface area for the exchange of nutrients and wastes. Diffusion is the most important mechanism for exchange across capillary walls.

There are four Starling forces for filtration:

The balance of forces changes along the length of a typical capillary as capillary hydrostatic pressure decreases. Net fluid filtration out of plasma occurs at the arterial end of capillary beds, and net fluid reabsorption into plasma occurs toward the venous end. 

Common Causes of Edema