Fluid Exchange - Starling Forces Normally there is nearly as much fluid reabsorbed as there is filtered. At the arterial end, net pressure is outward at 10 mmHg and fluid leaves the capillary (filtration). At the venous end, net pressure is inward at –9 mmHg (reabsorption). On average, about 85% of fluid filtered is reabsorbed.

Fluid Exchange - Starling Forces Fluid that is not reabsorbed (about 3L/ day for the entire body) enters the lymphatic vessels to be eventually returned to the blood.

Gas And Nutrient Exchange In contrast to the bulk flow of fluids at the capillaries, the exchange of gases and small particles (like certain nutrients and wastes) is a purely passive diffusion process. Gases and these other substances simply move into or out of the capillary down their concentration gradient.

Venous Reserve Because systemic veins and venules contain a large percentage of the blood volume (about 64% at rest), they function as blood reservoirs from which blood can be diverted quickly if needed. To counteract a drop in BP, stimulation of the sympathetic NS will cause venoconstriction, allowing a greater volume of blood to flow to skeletal muscles.

Venous Return The volume of blood returning through the veins to the right atrium must be the same amount of blood pumped into the arteries from the left ventricle – this is called the venous return. Besides pressure, venous return is aided by the presence of venous valves, a skeletal muscle pump, and the action of breathing.

Venous Return The skeletal muscle pump uses the action of muscles to milk blood in 1 direction (due to valves). The respiratory pump uses the negative pressures in the thoracic and abdominal cavities generated during inspiration to pull venous blood towards the heart.

Proximal valve Distal 1 2 3 Proximal valve Distal 1 2 Proximal valve Distal 1

Venous Return Although the venous circulation flows under much lower pressures than the arterial side, usually the small pressure differences (venule 16 mmHg to right atrium 0 mmHg), plus the aid of muscle and respiratory pumps is sufficient.

Pressure, Flow, And Resistance Blood pressure is a measure of the force (measured in mmHg) exerted in the lumen of the blood vessels. Blood flow is the amount of blood which is actually reaching the end organs (tissues of the body). Resistance is the sum of many factors which oppose the flow of blood.

Pressure, Flow, And Resistance Cardiovascular homeostasis is mainly dependent on blood flow… but blood flow is hard to measure. Clinically, we check blood pressure because it is easier to measure, and it is related to blood flow. The relationship between blood flow, blood pressure, and peripheral resistance follows a simple formula called Ohms Law. BP = Flow x Resistance

Pressure, Flow, And Resistance In an effort to meet physiological demands, we can increase blood flow by: Increasing BP Decreasing systemic vascular resistance in the blood vessels Usually our body will do both – when we exercise, for example. figure adapted from http://www.learnhemodynamics.com/hemo/basics.htm