Module Overview

• Explain the functions of the various components of the circulatory system
• Explain the generation of a "blood reservoir" and its function
• Explain how high pressures are generated in the arterial circulation
• Apply principles of Ohm's law and Poiseuille Hagen formula to understand the regulation of cardiac output and blood pressure in the circulation.
• Explain how blood flow to the capillary network is regulated
• Describe forces governing capillary exchange

Prelecture reading CVSL7 – Circulatory system

1. The circulatory system comprises the heart, arteries, arterioles, capillaries and veins.
2. The reason why there is more blood in the venous system is because of the low compliance of the veins.
3. High pressures in the arterial system are caused by 1) cardiac output and 2) the resistance at the arterioles.
4. Arterioles are high resistance vessels and a key site for the regulation of the circulatory system. They regulate arterial pressure upstream and flow of blood into the capillary network downstream. In a sense there is a “conflict” between maintaining arterial pressure vs ensuring adequate blood flow into tissues.
5. Ohm’s law govern rate of flow (L/min) in the circulation, relating flow, pressure and resistance. Poiseuille Hagen formula governs resistance. The most important parameter regulating resistance in the circulation is the radius of the arteriole.
6. The capillaries are sites where exchanges of nutrients, water, gases, hormones and waste products occur between blood and the cells.
7. Transcapillary fluid exchange is regulated by capillary pressure, plasma colloid osmotic pressure (oncotic pressure), tissue colloid osmotic pressure and tissue hydrostatic pressure.
8. Capillary pressure is the single most important factor ensuring adequate perfusion of tissues.
9. Edema occurs when there is an accumulation of fluid in the interstitial space.
10. Edema is caused by either an increase in venous pressure, a decrease in oncotic pressure, an obstruction to lymph flow or an increase in capillary permeability.