Overview
This lecture covers the different circulatory routes in the body, types of vascular anastomoses, and the principles of hemodynamics, including blood pressure, blood flow, and factors affecting peripheral resistance.
Circulatory Routes and Anastomoses
- Anastomosis is a connection between blood vessels, creating collateral pathways for blood flow.
- Arterial anastomoses connect arteries, commonly found in the heart, brain, and joints for reliable blood supply.
- New anastomoses can form if an artery is blocked to maintain tissue perfusion.
- Venous anastomoses, the most common type, connect veins into complex web-like structures to drain deoxygenated blood.
- Arteriovenous anastomosis (shunt) allows blood to flow directly from arteries to veins, bypassing capillaries.
Simple and Portal Circulatory Routes
- The simplest route: heart → arteries → arterioles → capillaries → venules → veins → heart.
- Portal systems (e.g., hypophyseal portal between hypothalamus and anterior pituitary) have two capillary beds in sequence.
Hemodynamics and Pressure Gradients
- Blood flow depends on pressure gradients: high pressure near the heart, low pressure in veins.
- Arteries near the heart (e.g., aorta) have the highest pressure; veins near the heart (e.g., vena cava) have the lowest.
- Blood flows from areas of high pressure to low pressure through the circulatory system.
Blood Flow, Cardiac Output, and Perfusion
- Blood flow is directly proportional to pressure gradient; higher gradient means higher flow.
- Blood flow is inversely proportional to resistance; higher resistance means lower flow.
- Cardiac output (CO) = heart rate × stroke volume; normal CO is about 5–6 liters per minute.
Blood Pressure and Its Determinants
- Blood pressure is the force of blood against vessel walls, measured in mmHg.
- Systolic pressure (ventricle contraction): ~120 mmHg; Diastolic pressure (ventricle relaxation): ~75 mmHg for healthy adults.
- Three main factors influence blood pressure: peripheral resistance, cardiac output, and blood volume.
Factors Affecting Peripheral Resistance
- Peripheral resistance opposes blood flow, highest in long, narrow, or obstructed vessels.
- Major variables: vessel diameter (smaller radius increases resistance), viscosity (thicker blood increases resistance), vessel length (longer vessel increases resistance).
- Obstructions such as plaque, clots, or tumors increase resistance.
- Laminar flow: blood flows faster in the vessel center and slower near the walls.
Blood Volume and Vascular Compliance
- Blood volume affects blood pressure; more water in plasma increases volume and pressure.
- Veins are the most compliant vessels; they expand easily with increased blood volume.
- Systemic arteries have much higher pressure than pulmonary arteries; pressure decreases further from the heart.
Key Terms & Definitions
- Anastomosis — a connection between blood vessels.
- Collateral vessels — alternate pathways formed by anastomoses.
- Arteriovenous anastomosis (shunt) — direct artery-to-vein connection bypassing capillaries.
- Pressure gradient — difference in blood pressure that drives blood flow.
- Cardiac output (CO) — volume of blood pumped by the heart per minute.
- Peripheral resistance — opposition to blood flow in vessels, mainly in smaller or longer vessels.
- Compliance — ability of a vessel to expand with increased volume.
Action Items / Next Steps
- Review textbook graphics/diagrams on circulatory routes and blood pressure regulation.
- Memorize normal systolic and diastolic pressures and factors influencing blood pressure.
- Be prepared to define and give examples of different anastomoses.