Biomechanics 2
Passive mechanical properties of blood vessels - Lamé's equation, volume-pressure dependencies. Pulse wave. Mechanics of breathing
Components causing passive mechanical properties of blood vessels:
- 1. Collagen
- 2. Elastin
- 3. Muscle layer
Lamé's equation:
Transmural pressure; the pressure difference between the inside and the pressure outside the walls of the blood vessels.
The pressure inside the blood vessels is divided into;
- 1. Tangential - constant
- 2. Normal \sigma - changing human vessel
Volume-pressure dependencies: Compliance C - refers to the ability of an elastic structure to expand. It is calculated by volume change per unit to the pressure changes across the structure.
Specific compliance:
Pulse wave:
The pulse wave is a pressure wave that travels along the elastic arterial walls with each heartbeat. It moves much faster (about 5 to 10 m/s) than the blood itself, and stiffer arteries carry it faster.
Mechanics of breathing:
Transpulmonary pressure is a term used to describe the difference between the values of alveolar pressure and intrapleural pressure in the lungs. During human ventilation air flows because of: pressure gradients.
The action of breathing in and out is due to changes of pressure within the thorax, in comparison with the outside. This action is also known as external respiration. When we inhale the intercostal muscles and diaphragm contract to expand the chest cavity. The diaphragm flattens and moves downwards and the intercostal muscles move the rib cage upwards and out.
This increase in size decreases the internal air pressure and so air from the outside (now at a higher pressure than inside the thorax) rushes into the lungs to equalise the pressures.
When we exhale the diaphragm and intercostal muscles relax and return to their resting positions. This reduces the size of the thoracic cavity, thereby increasing the pressure and forcing air out of the lungs.