Transmural pressure (TMP) is a fundamental physiological concept that governs the shape and volume of hollow organs and tubular structures throughout the body. It represents the difference in pressure measured across the wall of a structure, such as a blood vessel, a heart chamber, or an airway. This pressure difference is the working force that determines whether a flexible biological container will expand or collapse. TMP is directly responsible for the structural integrity and mechanical behavior of many internal systems, making it a key factor in understanding normal function and disease states.
Defining the Pressure Gradient
The transmural pressure determines the net force acting perpendicular to the wall of a hollow structure. It is mathematically defined as the pressure inside the structure (\(P_{in}\)) minus the pressure outside the structure (\(P_{out}\)), or TMP = \(P_{in}\) – \(P_{out}\). This calculation provides the actual distending pressure that stretches the wall material.
A positive transmural pressure occurs when the internal pressure is greater than the external pressure, causing the structure to distend or expand. Conversely, if the internal pressure drops below the external pressure, the resulting negative TMP promotes collapse. The magnitude of this pressure directly determines the tension or stress exerted on the wall material, which, according to principles like the Law of Laplace, influences the structural integrity of the vessel or chamber.
Role in the Circulatory System
In the cardiovascular system, transmural pressure dictates blood vessel diameter and the filling of heart chambers. For a blood vessel, TMP is the difference between the intraluminal blood pressure and the surrounding tissue pressure, which influences vascular tone.
When TMP increases in an artery, the smooth muscle in the vessel wall responds through the myogenic response, causing contraction to resist the stretch and maintain a constant diameter. High TMP places significant mechanical stress on the endothelium, contributing to pathological changes in the vessel wall. Changes in the pressure surrounding the heart chambers, such as the negative intrathoracic pressure during breathing, directly affect the TMP of the atria and ventricles. This influence impacts both cardiac filling and the resistance the heart must pump against.
Role in Pulmonary Mechanics
Transmural pressure is central to the mechanics of breathing, where it is termed Transpulmonary Pressure (TPP) when applied to the lungs. TPP is calculated as the alveolar pressure minus the pleural pressure. A positive TPP is the force that keeps the alveoli and airways open, opposing the lung’s natural elastic recoil tendency to collapse.
During a normal breath, TPP must be maintained at a positive value, typically peaking around 10 to 15 centimeters of water (cmH2O) during maximal inspiration. This sustained positive pressure ensures the patency of the smaller bronchi and bronchioles. Without this positive distending pressure, the elastic fibers within the lung tissue would cause deflation.
Clinical Consequences of Imbalance
Disruptions to the normal transmural pressure range can lead to serious pathological conditions in both the circulatory and respiratory systems. Dangerously high TMP in a blood vessel, where internal pressure greatly exceeds external pressure, increases wall tension to a point that can exceed the tensile strength of the vessel wall. This excessive force can lead to ballooning and potential rupture, known as an aneurysm.
Conversely, a severely low or negative TMP can cause a hollow structure to collapse. In the circulatory system, if external tissue pressure exceeds internal blood pressure in a vein or capillary, the vessel is compressed, leading to a loss of blood flow known as critical closing pressure. In the respiratory system, a loss of the TPP gradient, such as when air enters the pleural space during a pneumothorax, causes the pressure difference to drop to zero, resulting in collapse of the lung. Furthermore, excessive positive pressure applied during mechanical ventilation can cause overdistension and rupture of the alveolar walls, a form of lung injury called barotrauma.