Biofluid Mechanics of Special Organs and the Issue of System Control

Abstract

In the field of fluid flow within the human body, blood flow in the systemic circulation has been the main focus since the recognition by William Harvey that blood was in fact in continuous circulation and carried by a network of blood vessels. But beyond the systemic circulation, other fluids and other fluid flow phenomena pervade the body so totally that it would be hard to imagine a bodily function of any kind that does not involve fluids or fluid flow. In fact, the study of the systemic circulation is limited to not only the type of fluid involved but also to the type of service which the systemic circulation provides -the first being blood, of course, the second is transportthe principal function of the systemic circulation is to provide a means of transporting blood continuously from the heart to tissue cells and back again. Some of the most fascinating fluid flow phenomena within the human body involve fluids other than blood and a service other than transport-the lymphatic and pulmonary systems provide two striking examples. In this paper we outline the special fluid mechanics of these two systems. While transport is still involved in both cases, this is not the only service which they provide and blood is not the only fluid involved. In both systems, filtration, extraction, enrichment, and in general some "treatment" of the fluid itself is the primary function. In the pulmonary system, the liquid lining of the lungs plays a pivotal role, and the mechanical interaction between tissue and liquid is of key importance to lung viability. In disease states such as respiratory distress syndrome, the lining fluid can become dysfunctional, leading to reduced gas exchange and damage to sensitive pulmonary tissues. The study of the systemic circulation has also been conventionally limited to treating the system as if it were an open-loop system in which the main hemodynamic variables such as pressure and flow are governed by the laws of fluid mechanics independently from the physiological controls and regulations that govern these same variables. This implies that any failure of the system can be fully explained in terms of the laws of fluid mechanics, which of course is not the case. While a system failure due to a physical obstruction in a blood vessel can be readily explained in terms of the laws of fluid mechanics, a system failure due to arrhythmia cannot. In this paper we examine the clinical implications of these issues and of the special biofluid mechanics issues that arise in the lymphatic and pulmonary systems.

Application of microscale culture technologies for studying lymphatic vessel biology

“…Mechanical stimulation due to interstitial fluid flow or slowly moving convective flow through the tissue interstitium has emerged as a potent mediator of vascular morphogenesis . The driving forces for interstitial flow are hydrostatic and osmotic pressure differences between blood vessels, the interstitial space, and lymphatic vessels (Figure ). These forces (also known as the Starling forces ) determine the rate of transendothelial filtration (Figure ).…”

“…The level of transvascular volume flux (J v ) scales with the hydraulic conductivity (L p ), which is a function of vessel permeability. Fluid outside of blood vessels moves through the interstitium as interstitial flow that is collected by lymphatic vessels lymphatic vessels 55,56 (Figure 3). These forces (also known as the Starling forces 57 ) determine the rate of transendothelial filtration ( Figure 3).…”

Demonstration and Analysis of the Suction Effect for Pumping Lymph from Tissue Beds at Subatmospheric Pressure

“…This adhesion may have resulted from surface tension or direct adhesion. We have shown that these vessels show significant expression of integrin components necessary for adhesion to collagen 27 , 28 .While we are uncertain of the exact adhesion mechanism, the results do clearly illustrate that external tethering facilitates the suction effect in the presence of negative transmural pressures.…”

A continuum model of interfacial surfactant transport for particle methods

“…The study of modeling for surfactant transport is important in technological applications as well as biological/clinical aspects. Pulmonary surfactant plays a vital role in reducing the surface tension of the liquid lining alveoli and airways, and significantly influences the behavior of the liquid in lung [4][5][6].…”