Microcirculation Lab

The central focus of our laboratory is to apply engineering techniques and methods toward gaining an understanding of mechanical factors that affect the distribution of pressure and flow in the microcirculation in health and disease. The microcirculation is the business end of the vascular system and with its 60,000 miles of capillaries in the human body is responsible for the transport of nutrients and metabolites to the body's many tissues. Perfusion of the microvascular network (comprised of the capillaries and associated small vessels smaller than the thickness of a human hair) is strongly dependent upon the mechanical properties of blood cells and the vessel wall. Our goal is to quantitatively examine the structural features of blood cells and the microvessel wall at the cellular and molecular level to describe microvascular function in the normal flow state as well as a variety of pathological disorders, and thereby establish the basis for the design of new therapeautic strategies to combat the disease process. Some of the pathological disorders that are the subject of attention are those related to abnormalities in blood cell properties (such as sickle cell disease), inflammation, shock and the low flow state. To achieve these goals, studies are performed using the techniques of intravital microscopy to study blood flow in the microcirculation of the living anesthetized animal (mainly rats). A broad spectrum of electromechanical techniques are employed to measure intravascular pressures and flows and imaging processing techniques are applied to video images to acquire data which can then be integrated through the use of computer models to decipher the mysteries of microvascular function.

To study the mechanics of microvascular perfusion we invesitgate the rheology of blood and blood cell interction with the microvessel wall. Rheology, (from Greek rheo, flow) is the study of the flow and deformation of materials. The general aim of rheological studies is to characterize the intrinsic mechanical properties of a fluid or solid in terms of the resistance it offers to deformation under a given load, or shear at a prescribed rate . The viscous properties of blood in large bore tubes and viscometric instruments has provided a foundation for understanding the rheology of blood in microvessels. With the assumption that blood is a homogenous fluid with an intrinsic viscosity, these devices have revealed that blood viscosity falls as shear rates rise (shear thinning) from on the order of 0.1 to 1000 sec-1, in contrast to the behavior of Newtonian fluid whose viscosity is invariant with shear rate. At the microvascular level, the particulate nature of blood flow results in large departures from Newtonian fluid behavior.

To determine the factors that affect the viscosity of blood in microvessels, we must determine how red blood cells and white blood cells contribute to the resitance to flow in small blood vessels.

Hematocrit

The prinicipal factor affecting the viscosity of blood is hematocrit.

Shown above is the distribution of red blood cells at three successive bifurcations in the cremaster muscle of the mouse. The fraction of red cells present (hematocrit) and plasma vary due to the skimming of plasma into the left branch of the arteriole. The hematocrit in the capillary on the right is greatly reduced because the red cells speed up relative to the plasma as they squeeze through the capillary. Since they must travel faster than the plasma, there must be fewer of them present to maintain the same proportions of cells and plasma as blood exits the capillary. This is the so-called Fahraeus Effect.

The volume fraction of red cells present in a microvessel is the microvessel hematocrit, Hmicro. Microvessel hematocrit falls as blood courses its way from feeding arterioles to capillaries, and then increases again in the postcapillary venules. Hmicro was measured by photometric methods and normalized with respect to systemic hematocrit (Hsys) obtained in the large blood vessels.

White Blood Cells

White blood cells (leukocytes) may also dramatically affect the resistance to flow. As blood exits the capillaries, there is a phase separation of white blood cells (WBCs) and red blood cells (RBCs).

As RBCs exit the capillaries, they push the WBCs toward the wall and enable then to roll along the endothelium. Specific adhesion molecules on the surface of the endothelium and WBCs enhance their adhesiveness and enable them to roll along the wall for considerable distances.

During inflammation, the adhesiveness of the venule wall increases and WBC rolling is increased leading to their subsequent firm adhesion. Shown above is the rapid increase in WBC rolling and adhesion in response to the peptide fMLP that mimics inflammatory products released within the tissue.

As the number of WBCs adhering to the walls of venules increases, the resistance to flow increases dramatically. As few as 10 WBCs adhering per 100 microns of venule length can result in a two-fold increase in hemodynamic resistance.


With reductions in upstream to downstream pressure drop flow falls in a nonlinear manner. At very low pressure gradients, flow stops completely. The apparent viscosity of the blood rises as mean velocity falls (lower panel).	As shear rates are reduces, the number of WBCs adhering to the walls of postcapillary venules increases greatly. Shown above is the number of WBCs that are firmly adhered to the endothelium of postcapillary venules as flow is mechanically reduced by compressing the feeding vessel upstream and the endothelium is made "sticky" with fMLP..

As shear rates are reduced, red cell aggregate and tend to obstruct the capillary entrance. With weak aggregation, red cells form rouleaux, which look like stacks of coins. As the strength of aggregation is increased, red cell clumps are formed which are more difficult to disrupt at the entrance to capillaries.


Red cell rouleaux are formed as shear rates are reduced and RBCs aggregate. They become jammed at the capillary entrance slowly break apart with time.	With increased strength of aggregation, clumps of RBCs are formed which lodge at the capillary entrance. They are more difficult to remove and may block the capillary permanently.

Blood Cell Deformability

An important determinant of the resistance to blood flow is blood cell deformability. Many diseases result in abnormal blood cell properties, such as sickle cell disease. Stiffer than normal cells may become trapped at the entrance to capillaries and obstruct flow, thereby reducing delivery of oxygen to tissue.

Cell deformability determines which capillaries will be perfused. Smaller diameter capillaries require greater deformations of red cells and white cells in order for them to enter a capillary. A bolus of fluorescently labeled plasma (A) easily passes through all capillaries in the cremaster muscle capillary network. In contrast, fluorescently labeled RBCs (B) are confined to the central portion of the network. The much sitffer leukocytes (C) travel from arteriole to venule through larger diameter vessels that comprise thoroughfare channels that run through the central portions of the capillary bed where the pressure gradients from arteriole to venule are the largest.

Microcirculation Laboratory

Director: Dr. Herbert H. Lipowsky

Mission