Chapter 8

In general terms, the topic of hemodynamics deals with flow and distribution of blood and fluids within the body. To maintain the correct amount of intravascular and extravascular volumes, the body must maintain both hydrostatic pressure and osmotic pressure. In vessels, hydrostatic pressure refers to the pressure pushing fluid out into the interstitial tissue. In interstitial tissue, hydrostatic pressure pushes fluid into the vessels. Osmotic pressure, which is imparted by the presence of dissolved solutes, pulls fluid into the vessels and into the interstitial tissue. An imbalance in either of these two pressures results in an abnormal distribution of fluid in the cells or interstitial tissues. The term used to describe excessive amounts of fluid within the interstitial tissues or within cells is edema.

The integrity of vessel walls plays a critical role in maintaining normal distribution of fluid in the vessels and interstitial tissues. The process of coagulation serves to maintain the integrity of the vasculature in the event of disruption of the vascular wall. Inappropriate coagulation can have deleterious consequences, however. For example, abnormal coagulation can result in vessel occlusion; thus the process must be closely controlled. An adequate supply of blood to the tissues is vital because it provides oxygen and nutrients to the cells and removes toxic metabolites from the cells. An inadequate amount of blood flow to an organ is termed ischemia. Ischemia is an important cause of cellular dysfunction and, if severe, often leads to cell death. The resultant area of necrotic cells is termed an infarct. In addition to localized ischemia due to occlusion of blood vessels, a more generalized ischemia can occur due to widespread hypoperfusion of the body. This generalized hypoperfusion of the organs and resultant organ damage is called shock. Shock can result from a decreased amount of blood (i.e., hypovolemic shock), failure of the heart to effectively pump the blood (i.e., cardiogenic shock), or generalized dilation of the vasculature system secondary to infection (i.e., septic shock). This chapter will discuss edema, hyperemia and congestion, hemorrhage, thrombi, emboli, infarcts, and shock.

Basic description: Accumulation of fluid within the cells, interstitial tissue, and body cavities.

Mechanisms of edema formation: Include increased vascular hydrostatic pressure, decreased plasma osmotic pressure, lymphatic obstruction, and inflammation. Increased vascular hydrostatic pressure is usually due to impaired venous return or arteriolar dilation.

Causes of increased vascular hydrostatic pressure

• Heart failure: The heart is not pumping blood as effectively as it should, so there is a back up of blood into the veins.
• Cirrhosis: Fibrous scarring of the liver that impairs return of blood through the portal vein, thereby increasing venous pressure in portal vein tributaries and causing fluid to leak into the peritoneal cavity.
• Venous obstruction: For example, a tumor pushing on a vein will cause back up of blood, eventually with leakage of fluid into the interstitium.

Causes of decreased plasma osmotic ...

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