Fluid Compartments Flashcards
Describe the major fluid compartments in the body
The major fluid compartments are the intracellular fluid volume (40% of body weight) and the extracellular fluid (20% of body weight). The extracellular fluid consists of the interstitial fluid (80% of ECF) and plasma (20% of ECF). These compartments are separated by a semi permeable membrane and each compartment has a distinctive pattern of electrolytes.
explain how water moves between fluid compartments and how water may enter and leave the body
Movement between compartments occurs because of hydrostatic pressure and osmotic pressure. Hydrostatic pressure is the force generated by the blood of the vessel (capillary) wall and forces fluid out of the capillaries into the interstitial space. Osmotic pressure is the pressure caused by the presence of plasma proteins within the capillaries (note that there is virtually no protein in the interstitial space) and forces fluid into the capillaries from the interstitial space. Fluid movement between intracellular space and the interstitial space is largely by diffusion.
Water may enter the body by drinking, which is regulated by the thirst centre in the hypothalamus as well as through metabolic water and ingestion of foods. Outflow of water is regulated by hormones affecting the kidney. Water can also be lost through vomiting, diarrhoea, perspiration and burns.
Discuss the pressures involved in the movement of fluid between capillaries and interstitial spaces
Capillary hydrostatic pressure filtration and osmotic pressure reabsorption are in near equilibrium. The near equilibrium between filtration from capillaries to the interstitial space and reabsorption from the interstitial space into the capillaries is referred to as Starlings Law of the Capillaries. The net outward force results in a flow of fluid that contributes to a moist interstitial space, skin turgor and a positive interstitial fluid pressure. The net outflow of fluid (3-4 litres a day) is collected by the lymphatic capillaries and returned to the cardiovascular system.
Explain the fluid shifts that occur in burns.
Burns lead to an increase in capillary permeability, loss of plasma proteins and an imbalance in Starlings Forces that promotes the formation of oedema and loss of plasma volume.
Outline what is meant by the microcirculation and how this relates functionally to the rest of the circulatory system.
Blood flow from arterioles to venules through capillaries is called the microcirculation.
Describe the control of blood flow in the peripheral circulation
Blood flow is controlled in the peripheral circulation by the diameter of the blood vessels. As diameter increases, the total cross sectional area increases and the velocity of blood flow decreases. Blood flow may be under local control; in most tissues blood flow is proportional to metabolic needs of tissues. In addition, blood flow into capillary beds can be changed by vasoconstriction or vasodilation of arterioles supplying the capillary bed and the opening and closing of precapillary sphincters. The nervous system is also responsible for routing blood flow and maintain blood pressure and sympathetic action potential stimulate adrenaline and noradrenaline.
Explain how diabetes can alter peripheral blood flow in such a way that damage occurs
Hyperglycaemic episodes leads to reduced nitric oxide (NO) production. This results in increased reactivity of smooth muscles to noradrenaline and accelerated atherosclerosis, reducing the ability to repair. This results in vasoconstriction and decreased perfusion
