Extracellular fluid

• Extracellular fluid is transported through the body in two stages. The first stage is movement of blood through the body in the blood vessels. The second is movement of fluid between the blood capillaries and the intercellular spaces between the tissue cells.
• The lungs remove Co2 and replace blood oxygen
• The gut gives nutrients and removes waste products from the body.
• The kidneys regulate electrolytes through excretion
• As blood passes through blood capillaries, continual exchange of extracellular fluid occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. 
• The capillary walls are permeable to most molecules in the blood plasma, with the exception of plasma proteins, which are too large to pass through capillaries readily.
• The direction of capillary diffusion is governed by osmotic pressure (the difference in water potential between the blood and the capillary wall) ie if the interstitial fluid is pressing on the capillary it will absorb, if the capillary is pressing on the interstitial fluid, it will lose fluid
• fluid and dissolved molecules are continually moving and bouncing in all directions in the plasma and fluid in the intercellular spaces, as well as through capillary pores, thus, extracellular fluid is continually being mixed by kinetic forces, maintaining its homogeneity
• The blood picks up oxygen in alveoli. The alveolar membrane between the alveoli and the lumen of the pulmonary capillaries, is only 0.4 to 2.0 micrometers thick, so oxygen rapidly diffuses by molecular motion through this membrane into the blood.
• in the gut carbohydrates, fatty acids, and amino acids, are absorbed from ingested food into the extracellular fluid of the blood.
• Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested.
• Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells
• End products of cellular metabolism

  • Urea
  • Uric acid
• Excesses of ions


  • (not specified)
• Kidneys

  Remove excess water from the body
• Passage of blood through the kidneys

  1. Filters large quantities of plasma through the glomerular capillaries into the tubules

  2. Reabsorbs into the blood substances needed by the body

  3. Reabsorbs poorly most of the other substances that are not needed by the body

  4. Passes through the renal tubules into the urine
• Glomerular capillaries

  Capillaries in the kidneys where plasma is filtered
• Tubules

  Tiny tubes in the kidneys where filtrate is processed
• Substances needed by the body

  • Glucose
  • Amino acids
  • Appropriate amounts of water
  • Many of the ions
• Substances not needed by the body

  • Urea
  • Creatinine
• Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces.
• Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces.