lect 5 objs

• Cell membrane


  Separates ECF from ICF
• If volume changes

  The length of the baseline changes
• If the solute concentration changes

  The height of the rectangle changes
• ECF and ICF

  Are in osmotic equilibrium
• Na+


  Confined to the ECF
• Total body water in men and women
• Amount of Na+ in the body

  Determines the volume of the ECF
• The percentage of total body water varies depending on the amount of fat a person possesses
• 2/3 of the volume is ICF and 1/3 is ECF for both men and women
• Total body water distribution

  • Men ~60%
  • Women ~50%
• Major body fluid compartments

  • ECF (extracellular)
  • ICF (intracellular)
• Fluid balance control

  1. Regulation of ECF volume by maintaining salt balance

  2. Regulation of ECF osmolarity by maintaining water balance
• Plasma volume is about 1/4 of the ECF
• ECF and ICF compositions

  Differ due to the cell membrane barrier
• Plasma and interstitial fluid


  Are nearly identical in composition except for the lack of plasma proteins in interstitial fluid
• Changes in ECF volume


  Occur by changing Na+, the major ECF cation
• Changes in ECF volume

  Alter arterial blood pressure in the same direction
• Salt intake is not controlled in humans, but salt output in the urine is closely regulated
• Blood pressure regulating mechanisms

  1. Vary the GFR and Na+ filtration by adjusting the caliber of the afferent arterioles supplying the glomeruli

  2. Vary the secretion of aldosterone, which promotes Na+ reabsorption by the renal tubules
• Changes in ECF osmolarity occur by changing H2O balance to avoid osmotic shifts between ECF and ICF cells, particularly in brain neurons
• Excess free H2O in the ECF dilutes ECF solutes, driving H2O into the cells
• Volume contraction and expansion refer to the ECF volume
• Prevention of osmotic shifts


  Stimulation of ADH (primarily) and thirst (to a lesser extent) by hypothalamic osmoreceptors
• An ECF H2O deficit concentrates the ECF solutes, driving H2O out of the cells and into hypertonic ECF
• Types of volume changes

  • Isosmolar volume expansion or contraction
  • Hyperosmolar expansion or contraction
  • Hypo-osmolar expansion or contraction
• Kidneys have many functions. They are integral to the maintenance of body water and solute concentration, acid-base status and excretory function. Kidneys are also endocrine organs. They produce calcitriol, the active form of vitamin D, and erythropoietin. They also generate local paracrine and autocrine substances. Renal epithelial cells and vasculature themselves are highly responsive to endocrine, paracrine and autocrine and stimuli
• The filtration barrier has three layers. The first is the capillary endothelium. The second is the basement membrane. And the third is the epithelium, a layer of podocytes, specialized cells that derive from the epithelium lining Bowman's capsule
• The functional unit of the kidney is the nephron. A nephron is a long tube with a swelling at one end, Bowman's capsule. The glomerular capillaries sit in the swelling like a fist sunken into bread dough. A second set of capillaries, the peritubular capillaries, wraps around the tube itself. Plasma is filtered from the glomerular capillaries into Bowman's capsule. The cells that line the tube reabsorb selected solutes and water from the filtrate or secrete them into it
• Glomerular and peritubular capillaries represent an arterial portal system. Blood enters the glomerulus through the afferent arteriole and diverges into the glomerular capillary bed. These capillaries converge into the efferent arteriole that gives rise to the peritubular capillaries
• The basement membrane is composed of material secreted by both the endothelium and the epithelium. It consists of type IV collagen fibers and negatively charged extracellular matrix material. Like the endothelium, the basement membrane restricts the passage of intermediate to large sized molecules especially those with negative charge
• The endothelium is fenestrated. The openings are 70 to 100 nm in diameter. They are not highly selective. However, the endothelium has a negative surface charge that inhibits the passage of negatively charged solutes
• The epithelial layer is comprised of podocytes. These specialized epithelial cells have foot processes with secondary toe-like protrusions called pedicels. Slit diaphragms bridge the space between the interdigitating pedicels. Pedicels and slit diaphragms bear a negative surface charge that inhibits passage of negatively charged molecules. In addition, slit diaphragms are connected to contractile elements within the podocytes. This architecture permits control over the permeability
• The podocyte is important in determining the perm-selectivity of the glomerulus and in the progression of chronic renal disease
• Substances freely filterable by glomerular capillaries

  • Water
  • Electrolytes
  • Solutes with a molecular weight less than 5200
• Substances variably filtered by glomerular capillaries


  • Molecules with molecular weights up to 68,000 to 69,000
• Larger molecules and proteins are not filtered
• Pedicels and slit diaphragms

  • Pedicels and slit diaphragms bear a negative surface charge that inhibits passage of negatively charged molecules
  • Slit diaphragms are connected to contractile elements within the podocytes, permitting control over the permeability characteristics of the epithelial layer
• Starling forces govern glomerular filtration

  1. Glomerular capillary pressure is the highest in the body and is the driving force for filtration

  2. Hydrostatic pressure in the glomerular capillaries is the driving force for filtration

  3. Oncotic pressure and hydrostatic pressure in Bowman's space oppose filtration
• Podocytes restrict large soluble anions from filtration
• Filterability of larger molecules is related to their size, shape, and charge