1. Overview of osmolarity, comparing different animals:

• Ionic composition of the extracellular body fluids (EBF) of marine invertebrates is often similar to that of sea water (Eckert, Table 14-1-1), while the ionic composition of EBF of vertebrates has about 1/3 the concentration of some ions like Na⁺ and Cl^- as found in sea water. Magnesium concentrations in vertebrate EBF are quite low compared to those in sea water (Eckert, Table 14-1-2).

• Osmoregulators and Osmoconformers - The ionic differences between marine invertebrates and most vertebrates in the tables shown above highlight the fact that marine invertebrates are osmoconformers - the body fluids match the osmotic composition of the sea water. But, most vertebrates are osmoregulators and adjust the ionic composition and the total amount of solute in their body fluids (blood, lymph) to keep these properties constant.

• Note some interesting things about ionic compositions:
  • The lower ionic concentrations in terrestrial vertebrates as compared to marine vertebrates may be the result of the freshwater evolution of most vertebrates. (Marine teleosts, bony fish - most fish, excluding sharks, etc. - evolved in fresh water. Marine teleosts found their way back into the marine environment.)

  • Many enzymes require Mg⁺⁺ to work properly. Notice that Mg is readily available in sea water and even vertebrates retain some Mg⁺⁺ in their EBF.

2. Water and solute control.

• FIRST IMPORTANT IDEA: Total composition of all solutes (mostly ions, since even proteins have some charge) is balanced between the intracellular and extracellular compartments. Water can move between the intracellular and extracellular compartments 5 to 8 orders of MAGNITUDE more easily than most solutes. Thus, if osmolarity, the total amount of fixed solute (including sodium, that is pumped out from cells), isn't kept the same inside and outside cells, water will flow from cells or into them, causing shrinking or swelling. Solute concentrations will change and biochemistry will stop working.

• SECOND IMPORTANT IDEA: Not just total osmolarity is important. Cells produce nitrogen wastes from breakdown of proteins (urea, uric acid). These have to be removed. Ions must be in correct concentrations. If there is too much or too little sodium or chloride outside cells, membrane potentials will be affected So, we need some organs to remove the nitrogen waste and to regulate ionic composition of the extracellular fluid.

3. The human kidney - a sophisticated of water and ion balance organ.

• Overview: The human kidney consists of individual functional units called nephrons (Eckert, Fig. 14-18), arranged in a radial fashion to form the kidney (Eckert, Fig. 14-17). In addition to clearing the blood of nitrogen and other wastes and balancing water and ion concentrations, the kidney regulates blood pressure by producing renin, that helps convert angiotensiongen (produced by the liver) to to angiotensin II. And, the kidney is an endocrine organ, producing erythropoietin that simulates red blood cell production in the bone marrow.

• The kidneys receive about 20% of the blood flow from the heart, about 1.25 liters of blood per minute. All the blood in the body is filtered in about 5 minutes. The kidney is a filter that produces about 180 liters of filtrate per day. Most of this is reabsorbed, yielding only 1 - 1.5 liters of urine per day for the average human. (You should know the typical values for blood flow, volume, urine flow, etc., given here!) As we saw previously, ADH is necessary for normal reabsorption of water back into the blood. If we drink ethanol, ADH production is inhibited and urine output increases significantly.

• Blood circulation in the kidney uses a portal system. (Can you describe what characterizes a "portal system"?) (Comparison of capillary networks)

• The anatomy of a kidney tubule shows filtrate flow through the glomerulus, into Bowman's Capsule, down the descending tubule, into the Loop of Henle, up the ascending tubule, past the macula densa, into the distal convoluted tubule and the collecting duct and tubule. (The nephron)

• There are two kinds of nephrons - cortical nephrons and juxtamedullary neurons. Only birds and mammals have the long juxtamedullary nephrons necessary for extreme concentration of urine. (Eckert, Fig. 14-18)
  
  Look at how birds and mammals are not closely related. However, both have many similar physiological adaptations.

• High resistance in the outlet to the glomerulus, the efferent arteriole, causes filtration into Bowman's Capsule (Eckert, Fig. 14-20).

• Filtration involves positive pressure from the glomerulus and opposing osmotic pressure from solutes in the blood that are impenetrable to the glomerular membranes (Eckert, Fig. 14-22).

• This figure shows the details of the glomerulus and Bowman's Capsule.

• The glomerulus filters the blood in the first step towards forming urine --> Endothelial cells lining the capillaries of the glomerulus contain negatively charged glycoproteins that repel blood proteins and prevent protein loss. Secondly, a sophisticated system mechanically filters blood through a diaphragm containing nephrin, a cell adhesion molecule. (Details of the filtration process.)

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2009, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.