Osmoregulation and Excretion [Important words are in bold]

The Basics Osmosis is the diffusion of water across a selectively permeable membrane. Osmosis is driven by changes in osmolarity (moles of solute per liter). When two solutions differ in osmolarity, the one with the greater osm. is called hyperosmotic and the one with smaller osm. is called hypoosmotic. If the osmolarities are the same, they’re called isoosmotic.

Osmotic Challenges Osmoconformers choose to regulate their internal osmolarity little or none. Osmoregulators do regulate their internal osmolarity. This can be beneficial in some situations, but comes with a hefty energy cost, in some animals taking up almost thirty percent of their energy budget.

Also, this is important: Animals that can’t tolerate great changes in external osmolarity are called stenohaline. Those that can are called euryhaline.

The next few slides are basically a list of a few adaptations some animals have to deal with differences in osmolarity.

Marine Animals Need to keep water in their bodies. They are osmoregulators. Fish are hypoosmotic to their surroundings and thus have mechanisms for transporting salt out of their systems.

Freshwater Animals… Are hyperosmotic to their environment. They face the problem of keeping water out. A notable adaptation is the ability to secrete large amounts of very dilute urine.

Fish like salmon… These fish are euryhaline, and as such are able to switch gears between water uptake-salt release and water release- salt uptake. Just thought I’d throw that in there.

Animals that live in temporary environments… Some aquatic animals have the ability to survive in a near water-less dormant state if their environment dries up. This state is called anhydrobiosis, and is currently an area of research.

Land Animals… These animals face the problem of living in a dry environment, therefore they must have adaptations that favor water retention and that reduce water loss. In these animals, lost water can be regained through a few methods such as metabolism or ingestion.

Transport Epithelia These are basically special types off epithelial cells that regulate transport in cells. They are arranged in tubule networks.

The next few slides will be focusing on the production and excretion of nitrogenous wastes.

Nitrogenous Wastes? They result from the body’s breaking down of macromolecules. The basic product is known as ammonia (NH 3 ) but many organisms secrete different substances due to ammonia’s great toxicity.

Ammonia- Must be diluted due to its toxicity. Usually only secreted in invertebrates. Urea- The molecular formula is CH 4 N 2 O. It is the product of the combination of ammonia with CO 2 It costs a bit more energy to produce, but is approx. 100,000 times less toxic than ammonia. Doesn’t need to be too dilute. Uric Acid- Its molecular formula is C 5 H 4 N 4 O 3 and it requires even less water to dilute it in excretion. Its primary disadvantage is its relatively high cost to make (energy).

The pattern here is that different mediums for nitrogen secretion require more or less water to dilute them with due to their toxicity. Evolutionarily, these adaptations were developed based upon the availability of water.

The next few slides will focus on the process of excretion and various methods by which it is completed

Steps of the Excretory Process. Filtration- Excretory system collects filtrate from the blood. Reabsorption- Transport epithelium reclaims valuable substances from the filtrate (water, etc.) Secretion- Toxins and excess ions are extracted from body fluids. Excretion- Filtrate leaves the system via whatever.

Protonephridia (Flame bulb Systems) Those beating cilia in the highlighted circle draw water and solutes in and move the filtrate outward until it leaves via a nephridiopore.

Metanephridia Fluid enters a nephrostome and passes through a collecting tubule which then opens to a nephridiopore and empties the contents of the lumen.

Malphigian Tubules These tubules open up into the digestive tract and culminate in tips that are coated in hemo- lymph. From here, nitrogen- ous wastes Diffuse into the tubule and head towards the rectum. Before passing through the rectum, H2O and valuable organic molecules are removed.

The next few slides have to do with the mammalian kidney.

Kidneys are supplied with blood via a renal artery and drained by a renal vein. Urine exits the kidney via a duct called the ureter. Both ureters drain into the urinary bladder. During urination, urine is expelled through a urethra. The kidney is comprised of nephrons: “units” which do the actual filtration. The Basic Process

The Nephron Each nephron contains a ball of capillaries called a glomerus. The glomerus is contained in the Bowman’s capsule, which is full of interstitial fluid. The Bowman’s capsule is the beginning of a long tubule. This long tubule is the site where much filtration and adjustment of filtrate osmolarity occurs.

The Nephron Pathway From the Bowman’s Capsule, the filtrate passes through the proximal tubule, down the loop of Henle, back up the distal tubule, and back down the Collecting Duct. The filtrate then flows into the renal pelvis, where it is drained by the ureter.

A bit more on nephron form: Every nephron is supplied by an afferent arteriole. Arterioles converge as they leave the glomerus, forming an efferent arteriole. The capillaries that extend downward around the loop of Henle are called the vasa recta.

Nephrons alter the osmolarity of the filtrate Osmolarity is about 300mosm/L Osmolarity at the bottom of the loop of Henle is about 1200mosm/L Osmolarity, by the time it has reached the top of the loop, has returned to about 200mosm/L Osmolarity is reduced even further in the Distal tubule to about 100mosm/L Osmolarity increases down the collecting duct all the way back to 1200mosm/L

Regulation of Kidney Function An important regulator in this process is ADH (antidiuretic hormone). ADH enables the collecting duct and distal tubule to increase in permeability. An increase in ADH is a decrease in urine volume. The body can do this when there is a water shortage. It also works both ways, a decrease in ADH resulting in a decrease in permeability and an increase in urine volume.

Kidney Regulation (Pt. II) The juxtaglomerular apparatus can produce renin. Renin activates Angiotensin II, which constricts blood flow to the kidneys. Angiotensin II also stimulates the adrenal glands to produce aldosterone, which increases Na + and H 2 O reabsorption.

The End!