Osmotic Regulation in Cells

Osmotic Regulation

• Osmosis is the movement of water molecules across a cell membrane from areas of high water concentration to areas of low water concentration.
• The environment can be classified as hypertonic (higher solute and lower water concentration), hypotonic (lower solute and higher water concentration), or isotonic (equal water and solute concentration).
• Isotonic environment is ideal for most animal cells, and osmotic regulation helps maintain it internally.

Osmotic Regulation in Marine Invertebrates

• Osmotic conformers maintain an equal solute concentration to seawater, have limited osmotic regulation, and are stenohaline (tolerate little changes in salinity).
• Euryhaline species, on the other hand, can tolerate a wider range of salinity.

Osmotic Regulation in Fish

• Marine fish live in a hypertonic environment and deal with water loss and salt uptake by hypoosmotic regulation.
• They do this by drinking water, producing a concentrated urine, and excreting salt through gills by active transport.
• Freshwater fish live in a hypotonic environment and deal with salt and minerals loss and water uptake by hyperosmotic regulation.
• They do this by drinking limited water and producing a diluted urine.

Life in Temporary Pools

• Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state called anhydrobiosis.
• Examples include tardigrades.

Osmotic Regulation in Terrestrial Animals

• Terrestrial animals lose water by evaporation, excretion, and replace it by water in food, drinking, and metabolic water.
• They regulate the solute content of body fluid that bathes their cells.

Transport Epithelia

• Transport epithelia are epithelial cells that are specialized for moving solutes in specific directions.
• They are typically arranged in complex tubular networks.
• An example is in nasal glands of marine birds, which remove excess sodium chloride from the blood.

Excretory Systems

• Nitrogenous wastes are produced by animals and need to be excreted.
• Aquatic organisms release ammonia across the whole body surface or through gills.
• Mammals and most adult amphibians convert ammonia to urea in the liver, and the circulatory system carries it to the kidneys.
• Insects, land snails, and many reptiles, including birds, mainly excrete uric acid.

Functions of Excretory Systems

• Most excretory systems produce urine by refining a filtrate derived from body fluids.
• Key functions include filtration, reabsorption, secretion, and excretion.

Types of Excretory Systems

• Protonephridia in acoelomates (flatworms) and pseudocoelomates (nematodes and rotifers) collect body fluids through collecting tubules and the action of a flame cell.
• Metanephridia in annelids is a more sophisticated system, having two openings and being surrounded by a network of blood vessels.
• Antennal glands (green glands) take a protein-free ultrafiltrate from the blood, and reabsorbs salts prior to excretion.
• Malpighian tubules in insects and spiders operate in conjunction with the rectum to secrete insoluble uric acid.
• Vertebrate kidneys have three physiological functions: filtration, reabsorption, secretion, and excretion.
• The functional unit of the kidney is the nephron.

Osmoregulation

• Osmoregulation is the process by which animals regulate the concentration of solutes in their cells and bodily fluids.
• Osmoregulation is crucial as it helps maintain proper cellular function and prevents dehydration or overhydration.

Osmosis

• Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane.
• Osmosis helps achieve equilibrium, a state in which the concentration of solutes is equal on both sides of the membrane.

Types of Environments

• Hypotonic environment: a solution with a lower concentration of solutes than the cell.
• Hypertonic environment: a solution with a higher concentration of solutes than the cell.
• Isotonic environment: a solution with the same concentration of solutes as the cell.

Cellular Response to Osmotic Pressure

• In a hypotonic environment, water moves into the cell, causing it to swell and potentially lyse (burst).
• In a hypertonic environment, water moves out of the cell, causing it to shrink and potentially die.

examples of Osmoregulation in Animals

• Marine invertebrates: Some marine invertebrates are osmotic conformers, meaning they regulate their body fluid concentration to match that of the surrounding seawater.
• Spider crabs: can survive in a narrow range of salinity (400-650 micro moles).
• Shore crabs: can survive in a wider range of salinity (up to 600 micro moles) due to their ability to tolerate changes in salinity.

Fish

• Fish that live in the ocean: live in a hypertonic environment, and must drink water to replenish lost fluids and excrete excess salt through concentrated urine.
• Fish that live in freshwater: live in a hypotonic environment, and must excrete excess water through diluted urine and avoid drinking too much water.

Terrestrial Animals

• Terrestrial animals constantly lose water through evaporation and breathing, and must replenish it through drinking water and eating food with high water content.
• Desert animals: can survive without drinking water by relying on metabolic water produced during cell respiration.

Metabolic Water

• Metabolic water is water produced as a byproduct of cell respiration.

• In humans, metabolic water makes up 12% of our daily water intake.

• In desert animals, metabolic water can make up to 90% of their daily water intake.### Metabolic Water in Desert Animals

• Fats contain a lot of hydrogen atoms that can combine with oxygen to form water during metabolic breakdown.

• When desert animals eat fatty foods, they also secure the water that will be produced during metabolism.

• This metabolic water is essential for desert animals to compensate for water losses.

Water Conservation in Desert Animals

• Desert animals minimize water loss through urine by not urinating much.
• Evaporation is a significant problem for desert animals.
• Feces contain a minimal amount of water.
• Desert animals must regulate their water intake to compensate for losses and eliminate excess water and minerals.

Nitrogenous Wastes

• Nitrogenous wastes result from protein digestion and must be removed to prevent toxicity.
• Animals have different strategies for removing nitrogenous wastes, depending on their environment and physiology.

Strategies for Nitrogenous Waste Removal

• Aquatic animals convert amino groups to ammonia (NH3) and excrete it through skin, gills, or body coverings.
• Mammals convert amino groups to urea, which is less toxic than ammonia, and excrete it in urine.
• Birds, reptiles, and land snails convert amino groups to uric acid, which is non-toxic and can be stored in the body until excreted.

Excretory Systems

• Excretory systems produce urine by refining a filtrate that collects excess water, salts, minerals, and nitrogenous wastes.
• Filtrate undergoes reabsorption, where the animal reclaims water and other essential materials.
• Remaining waste is then secreted and excreted.

Primitive Excretory Systems

• Proton nephridia (flatworms) use flame cells to push water and wastes out of the body.
• Metanephridia (annelids, e.g., earthworms) have a network of capillaries for reabsorption and secretion.

Advanced Excretory Systems

• Crayfish have a green gland (antennal gland) that filters the hemolymph (open circulatory system).
• Insects and spiders have malpighian tubules that absorb and secrete ions, water, and nitrogenous wastes into the intestine.
• Vertebrates have kidneys with nephrons, which filter, reabsorb, secrete, and excrete waste.

Kidney Function

• Nephron is the functional unit of the kidney, responsible for filtration, reabsorption, secretion, and excretion.
• Nephron consists of:
  • Afferent arteriole
  • Efferent arteriole
  • Bowman's capsule
  • Glomerulus
  • Proximal convoluted tubule
  • Descending branch of Henle's loop
  • Ascending limb of Henle's loop
  • Distal convoluted tubule
  • Collecting duct

Nephron and Urinary System

• The nephron is the functional unit of the kidney responsible for urine formation.
• The urinary system is closely associated with the cardiovascular system.

Cardiovascular System and Nephron

• The cardiovascular system contains blood, which has plasma with electrolytes, salts, and waste products.
• Arteries are blood vessels that carry blood away from the heart, and veins are blood vessels that carry blood to the heart.
• 99% of the time, arteries are associated with oxygen-rich blood, and veins are associated with oxygen-poor blood.
• Cardiovascular system structures associated with the nephron include the arteriole, afferent arteriole, glomerulus, efferent arteriole, and peritubular capillaries.

Nephron Structure

• The nephron consists of a Bowman's capsule, glomerulus, proximal convoluted tubule, descending limb of the loop of Henle, ascending limb of the loop of Henle, distal convoluted tubules, and collecting duct.
• The glomerulus plus Bowman's capsule together form the renal corpuscle.

Urine Formation

• There are three processes involved in urine formation: glomerular filtration, tubular reabsorption, and tubular secretion.
• Glomerular filtration occurs at the renal corpuscle, where smaller substances in the plasma are separated from larger substances.
• Tubular reabsorption is the movement of substances from the tubules of the nephron into the peritubular capillaries.
• Tubular secretion is the movement of substances from the peritubular capillaries into the tubules of the nephron.

Urine Formation Processes

• Glomerular filtration: smaller substances in the plasma are separated from larger substances, and the smaller substances are delivered to the nephron via the afferent arteriole.
• Tubular reabsorption: substances are reabsorbed back into the body via the peritubular capillaries, such as water, glucose, amino acids, ions, and sodium.
• Tubular secretion: substances are added to the filtrate to be eliminated as waste, such as potassium ions, hydrogen ions, ammonia, and drugs.

Urine Composition and Expulsion

• After the distal convoluted tubules, the substance is now called urine and enters the collecting duct.
• At the collecting duct, the last of tubular reabsorption of water occurs, and the dilution or concentration of urine occurs.
• Once urine has been produced by the kidney and transported to the urinary bladder, it's time for urine to be expelled from the body through micturition (urination).
• The urinary bladder is composed of transitional epithelium and smooth muscle, and embedded in the wall are sensory receptors or stretch receptors that are attached to sensory neurons.

Micturition

• When the bladder becomes distended due to filling with urine, the stretch receptors within the walls of the bladder send an action potential down the sensory neuron, causing the smooth muscles of the urinary bladder to contract.
• Another action potential from the sensory neuron will be sent up the spinal cord to the motor cortex of the cerebrum, and then to another motor neuron to stimulate the opening of the urinary sphincter.
• There are two urinary sphincters: the internal sphincter, which is composed of smooth muscle and under involuntary control, and the external sphincter, which is composed of skeletal muscle and under voluntary control.