Organismal Biology: Fluid Regulation

• Organismal Biology focuses on the regulation of body fluids.
• Including an introduction to osmoregulation and invertebrate excretory systems
• And the mammalian excretory system

Osmoregulation and Invertebrate Excretory Systems

• Lesson 13.1 involves enumerating structures and processes involved in the regulation of body fluids in animals.
• Lesson 13.1 also describes excretory systems in animals, especially the human urinary system.
• Key terms related to osmoregulation are defined.
• Different types of animals are described based on the osmolarity of their body fluids.
• Excretory systems in invertebrates are enumerated.

The Internal Environment: Extracellular Fluid

• Water makes up approximately 60% of the body.
• Intracellular fluid, or cytosol, accounts for two-thirds of the total body water.
• The remaining portion is the extracellular fluid, also known as the internal environment.
• The extracellular fluid (ECF) includes the fluid outside cells and in spaces between blood vessels.
• ECF can be blood plasma, transcellular fluid, or interstitial fluid.
• The interstitial fluid between blood vessels and cells makes up about 11 liters of the ECF and 26% of the total water in the body.
• Interstitial fluid contains water, biomolecules, ions, cell wastes, and white blood cells.
• Blood plasma, or intravascular fluid, is the yellowish liquid part of the blood
• Blood plasma is about half the total volume of blood and three liters of the ECF in the body.
• Transcellular fluid fills spaces in the body where fluid does not have biological functions or does not flow in large volumes; examples include water in eyes (but not tears).
• Dissolved solutes in the ECF are expressed as osmolarity, measured in milliosmoles per liter of solution (mOsm/L).
• Water flows from areas of higher to lower osmolarity across a semi-permeable membrane in a process known as osmosis.
• Water and ion balance is regulated by osmoregulation.
• Metabolic processes generate products used by cells, and wastes are eliminated through excretion.

Osmolarity of Body Fluids

• Osmoconformers allow the osmolarity of their body fluids to equal the environment.
• Osmoconformers create an isosmotic condition, seen in most marine invertebrates, requiring less energy for osmoregulation.
• Osmoregulators maintain their body fluids' osmolarity unequal to the environment.
• Osmoregulators either discharge water in hypotonic conditions or take in it in hypertonic conditions.
• Ammonia comes from aquatic invertebrates and larval amphibians.
• Ammonia is soluble in water but highly toxic, and it is released in dilute amounts.
• Urea is from most amphibians, reptiles, and mammals
• Urea requires more energy to form by combining ammonia with a bicarbonate ion (HCO3⁻).
• Urea is less toxic and needs less water to excrete.
• Uric acid is from birds, insects, and some reptiles.
• Uric acid is non-toxic but requires more energy to create and is insoluble in water.

Excretion in Simple Organisms

• Cell membranes in unicellular organisms facilitate waste exit.
• Freshwater protists like Paramecium expel excess water with wastes via an organelle in the cytoplasm, known as the contractile vacuole.

Invertebrate Excretory Systems

• Flatworms found in hypotonic conditions like Dugesia use a network of tubules called protonephridia, ending in flame cells
• Water enters and is filtered by the flame cell
• Wastes exit via an external pore or the nephridiopore.
• Most annelids and adult mollusks have metanephridia or a tubular network, with a funnel-shaped internal opening, known as a nephrostome, to collect body fluids.
• Wastes are stored as urine in the bladder and later excreted through a nephridiopore.
• Terrestrial arthropods have Malpighian tubules, which do not filter water.
• They secrete uric acid, Na+, and K+ ions so that osmosis of fluids can occur from hemolymph to the hindgut for absorption.
• Urine and feces are then excreted at the anal pore.
• Osmoregulation maintains the balance of body fluids, involving fluid movement via osmosis and metabolic waste excretion.
• The extracellular fluid, comprising blood plasma, transcellular fluid, and interstitial fluid, surrounds the cells.
• Animals have diverse excretory structures and produce different types of nitrogenous wastes.

Mammalian Excretory System

• Lesson 13.2 focuses on characterizing the mammalian urinary or excretory system.

Excretion in Mammals

• The mammalian excretory system has two kidneys, each with a ureter that leads to a urinary bladder for urine storage.
• Urine is expelled from the bladder through the urethra.
• Kidneys are bean-shaped and have an outer coat of connective tissue, that being the renal capsule.
• The kidney structure includes the cortex (middle layer) and the inner zone or medulla, which contain nephrons and blood vessels.
• Each kidney has approximately 1.3 million nephrons, with a total length of 80 km if connected.
• The nephron serves as the functional unit for filtering blood.
• About 1,600 L of blood pass through the kidneys daily.
• Filtration by the nephron beings in the glomerulus, a ball of capillaries enclosed by the Bowman’s capsule.
• High blood pressure forces filtrate out of the capillaries, which is then collected by the capsule.
• Larger solutes, proteins, and cells remain in capillaries
• Nutrients and ions mix in the glomerular filtrate, which then passes by the Bowman’s capsule and into the renal tubules.
• The renal tubules filter and reabsorb ions
• The renal tubules include the proximal tubule (has brush borders for greater surface area), the Loop of Henle, the distal tubule, and the collecting duct.

Tubular Reabsorption in the Kidneys

• Useful materials like salts, water, glucose, and amino acids are taken to the peritubular capillaries around the renal tubules and glomerulus during tubular reabsorption.
• Receptors in the juxtaglomerular apparatus regulate blood pressure, blood flow volume, and filtration rate.
• Osmolarity of fluids increases when the body is dehydrated
• Vasopressin, or antidiuretic hormone (ADH), is secreted by the hypothalamus via the pituitary so the renal tubules are more permeable and reserve/save water.
• The kidneys release renin when pressure receptors detect low ECF volume, which means reduced sodium
• Renin indirectly starts the secretion of aldosterone for Na+ reabsorption.
• Saliva production is restricted by the hypothalamus thirst center when there is a high solute concentration in the ECF to stimulate thirst and the urge to drink.
• The concentration of hydrogen ions, or pH, in the ECF, varies due to various cellular processes.
• The kidneys can eliminate excess H+ ions and reabsorb HCO₃ ions into the blood to maintain acid-base balance.
• The mammalian excretory system consists of the kidneys, ureter, urinary bladder, and urethra.
• Kidney functions include blood filtration, urine formation, regulation of body fluids, and acid-base balance for homeostasis.
• A nephron is the kidney's functional unit.
• Blood completes about 300 cycles in the kidneys per day.
• The hypothalamus and associated glands and hormones regulate the urinary system's fluid balance.