Osmoregulation and Body Fluid Composition

Osmoregulation

• Osmotic pressure is the pressure needed to offset the movement of pure solvents (e.g., water) across a semipermeable membrane.
• Hydrostatic pressure is the pressure exerted by a fluid on its surroundings.
• Blood pressure on vessel walls contributes to hydrostatic pressure.
• Pressure in fluid increases with depth.

Ionic Concentration and -tonic Solutions

• Ionic concentration refers to the concentration of ions in a solution.
• Semipermeable membranes allow certain substances to pass through.
• Initially, there is a net movement of water from one side to the other. 
• At equilibrium, there is no net movement of water.

Electrolyte Composition of Body Fluids

• Serum and ECF (extracellular fluid) have similar electrolyte compositions.
• Key electrolytes like Na+, K+, Ca2+, and Cl- are listed along with their respective concentrations in different body fluids.
• Active transport and diffusion are methods used during metabolism in maintaining the electrolyte balance in the body.

Homeostasis Challenges

• Animals maintain homeostasis depending on their environment.
• Blood concentration relative to the environment influences urine concentration and osmoregulatory mechanisms.
• Different animals have varied strategies like drinking seawater or excreting excess salt.
• Examples include marine elasmobranchs, marine teleosts, freshwater teleosts, amphibians, and marine reptiles.

Water Gain and Loss

• Mammals, including kangaroo rats, maintain balance between water gain and loss in their environment.
• Water gain pathways and losses include intake of water, drinking, and metabolic water production.
• Water loss pathways include urine, feces, evaporation/perspiration, and respiration.

Respiratory Evaporative Water Loss

• Respiratory water loss is affected by relative humidity, air temperature, and breathing rate.
• Inhaled air warms and humidifies as it passes through the nasal passages and lungs.
• Exhaled air cools and loses moisture, which is a form of evaporative loss.
• There is a correlation between water loss through respiration and ambient air temperature.

Mammalian Kidney Adaptation

• Kidney structure relates to environment.
• Beaver's have short loops of Henle, while desert dwelling animals like kangaroo rats have long loops of Henle.
• These adaptations reflect their respective environment's water availability.

Metabolic Water

• By definition, metabolic water is water produced as a result of metabolic processes, rather than from food intake or external sources.
• Oxidation of foodstuffs in the body produces metabolic water.
• Carbohydrates produce the most metabolic water (0.56 g of water per gram).
• Lipids are more water-efficient than Carbohydrates and proteins in producing metabolic water.

Fat and Protein Stores as Water Sources

• Migratory birds rely on stored fat for energy and water.
• Fat stores can release more water than glycogen stores.
• Protein degradation can release bound water.
• The need for energy and water is balanced with storage mechanisms in organs.

Hummingbirds and Dehydration

• Contrary to some assumptions, hummingbirds' dehydration regulation is not a daily problem.
• Frequent water intake and maximum glucose reabsorption offset urine dilution.
• Concentrated glucose absorption has significant roles in water retention.

Carb Absorption

• Carbohydrate absorption in the intestine involves several stages.
• Maltose is broken down to glucose.
• Glucose is absorbed via active transport with Na+ entering the cell.
• Glucose then leaves enter the blood by facilitated transport.

Glucose Re-absorption

• Active transport, facilitated diffusion, and sodium-potassium pumps assist in glucose reabsorption within the kidneys.

Type II Diabetes

• Hyperglycemia (high sugar) is a common symptom.
• Sweet-tasting urine was an historical diagnostic indicator.

Blood pH Regulation

• Blood pH is regulated by maintaining appropriate levels of bicarbonate and hydrogen ions.
• The kidneys and carbonic anhydrase play a role in maintaining an appropriate balance between these.
• The kidneys help maintain an internal equilibrium of these levels.

Urine Acidification

• Ammonia and phosphates act as buffers for urine.
• Urine pH is generally lower than blood pH.
• The protein in our diet affects the acidity of urine.

Vasopressin

• Also known as antidiuretic hormone (ADH), vasopressin regulates water reabsorption in the kidneys.
• Released in response to high plasma osmolarity or low blood pressure.
• Stimulates aquaporins which are proteins, to increase water permeability in collecting ducts, allowing more water to be absorbed.
• High blood osmolality or low blood pressure stimulates vasopressin production and release.

Hypertension

• Body regulates blood osmotic pressure by adjusting sodium levels in the kidneys.
• Excess sodium intake can lead water to be retained in the blood, which increases blood pressure.
• Medications such as ACE inhibitors and Angiotensin Receptor blockers can be used to manage high blood pressure.

Renin-Angiotensin-Aldosterone (RAA) Pathway

• Juxtaglomerular cells release renin in response to low blood pressure or low glomerular filtration rate (GFR).
• Renin converts angiotensinogen to angiotensin I, and then to angiotensin II , which has various effects on the cardiovascular system, including vasoconstriction and secretion of aldosterone.
• Aldosterone promotes sodium and water retention, while raising blood pressure.