Body Water Composition and Balance

Body Water Composition

• Total body water constitutes about 60% of the total body weight
• All body fluids are either extracellular fluid (ECF) or intracellular fluid (ICF)
• ECF makes up about 20% of body weight and includes plasma, interstitial fluid, and transcellular water
• ICF makes up about 40% of body weight
• Transcellular water constitutes a small portion of the ECF, about 1%, and is localized in specific body areas
• Lymph is a type of transcellular water found in lymphatic vessels
• Cerebrospinal fluid is a type of transcellular water found in the brain
• Synovial fluid is a type of transcellular water found in joints
• Aqueous humor and vitreous body are types of transcellular water found in eyes
• Endolymph and perilymph are types of transcellular water found in the ears
• Pleural, pericardial, and peritoneal fluids are transcellular fluids located between serous membranes
• Glomerular filtrate is a type of transcellular water found in the kidneys

Water Balance

• Water intake occurs through ingested liquids, ingested food, and metabolic water (water of oxidation)
• Water losses occur through urine, skin (sweat), lungs (exhaled water vapor), and the gastrointestinal tract (feces)

Kidney Function

• Cells require the extracellular fluid to remain within defined limits to survive
• Kidneys primarily stabilize the volume and ion concentrations of the extracellular fluid.
• Kidneys enable mammal survival with highly variable access to water and salts
• Urine production is one of the kidney functions
• Blood filtration is one of the kidney functions
• Metabolic waste and xenobiotics excretion is one of the kidney functions
• Kidneys retrieve needed metabolic substances, including water, glucose, electrolytes, and low molecular weight proteins
• Kidneys respond to water, electrolyte, and acid-base disturbances
• Gluconeogenesis, or glucose production, is one of the kidney functions
• Hormone production associated with systemic blood pressure, red blood cell production, and calcium metabolism is one of the kidney functions

Kidney Anatomy

• Urinary system: kidneys, ureters, urinary bladder, and urethra
• The kidney regulates composition and volume of the extracellular fluid
• Only the kidney is of physiological relevance

Endocrine Functions of the Kidney

• Endocrine functions involve production and secretion of hormones into circulation
• Calcitriol, or 1,25-(OH)2-D3, is produced and stimulated by PTH to respond to hypocalcemia
• Renin is a hormone activator that regulates blood pressure and is essential to the renin-angiotensin-aldosterone system (RAA)
• Erythropoietin is a hormone that is a glycoprotein/growth factor and is essential for erythropoiesis

The Nephron

• Nephrons are the functional units of the kidney
• Humans have about 1 million nephrons per kidney; cows have about 4 million; dogs have 400,000; cats have 190,000
• Nephrons cannot be replaced and are only partially regenerated
• Kidneys contain two types of nephrons that differ with respect to their location and capillary networks
• Cortical nephrons have glomeruli located far from the cortex/medulla junction, short loops of Henle, and blood supply by peritubular capillaries
• Juxtamedullary nephrons have glomeruli located near the cortex/medulla junction
• Juxtamedullary nephrons have efferent arterioles giving rise to long straight capillaries (vasa recta) that descend into the renal medulla
• Juxtamedullary nephrons have extremely long loops of Henle necessary for urine concentration

Nephron Structures

• Renal corpuscle or Malpighian body is in the renal cortex
• Renal corpuscle consists of a glomerulus surrounded by Bowman's capsule
• Proximal tubule is longest part of the nephron, about 10 mm
• Proximal tubule consists of a proximal convoluted tubule (PCT) and a straight part (PST)
• Loop of Henle consists of a thick descending limb, a thin descending limb, a thin ascending limb (only in nephrons with long loops), and a thick ascending limb (TAL, macula densa)
• Distal tubule refers to the convoluted part (DCT) and straight part (DST)
• Collecting ducts (CD) extend from the renal cortex through the medulla
• The collecting ducts open at the renal papilla into the renal pelvis, then to the ureter, urinary bladder, and urethra for excretion
• The nephron consist of the proximal tubule (PCT and PST), thick and thin descending limb (DL), thick and thin ascending limbs (AL), the distal tubule (DCT and DST), Loop of Henle and the collecting duct (CD).

Glomerular Filtration

• Glomerular filtration is the first step in urine formation
• Large amounts of fluid filter through the glomerular capillaries into Bowman's capsule
• The glomerulus is a compact network of capillaries that retains cellular components and medium-to-high molecular weight proteins
• The glomerulus is semipermeable
• Most of what is filtered (99%+) is reabsorbed, leaving less than 1 liter to be excreted which is highly variable

Glomerular Filtration Rate (GFR)

• Adult dogs produce approximately 20 to 40 ml of urine per kilogram of body weight per 24 hours (~1.0 to 2.0 ml/kg/hour)
• Adult cats produce an average of 28 ml of urine per kilogram of body weight per 24 hours (~1.2 ml/kg/hr)
• Electrolytes like sodium and small organic compounds like glucose are filtered as freely as water
• GFR is 2.4 – 3.7 mL/min/kg, up to 5,328 mL/kg/day
• Urine is formed through the filtration of the blood
• The primary urine filtered from the blood and passes into the Bowman's capsule

Primary Urine and Tubular Fluid

• Primary urine is the ultrafiltrate accumulating in the capsular space between the glomerulus and Bowman's capsule
• Primary urine contains the same concentrations of salt and glucose as plasma
• Tubular fluid is the filtrate within the tubular system
• The tubular fluid is continuously modified through tubular reabsorption to become "final urine"
• Tubular reabsorption recovers most filtered substances, including 100% of glucose and 99% of H2O, Na+, Ca++, Cl-, and HCO3-

Glomerular Filterability and Selectivity

• Filterability of substances depends on molecular weight
• Water (18 Da) has a filterability of 1.0
• Sodium (23 Da) has a filterability of 1.0
• Glucose (180 Da) has a filterability of 1.0
• Myoglobin (17,000 Da) has a filterability of 0.75
• Hemoglobin (68,000 Da) has a filterability of 0.03
• Albumin (69,000 Da) has a filterability of <0.01
• Cationic substances are more easily filtered than anionic substances due to the negative charge of the glomerular basement membrane
• Plasma protein binding acts as a protective mechanism for some solutes such as calcium. Some drugs may be retained in the circulation for a defined time period before being eliminated

Osmolality

• Osmolality is the number of osmoles of solute per kilogram of solvent
• Osmolality is more suitable for living organisms because volume is temperature-dependent

Fluid Movement

• Hydrostatic/Hydraulic Pressure is a force that contributes to fluid movement
• Osmotic or oncotic pressure is a force that contributes to fluid movement
• The permeability of the capillary membranes in the glomerulus influences both forces. The main driving force for filtration is the glomerular capillary hydrostatic pressure. In opposition is the hydrostatic pressure in Bowman's space and the oncotic pressure of the blood plasma
• The oncotic pressure of the filtrate is essentially non-existent.

Kidney Function

• Kidneys maintain water content and plasma tonicity.
• The water is reabsorbed in the kidneys
• Terrestrial must avoid dehydration
• The water is reabsorbed by: PT, thin descending limb, and collecting duct

Water Reabsorption

• The proximal tubule (PT) reabsorbs water through aquaporins (AQP's) and solute reabsorption
• Na+ reabsorption favors water movement into the cells and interstitium
• The thin descending limb of Henle's loop reabsorbs water by osmosis
• The collecting ducts do the last big water reabsorption regulated by antidiuretic hormone (ADH)-sensitive AQP's
• ADH allows the collecting ducts to reabsorb a lot more water
• If plasma osmolality is increased, ADH is released, leading to concentrated urine.
• In the absence of ADH, the collecting duct will be relatively impermeable to water and will produce diluted urine.
• Urine concentration/dilution id defined by te permeability of water which is determined by osmotic pressure. Osmoreceptors are sensitive to changes in extracellular osmolality; stimulating the water channels.