Urinary Dr.V

Functions of the Urinary System

• The urinary system includes six organs: two kidneys, two ureters, the urinary bladder, and the urethra.
• The urinary tract intricately relates spatially to the vagina and uterus in females, and the prostate gland in males.
• Kidneys filter blood plasma and remove wastes.
• Kidneys regulate blood volume, pressure, and osmolarity via water elimination or conservation
• Kidneys also regulate the electrolyte and acid-base balance of body fluids.
• Kidneys secrete erythropoietin which stimulates RBC production.
• Kidneys regulate calcium homeostasis and bone metabolism by synthesizing calcitriol.
• Kidneys clear hormones and drugs from the blood, limiting their effects and detoxify free radicals.
• During extreme starvation, kidneys support blood glucose levels by synthesizing glucose from amino acids.
• Waste includes any useless or excessive substance, while metabolic waste is produced by body processes.
• Nitrogenous wastes are highly toxic
• About 50% of nitrogenous waste is urea, a protein catabolism byproduct.
• Proteins break down into amino acids, and then the -NH2 group is removed.
• The -NH2 forms toxic ammonia (NH3), converted by the liver into less toxic urea, CO(NH2)2.
• Other nitrogenous wastes in urine are uric acid and creatinine.
• Blood urea nitrogen (BUN) measures the level of nitrogenous waste in the blood.
• Normal BUN ranges from 10 to 20 mg/dL.
• High BUN indicates azotemia and possible renal insufficiency.
• Uremia arises from nitrogenous waste toxicity, causing diarrhea, vomiting, dyspnea, and cardiac arrhythmia.
• Convulsions, coma, and death can occur within days with uremia.
• Excretion separates wastes from body fluids, with four organ systems involved.
• The respiratory system excretes carbon dioxide, small amounts of other gases, and water.
• The integumentary system excretes water, inorganic salts, lactic acid, and urea in sweat.
• The digestive system eliminates food residue (not excretion) and actively excretes water, salts, carbon dioxide, lipids, bile pigments, cholesterol, etc.
• The urinary system excretes metabolic wastes, toxins, drugs, hormones, salts, hydrogen ions, and water.

Anatomy of the Kidney

• Kidneys are located against the posterior abdominal wall at the level of vertebrae T12 to L3.
• The right kidney sits lower due to the liver's space occupation.
• Rib 12 crosses the approximate middle of the left kidney.
• Kidneys are retroperitoneal, along with ureters, urinary bladder, renal artery/vein, and adrenal glands.
• Grossly, each kidney weighs about 150g, measuring 11 cm long, 6 cm wide, and 3 cm thick.
• The kidney's lateral surface is convex, while the medial surface has a concave slit, the hilum that admits renal nerves, blood vessels, lymphatics, and a ureter.
• Kidneys have three protective connective tissue layers.
• A fibrous renal fascia, deep to the parietal peritoneum, binds the kidney and associated organs to the abdominal wall.
• The perirenal fat capsule cushions the kidney and holds it in place.
• The fibrous capsule encloses the kidney, protecting it from trauma and infection.
• Kidneys are suspended by collagen fibers extending from the fibrous capsule through the fat to the renal fascia.
• The renal fascia is fused with the peritoneum anteriorly and with the fascia of lumbar muscles posteriorly.
• Kidneys drop about 3 cm when transitioning from lying down to standing.
• Occasional detachment can lead to displacement and pathological results.
• The renal parenchyma-glandular tissue forms urine and appears C-shaped in frontal section.
• It encircles the renal sinus, a medial cavity with blood/lymphatic vessels, nerves, urine-collecting structures, and adipose tissue.
• The parenchyma divides into the outer renal cortex (1 cm thick) and inner renal medulla (facing the sinus).
• Extensions of the cortex, renal columns, project toward the sinus, dividing the medulla into 6-10 renal pyramids.
• Each pyramid is conical with its base facing the cortex and a blunt point, the renal papilla, facing the sinus; one pyramid and its overlying cortex comprise a kidney lobe.
• Each renal pyramid's papilla is nestled in a minor calyx, which collects urine.
• Two or three minor calyces converge into a major calyx, and two or three major calyces converge in the sinus to form the renal pelvis.
• The ureter, a tubular continuation of the renal pelvis, drains urine to the urinary bladder.
• Kidneys receive about 21% of the cardiac output (renal fraction).
• A renal artery from the aorta supplies each kidney.
• Before or after entering the hilum, the renal artery divides into segmental arteries and then into interlobar arteries.
• An interlobar artery penetrates each renal column, traveling between pyramids to the corticomedullary junction.
• It branches into arcuate arteries, forming 90° bends and traveling along the pyramid base.
• Each arcuate artery branches into cortical radiate arteries that pass upward into the cortex.
• Finer branches of renal circulation start with cortical radiate arteries.
• Afferent arterioles branch from each cortical radiate artery at right angles.
• Each afferent arteriole supplies a nephron
• The afferent arteriole leads to a glomerulus, a ball of capillaries enclosed in the glomerular capsule.
• Blood exits the glomerulus through an efferent arteriole.
• Efferent arterioles usually lead to peritubular capillaries that form a network around the nephron's renal tubule.
• The renal tubule reabsorbs water and solutes filtered from blood at the glomerulus, returning them via peritubular capillaries.
• Blood in peritubular capillaries flows into cortical radiate veins, arcuate veins, interlobar veins, and the renal vein, with no segmental veins present.
• The hilum is where the renal vein leaves, draining into the inferior vena cava.
• The renal medulla receives only 1-2% of the total renal blood flow, supplied by the vasa recta network.
• Vasa recta arise from nephrons in the deep cortex, close to the medulla.
• Efferent arterioles descend into the medulla and become vasa recta instead of peritubular capillaries.
• They lead into venules that ascend and empty into the arcuate and cortical radiate veins.
• Vasa recta capillaries are wedged tightly between medullary tubule parts, carrying away reabsorbed water and solutes.
• Each kidney contains ~1.2 million nephrons.
• Each nephron has a renal corpuscle to filter blood plasma and a renal tubule to convert filtrate into urine.
• The renal corpuscle includes the glomerulus and a two-layered glomerular capsule.
• The capsule's parietal (outer) layer is simple squamous epithelium
• The visceral (inner) layer contains podocytes that wrap around glomerular capillaries.
• Two layers are separated by a filtrate-collecting capsular space appearing as an empty circular or C-shaped area around the glomerulus in tissue sections.
• Renal corpuscle sides are called vascular and urinary poles.
• At the vascular pole, the afferent arteriole enters the capsule, bringing blood to the glomerulus.
• The efferent arteriole exits the capsule, carrying blood away.
• The afferent arteriole is significantly larger, with the glomerulus having a large inlet and a small outlet.
• At the urinary pole, the parietal wall turns away and becomes the renal tubule; the capsule's simple squamous epithelium becomes simple cuboidal in the tubule.
• The renal tubule is a duct from the glomerular capsule to the medullary pyramid tip (~3 cm long in segments).
• The proximal convoluted tubule (PCT) arises from the glomerular capsule and is the longest and most coiled region.
• It has a simple cuboidal epithelium with shaggy, brush border microvilli.
• A great quantity of absorption occurs through the microvilli.
• The nephron loop, U-shaped, resides mostly in the medulla.
• The PCT straightens out, forming the descending limb and dips toward or into the medulla.
• The loop turns 180° at the deep end and forms the ascending limb, which returns to the cortex.
• The loop is divided into thick and thin segments.
• Thick segments have simple cuboidal epithelium for the initial descending limb and part of the ascending limb.
• Their cells actively transport salts, have high metabolic activity, and are loaded with mitochondria.
• The thin segment has simple squamous epithelium and forms the lower descending limb, continuing partway up the ascending limb in some nephrons.
• Its cells have low metabolic activity, but the segment is permeable to water.
• The distal convoluted tubule (DCT) starts shortly after the ascending limb reenters the cortex.
• It is shorter and less coiled than the proximal convoluted tubule.
• It features a cuboidal epithelium with smooth cells nearly devoid of microvilli; it is located at the end of the nephron.
• Fluid drains from the DCTs of several nephrons as the collecting duct passes back into the medulla.
• Numerous collecting ducts converge towards the tip of a medullary pyramid, where they merge to form a papillary duct near the papilla.
• Around 30 papillary ducts end in pores at the conical tip of each papilla.
• Urine from these ducts drains into the minor calyx that encloses the papilla; the ducts have simple cuboidal epithelium.
• Fluid flows in the following route: glomerular capsule → proximal convoluted tubule → nephron loop → distal convoluted tubule → collecting duct → papillary duct → minor calyx → major calyx → renal pelvis →ureter → urinary bladder → urethra.
• Cortical nephrons lie close to the kidney surface, beneath the renal capsule.
• They have short nephron loops that dip slightly into the outer medulla or turn back before leaving the cortex; some lack loops entirely.
• Juxtamedullary nephrons are close to the medulla.
• They have long nephron loops extending to the renal pyramid apex.
• Juxtamedullary nephrons account for only 15% of all nephrons but maintain the osmotic gradient in the medulla.
• A renal plexus of nerves and ganglia wraps around each renal artery.
• The plexus follows the renal artery branches into the kidney's parenchyma - with nerve fibers issuing to blood vessels and convoluted tubules.
• The renal plexus provides sympathetic innervation from the abdominal aortic plexus and parasympathetic innervation from vagus nerve branches.
• It also carries afferent pain fibers from the kidneys en route to the spinal cord.
• Sympathetic fiber stimulation reduces glomerular blood flow/urine production.
• Sympathetic fibers respond to blood pressure decrease by stimulating the kidneys to secrete renin.
• The function of parasympathetic innervation is unknown.

Urine Formation I: Glomerular Filtration

• The kidney changes blood plasma into urine via glomerular filtration, tubular reabsorption, tubular secretion, and water conservation.
• Fluid is named differently as it flows through the nephron to show its changing composition.
• Glomerular filtrate is similar to blood plasma but contains almost no protein; it is found in the capsular space.
• Tubular fluid, from the proximal convoluted tubule through the distal convoluted tubule, has had substances removed and added by tubule cells.
• Urine, found in the collecting duct, changes little other than water content.
• Glomerular filtration is a special capillary exchange case, taking place across three barriers making up the filtration membrane.
• The fenestrated endothelium of the capillary's endothelial cells of glomerular capillaries is honeycombed with filtration pores (70-90 nm diameter).
• These capillaries are highly permeable, but blood cells do not pass through the pores.
• The basement membrane is a proteoglycan gel that holds back particles larger than 8 nm.
• Smaller anionic particles are also held back because of negative charge repulsion; normally comprises only 0.03% protein versus 7% protein in blood plasma.
• Filtration slits: A podocyte of glomerular capsule has a bulbous cell body and thick arms with foot processes (pedicels) wrapped around capillaries, interdigitating with each other.
• The foot processes have negatively charged filtration slits (30 nm wide) that serve as an additional obstacle to large anions.
• Molecules smaller than 3 nm pass through the filtration membrane into the capsular space.
• Substances that pass through have about the same concentration in glomerular filtrate as plasma.
• Some substances with low molecular weight are retained because they are bound to plasma proteins like calcium, iron, and thyroid hormones.
• Kidney infections and trauma can damage the filtration membrane, allowing albumin or blood cells to filter through.
• Kidney diseases can be marked by protein presence (especially albumin) or blood in the urine, conditions called proteinuria (albuminuria) and hematuria, respectively.
• Distance runners/swimmers can temporarily experience proteinuria/hematuria.
• Strenuous exercise reduces kidney perfusion, glomerular deterioration under prolonged hypoxia, leaking components into the filtrate.
• Glomerular filtration follows pressure principles governing filtration in other capillaries, but has significant force differences.
• Blood hydrostatic pressure (BHP) is higher than elsewhere (~60 mm Hg vs. 10–15 mm Hg in other capillaries) due to the afferent being larger than the efferent arteriole.
• Hydrostatic pressure in the capsular space is ~18 mm Hg.
• Colloid osmotic pressure (COP) of the blood is ~32 mm Hg.
• Because glomerular filtrate is protein-free, it contains no significant COP.
• High outward pressure of 60 mm Hg is countered by two inward pressures of 18 and 32 mm Hg, creating a net filtration pressure of 10 mm Hgout.
• BHP drops low enough in most blood capillaries at the venous end for osmosis to override filtration and the capillaries to reabsorb fluid.
• The BHP remains high so that they engage solely in filtration in the glomerular capillaries.
• High glomerular blood pressure makes kidneys particularly susceptible to hypertension.
• This ruptures glomerular capillaries, leads to scarring (nephrosclerosis), promotes renal blood vessel atherosclerosis, and leads to renal failure.
• The glomerular filtration rate (GFR) is the amount of filtrate created by both kidneys per minute.
• Produces ~12.5 mL of filtrate per minute for every 1 mm Hg of net filtration pressure: the filtration coefficient (Kf).
• The Kf relies on filtration barrier permeability and surface area, being about 10% lower in women.
• The reference man GFR = NFP × Kf = 10 × 12.5 = 125 mL/min., while the reference woman GFR = 105 mL/min.
• This equals 180 L/day in males and 150 L/day in females (60X the body’s blood volume), with a small fraction eliminated as urine.
• Adults reabsorb 99% of the filtrate, excreting 1-2 L of urine daily.
• Glomerular filtration must be wellControlled for proper filtration and reabsorption.
• Adjusting GFR relies on changing glomerular blood pressure via renal autoregulation, sympathetic control, and hormonal control.
• Renal autoregulation adjusts nephron blood flow/GFR without external control.
• It allows nephrons to keep GFR stable despite arterial blood pressure changes; autoregulation involves the myogenic mechanism and tubuloglomerular feedback.
• The myogenic mechanism contracts smooth muscle when stretched.
• Increased BP stretches the afferent arteriole, constricting it to prevent blood flow into the glomerulus from changing much.
• Decreased BP relaxes the afferent arteriole, allowing for blood flow more easily.
• Tubuloglomerular feedback occurs as the glomerulus gets feedback on downstream tubular fluid, adjusting filtration.
• It involves the juxtaglomerular apparatus at the very end of the nephron loop, where the loop connects with afferent and efferent arterioles at the vascular pole of the renal corpuscle.
• Tubuloglomerular feedback starts with the macula densa, sensory cells on one side of the loop.
• Macula densa absorb Na+, K+, and Cl¯; swell; and secrete ATP and mesangial cells break down the ATP to adenosine.
• Adenosine, as a paracrine messenger, stimulates nearby granular cells, which are changed smooth muscle cells wrapping around the afferent/efferent arterioles.
• Granular cells respond to adenosine by constricting the afferent arteriole, reducing blood flow/GFR.
• Granular cells produce renin in response to blood pressure decrease.
• Renin starts a renin-angiotensin-aldosterone mechanism to restore blood pressure/volume
• Renal autoregulation causes GFR to fluctuate within narrow limits to maintain a dynamic equilibrium.
• Renal autoregulation does not compensate for extreme blood pressure variations.
• Below a MAP of 70 mm Hg, glomerular filtration ceases.
• During strenuous exercise, sympathetic nerve fibers work with epinephrine to constrict the afferent arterioles.
• This reduces GFR/urine production, redirecting blood to the heart, brain, and skeletal muscles, with GFR lowered to few mL per minute.
• Hormonal control happens via the renin-angiotensin-aldosterone mechanism.
• A drop in blood pressure is detected by baroreceptors in the aorta/carotid arteries.
• Baroreceptors signal the brainstem, activating corrective sympathetic reflexes.
• Sympathetic fibers stimulate granular cells to secrete renin, which acts on angiotensinogen in plasma to split off a 10 amino acid peptide, angiotensin I.
• Angiotensin-converting enzyme (ACE) in the lungs and kidneys removes two amino acids, changing it into angiotensin II- a hormone.
• Angiotensin II restores fluid volume and blood pressure, acts as a vasoconstrictor, and raises mean arterial blood pressure.
• This constricts the efferent arterioles, raising glomerular blood pressure/GFR; filtrate is able to continue
• By lowering blood pressure in the peritubular capillaries it enhances water/sodium chloride reabsorption.
• It also stimulates the adrenal cortex to secrete aldosterone, stimulating sodium/water reabsorption in the distal convoluted tubule; angiotensin II has a similar effect on the proximal convoluted tubule.
• In relation to ADH, angiotensin II stimulates the pituitary gland to secrete antidiuretic hormone and stimulates thirst to encourage water intake.

Urine Formation II: Tubular Reabsorption and Secretion

• The process by which glomerular filtrate becomes urine includes tubular reabsorption and secretion.
• The proximal convoluted tubule (PCT) reabsorbs about 65% of the glomerular filtrate, while also removing some substances and secreting them into the tubule.
• PCT length and microvilli increase absorptive surface area.
• PCT cells have many mitochondria for active transport, using resting ATP/calorie consumption.
• Tubular reabsorption is reclaiming water/solutes from the tubular fluid, returning them to the blood.
• Reabsorption occurs via two routes.
• The transcellular route passes substances through the cytoplasm and base of epithelial cells.
• The paracellular route travels between cells.
• Solvent drag occurs with water which carries a variety of solutes.
• Regardless of the route used, these materials enter tissue fluid at the base of the epithelium, uptaken by peritubular capillaries.
• Both solutes and water are carried through the tubule epithelium by mechanisms.
• Sodium reabsorption creates an electrical/osmotic gradient that drives water/solute reabsorption.

• Nat has a concentration of 140 mEq/L versus only 12 mEq/L within epithelial cells, favoring diffusion into the cells.
• Transport proteins located on the apical cell surface help transport sodium.
• These transport proteins include: symports to which another solutes binds and Na*–H* antiport, that pulls in Na⁺ to pump H⁺ out.
• Angiotensin II activates the antiport, exerting strong influence on sodium reabsorption.
• The Na*-K+ pumps in the basal surface actively utilize ATP to pump sodium and prevent accumulation.
• Apical symports don't consume ATP but are secondary active transport because of their dependance on basal Na*–K+ pumps.
• Chloride, being attracted to Na⁺, uses electrical attraction, and various antiports absorb Cl that is exchanged for other anions.
• Cl¯ and K⁺ are driven out through the basal cell by a K⁺–Cl¯ symport as Both Na¹ and Cl ¯ diffuse through the tubule epithelium by the paracellular route.
• Other electrolytes diffuse through the paracellular route.
• Potassium, magnesium, and phosphate ions diffuse through the paracellular route aided by water.
• Phosphate is also cotransported by Na⁺
• Calcium reabsorption takes place mostly through the paracellular route in the PCT and then more later in the nephron.
• Glucose is cotransported with Na⁺ by symports called sodium-glucose transporters (SGLTs).
• It can be removed from the basolateral surface via facilitated diffusion, with no glucose remaining in the urine.
• Nitrogenous wastes primarily diffuse through epithelium with water.
• Urea diffuses by water while the nephron reabsorbs 40-60% of the urea at 99% of water meaning that urine urea concentration is increased versus glomerular filtrate/blood rates.
• Nearly all PCT uric acid is reabsorbed, but later parts of the nephron secrete it back into the tubular fluid.
• Creatinine is not reabsorbed and is passed into the urine.
• Water reabsorption represents a significant process of the kidneys.
• About two-thirds is reabsorbed by the PCT as solutes are followed by osmosis through routes.
• Transcellular absorption utilizes water channels called aquaporins.
• PCT osmolarity remains unchanged as solute/water reabsorption is proportionate.
• In the PCT, constant water reabsorption is known as obligatory water reabsorption with water reabsorption being modulated by hormones in other sections of the nephron.
• Water and solutes reabsorbed from basal surface of tubule epithelium into blood by peritubular capillaries thanks to solvent drag/osmosis.
• Various factors promote osmosis into capillaries.
• Reabsorbed fluid accumulation creates high interstitial fluid pressure.
• Efferent arteriole narrowness lowers blood hydrostatic pressure to 8 mm Hg in peritubular capillaries.
• As blood flows through glomerulus, proteins increases blood's colloid osmotic pressure (COP).
• Higher reabsorption takes place by constricting the arterioles with angiotensin II, reducing pressure within the peritubular capillaries.
• The transport maximum (Tm) is the maximum reabsorption rate, occurs transporters are saturated and each organic solute has its own Tm
• Glucose (Tm of 320 mg/min.) enters the renal tubule normally at 125 mg/min and is entirely reabsorbed.
• This causes glycosuria with a blood glucose level greater than 220 mg/dL; excess will pass into the urine.
• Untreated diabetes mellitus has plasma glucose greater than 400 mg/dL and glycosuria serves as one of its signs.
• Tubular secretion extracts chemicals from the blood and secretes them into the tubular fluid for acid base balance maintains via secretion of hydrogen/bicarbonate ions.
• It also compensates for reabsorption by clearing blood of drugs and contaminants by tubular secretion and compensates for PCT reabsorption of uric acid
• The clearance of a drug often requires dosages of three or four day to keep up.

Urine Formation III: Water Conservation

• The nephron loop generates an osmotic gradient to concentrate urine/conserve water.
• It also reabsorbs 25% of Na+, K+, Cl¯, and 15% glomerular filtrate water.
• Thick segment proteins simultaneously bind 1 Na+, 1 K+, and 2 Cl from tubular fluid and cotransport them into the cytoplasm as ions leave basolateral cell surfaces via diffusion K+)
• Potassium reenters via Na*–K+ pump and the reenters the tubular fluid while NaCl stays in renal medulla tissue fluid.
• Water doesn’t follow as the thick segment is impermeable as electrolyte content passes into the distal convoluted tubule.
• Fluid in the distal convoluted tubule (DCT) holds ~20% water/7% salts from its respective glomerular filtrate; most water and some salts are reabsorbed through the collection & DCT.
• The amounts reabsorbed is regulated by: aldosterone, natriuretic peptides, antidiuretic and parathyroid
• The DCT/collecting duct contain two types of cell; principal cells contain more receptors for the hormones also take part in salt and water balance, intercalated cells reabsorb K+/secrete h+ into lumen/acid balance.
• Released as a steroid from the renal and when lower blood Na+, aldosterone is the "salt retaining" hormone/increase K+ concentration.
• Lowered blood stimulates the kidney to secrete renin for secondary aldosterone secretion stimulation: aldosterone acts on the nephron loop/DCT with the ascending limb/cortical portion stim Na water follows, results in NaCl retention with lowered urine volume, and increased of K concentration of urine.
• In response to to increased blood the heart releases natriuretic hormone, with four actiions for lowered of blooud volume:
  • Efferent arteriole’s constricts while the afferent arteriole dilates, increasing GFR.
  • Production of renin/aldosterone decreases by antagonizing the renin-angiotensin-aldosterone mechanism.
  • Secretion of Antidiuretic is decreased by inhibiting the respective secretion/action.
• Reabsorption of NaCl from the collecting duct is stopped with release of anti-diuretics.
• Released from the gland from posterior section from dehydration while loss of blood volume and increase blood stimulates baroreceptors.
• This increases collecting duct permeability with antidiuretic secretions.
• PCT osmolarity remains unchanged as solute/water reabsorption is proportionate.
• In the PCT, constant water reabsorption is known as obligatory water reabsorption with water reabsorption being modulated by hormones in other sections of the nephron.

Urine Formation III: Water Conservation

• The collecting duct (CD) begins in the cortex, receiving tubular fluid from numerous nephrons.
• As it travels through the medulla, reabsorbs water and concentrates urine.
• Entering the CD: urine is isotonic with blood plasma at 300 mOsm/L
• Leaving the CD: urine can be up to four times more concentrated (hypertonic)
• Concentrating wastes and controlling water loss is crucial for terrestrial animals.
• Two factors enable the collecting duct to produce hypertonic urine.
• Extracellular fluid osmolarity is four times greater in the lower medulla versus the cortex.
• The medullary CD portion is more permeable to water than to solutes.
• Descending the CD, water leaves by osmosis, leaving NaCl and wastes behind to concentrate the urine.
• Control of water loss relies on body hydration levels.
• With excessive water, water diuresis produces expansive volumes of hypotonic urine.
• Cortical regions of CD reabsorb NaCl but maintain impermeability to water.
• Salts are removed, water is retained, and urine osmolarity drops to 50 mOsm/L.
• Dehydration results in scant and highly concentrated urine output.
• High blood osmolarity causes ADH release, reabsorbing water via storage vesicles, also synthesizing aquaporins for increased water permeability.
• ADH secretion wanes, removing aquaporins if water becomes more available.
• Low blood pressure can reduce GFR and increase dehydration allowing more time for reabsorption and less urine production.
• The nephron loop regulates salinity through countercurrent multiplication which continues the loop of capturing returning salt to the loop to maintain the necessary gradient.
• By multipying the osmolarity at increasing depths of the medulla, a multiplier effect maintains gradient.
• It is a countercurrent because adjacent tubules carry movement up or down.
• Due to this, a multiplier feedback occurs which sustains the gradients between each side.
• Fluid enters environment which increasingly osmolar, driving water out into the ECF, increasing the descending limb's lumen concentrations.
• The lower portions have concentrations of around 1200, which becomes diluted on it returns upwards and leaves the salt pumps.
• This process is maintained partially because urea gets recycled through and the loop so it can keep a salt presence to supplement both molecules being approximately similar.
• Neither area however, is permeable, allowing concentration.
• The blood, being a salt recycling system, also prevents urea/salt through an active gradient.
• Flows downwards to have water diffuse out and absorbs to it, then reverses the direction.
• In effect, the vessels gives and takes water, never becoming as dense while it takes what's necessary, instead recycling.

Urine and Renal Function Tests

• Urinalysis tests urine and is the most common medical test.
• Body’s hydration determines the following properties.
• It ranges from deep amber to more clearer. This is caused by hemoglobins breakdown.
• Food/drug intake also alters test, but the bacterial growth influences normal rate .
• High pyuria suggests kidney if not clear.
• Hematuria also signifies issues as well usually.
• Ammonia is bacterial outcome, but some food can influence that.
• Certain smells indicates medical or genetic factor.
• Gravity also, the ratio of substances vs distilled waters, has a scale.
• Density gets an assumption.
• Body state influences the osmolarity between dehydration and normal intake.
• The concentration decides if it is hyper/hypotonic .
• PH also occurs, with it usually being slightly basic.
• Chemical concentration has water as its base.
• Urea is more active as is NA, CL which gets derived from other substances.
• Bile influences some levels as do glucose and other byproducts from breakdown.
• With tests, output influences average tests.
• Dehydration vs some sort of prescription and also more extreme cases like a blood disease.
• Some issues which might occur.
• High output: diuretics.
• Low output: a kidney failure, or dehydration, or circulatory shock.
• Diabetes.