Anatomy and Physiology 2 BIOL 244 Urinary, Fluids, Electrolytes and pH

What are the major functions of the kidneys?

Excrete metabolic wastes and drugs; regulate blood volume, blood pressure, osmolarity, electrolytes, and acid-base balance; produce erythropoietin and renin; activate vitamin D; and perform gluconeogenesis during prolonged fasting.

How do the kidneys regulate blood volume and pressure?

By adjusting sodium and water excretion and by releasing renin, which activates the renin-angiotensin-aldosterone system.

How do the kidneys regulate red blood cell production?

Low renal oxygen stimulates interstitial cells to release erythropoietin, which increases red blood cell production in bone marrow.

How do the kidneys activate vitamin D?

They convert calcidiol to calcitriol, the active form of vitamin D, mainly under stimulation by parathyroid hormone.

What are the major nitrogenous wastes excreted by the kidneys?

Urea, uric acid, creatinine, and smaller amounts of ammonia.

Where does urea come from?

The liver converts toxic ammonia produced during amino acid breakdown into urea.

Where does uric acid come from?

Breakdown of purine nucleotides.

Where does creatinine come from?

Breakdown of creatine phosphate in skeletal muscle.

What is azotemia?

An abnormal elevation of nitrogenous wastes, especially blood urea nitrogen and creatinine, in the blood.

What is uremia?

A symptomatic toxic state caused by severe retention of urinary wastes, often producing nausea, fatigue, confusion, itching, and other systemic effects.

Where are the kidneys located?

Retroperitoneally on the posterior abdominal wall, usually from about T12 to L3, with the right kidney slightly lower than the left.

What structures enter or leave at the renal hilum?

The renal artery, renal vein, nerves, lymphatics, and ureter.

What are the three protective layers around a kidney?

Fibrous capsule, perirenal fat, and renal fascia.

What is the renal cortex?

The superficial region containing renal corpuscles and convoluted tubules.

What is the renal medulla?

The deeper region composed of renal pyramids separated by renal columns.

What is a renal pyramid?

A cone-shaped medullary structure containing loops of Henle and collecting ducts.

What is a renal papilla?

The tip of a renal pyramid where collecting ducts empty urine into a minor calyx.

Trace urine flow through the kidney and urinary tract.

Collecting ducts → papillary ducts → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra.

What is a renal lobe?

One renal pyramid plus the cortical tissue overlying and adjacent to it.

Trace blood flow from the renal artery to the renal vein.

Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries or vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein.

What is unusual about renal circulation?

Blood passes through two capillary beds in series: glomerular capillaries followed by peritubular capillaries or vasa recta.

Why is the renal circulation called a portal system?

Because blood flows from one capillary bed to another through an efferent arteriole before returning to veins.

What is the role of the afferent arteriole?

It delivers blood to the glomerulus and helps regulate glomerular pressure and filtration rate.

What is the role of the efferent arteriole?

It carries blood away from the glomerulus and forms the peritubular capillaries or vasa recta.

What are peritubular capillaries?

Low-pressure capillaries surrounding cortical tubules that support reabsorption and secretion.

What are the vasa recta?

Long straight capillaries accompanying juxtamedullary loops that preserve the medullary osmotic gradient through countercurrent exchange.

How does sympathetic stimulation affect the kidneys?

It constricts renal arterioles, reduces renal blood flow and GFR during stress, stimulates renin release, and promotes sodium reabsorption.

What is a nephron?

The microscopic structural and functional unit of the kidney.

What are the major parts of a nephron?

Renal corpuscle, proximal convoluted tubule, nephron loop, distal convoluted tubule, and connecting tubule leading to a collecting duct.

What structures form the renal corpuscle?

The glomerulus and glomerular, or Bowman, capsule.

What is the vascular pole of the renal corpuscle?

The site where the afferent arteriole enters and the efferent arteriole exits.

What is the urinary pole of the renal corpuscle?

The site where filtrate leaves the capsule and enters the proximal tubule.

What percentage of nephrons are cortical nephrons?

About 85 percent.

What are the main features of cortical nephrons?

Renal corpuscles in the outer cortex, short loops extending only slightly into the medulla, and mainly peritubular capillaries.

What are the main features of juxtamedullary nephrons?

Renal corpuscles near the corticomedullary junction, long loops extending deep into the medulla, and associated vasa recta.

Why are juxtamedullary nephrons important?

Their long loops establish the medullary osmotic gradient needed to produce concentrated urine.

What epithelium lines the parietal layer of Bowman capsule?

Simple squamous epithelium.

What are podocytes?

Specialized cells of the visceral layer of Bowman capsule with foot processes forming filtration slits.

What is the histology of the proximal convoluted tubule?

Simple cuboidal epithelium with abundant microvilli, many mitochondria, and indistinct cell borders.

Why does the PCT have a brush border?

To greatly increase surface area for massive reabsorption.

What is the histology of the thin limb of the nephron loop?

Simple squamous epithelium.

What is the histology of the thick ascending limb?

Simple cuboidal epithelium rich in mitochondria and transport proteins.

What is the histology of the distal convoluted tubule?

Simple cuboidal epithelium with fewer microvilli than the PCT and a clearer lumen.

What cell types are found in the collecting duct?

Principal cells and intercalated cells.

What structures form the juxtaglomerular apparatus?

Macula densa cells of the distal tubule, juxtaglomerular granular cells of the afferent arteriole, and extraglomerular mesangial cells.

What does the macula densa sense?

Tubular sodium chloride concentration and flow.

What do juxtaglomerular cells secrete?

Renin.

What stimulates renin release?

Low afferent arteriolar pressure, low sodium chloride delivery to the macula densa, and sympathetic beta-1 stimulation.

What is the major function of the JGA?

It helps regulate GFR and systemic blood pressure through tubuloglomerular feedback and renin release.

What is renal functional reserve?

The ability of the kidneys to increase filtration and function above normal baseline when needed.

What happens to the remaining kidney after one kidney is lost?

Compensatory hypertrophy and increased filtration per nephron occur, often allowing near-normal total function.

What are the three basic renal processes?

Glomerular filtration, tubular reabsorption, and tubular secretion.

What is glomerular filtration?

Movement of water and small solutes from glomerular blood into Bowman capsule.

What is tubular reabsorption?

Movement of substances from tubular fluid back into blood.

What is tubular secretion?

Movement of substances from peritubular blood into tubular fluid.

What determines urinary excretion of a substance?

Excretion equals filtration minus reabsorption plus secretion.

What are the three main layers of the filtration membrane?

Fenestrated glomerular endothelium, fused basement membrane, and podocyte filtration slits with slit diaphragms.

What normally prevents blood cells from entering filtrate?

The intact glomerular endothelium and filtration barrier.

What normally limits plasma protein filtration?

The size and negative charge of the basement membrane and slit diaphragms.

What pressure favors glomerular filtration?

Glomerular blood hydrostatic pressure.

What pressures oppose glomerular filtration?

Capsular hydrostatic pressure and blood colloid osmotic pressure.

What is net filtration pressure?

The pressure favoring filtration minus the pressures opposing filtration; normally about 10 mmHg in many textbook examples.

What is glomerular filtration rate?

The volume of filtrate formed by all glomeruli per minute, typically about 125 mL/min in a healthy young adult.

Why is GFR important?

It determines how effectively wastes are cleared and how much fluid and solute enter the tubules for regulation.

What factors determine GFR?

Net filtration pressure, filtration membrane permeability, and filtration surface area.

What is renal autoregulation?

Intrinsic adjustment of afferent arteriolar resistance to keep renal blood flow and GFR relatively stable.

What is the myogenic mechanism?

Stretch of the afferent arteriole causes contraction; reduced stretch causes relaxation.

What is tubuloglomerular feedback?

The macula densa adjusts afferent arteriolar tone and renin release based on distal tubular sodium chloride delivery.

How does moderate sympathetic activity affect GFR?

It constricts renal vessels and tends to reduce renal blood flow and GFR while preserving blood pressure.

How does angiotensin II help maintain GFR during low blood pressure?

Preferential efferent arteriolar constriction helps maintain glomerular pressure, although high levels constrict both arterioles and reduce renal flow.

How does atrial natriuretic peptide affect GFR?

It increases filtration and promotes sodium and water excretion.

What is the transcellular route of reabsorption?

Movement through the apical membrane, cytoplasm, and basolateral membrane of tubular cells.

What is the paracellular route of reabsorption?

Movement between tubular cells through tight junctions and lateral spaces.

What is primary active transport in renal tubules?

Direct ATP-driven transport, especially by the basolateral sodium-potassium pump.

What is secondary active transport?

Transport powered by an ion gradient, such as sodium-glucose cotransport.

What is transport maximum?

The maximum rate at which carriers can transport a substance before they become saturated.

What is glucosuria?

Glucose in the urine.

Why does glucosuria occur in uncontrolled diabetes mellitus?

Filtered glucose exceeds the renal transport maximum, so not all glucose can be reabsorbed.

How does glucosuria cause polyuria?

Glucose remains in tubular fluid and retains water by osmotic diuresis.

What is obligatory water reabsorption?

Water reabsorption that occurs automatically as water follows solute, mainly in the PCT and descending limb.

What is facultative water reabsorption?

Variable water reabsorption in the late DCT and collecting ducts regulated mainly by ADH.

Where is aquaporin-1 found?

Constitutively in the PCT and thin descending limb, allowing obligatory water movement.

Where is aquaporin-2 found?

Inserted into the apical membrane of principal cells in response to ADH.

How does ADH increase water reabsorption?

It binds V2 receptors, increases cAMP, and inserts aquaporin-2 channels into principal-cell apical membranes.

What percentage of filtered sodium and water is reabsorbed in the PCT?

Roughly two-thirds under normal conditions.

Which nutrients are normally almost completely reabsorbed in the PCT?

Glucose and amino acids.

How does the PCT help regulate pH?

It reabsorbs filtered bicarbonate, secretes hydrogen ions, and produces ammonium from glutamine.

Why is ammonium production important?

It allows excretion of acid while generating new bicarbonate for the blood.

What effect does parathyroid hormone have on the DCT?

It increases calcium reabsorption.

What effect does PTH have on phosphate handling?

It decreases phosphate reabsorption in the PCT, increasing phosphate excretion.

What are the main functions of principal cells?

Reabsorb sodium and water and secrete potassium.

Which hormones act strongly on principal cells?

Aldosterone and ADH.

What do type A intercalated cells do?

Secrete hydrogen ions and reabsorb bicarbonate and potassium, helping correct acidosis.

What do type B intercalated cells do?

Secrete bicarbonate and reabsorb hydrogen ions, helping correct alkalosis.

Outline the renin-angiotensin-aldosterone system.

Renin converts angiotensinogen to angiotensin I; ACE forms angiotensin II; angiotensin II causes vasoconstriction and stimulates aldosterone, ADH, thirst, and sodium reabsorption.

What are the major effects of angiotensin II?

Vasoconstriction, increased proximal sodium reabsorption, stimulation of aldosterone and ADH, and increased thirst.

What does aldosterone do in the distal nephron?

Increases sodium reabsorption and potassium secretion; water follows sodium if ADH permits.

What primarily stimulates ADH release?

Increased plasma osmolarity and, at stronger levels, decreased blood volume or pressure.

What is the major renal effect of ADH?

Increased collecting-duct water permeability and water reabsorption.

What causes atrial natriuretic peptide release?

Stretch of the atria due to increased blood volume.