Lecture #6: Physiology: Organization of the Urinary System

What are the three essential functions of the kidneys?

Filtration and excretion of metabolic products/toxins; regulation of fluid status, electrolytes, and acid-base balance; and production or activation of hormones involved in erythropoiesis, Ca²⁺ metabolism, and blood pressure/flow regulation.

Where are the kidneys located?

They are paired, bean-shaped retroperitoneal organs located on either side of the vertebral column.

What is the renal hilus?

The site where the renal artery and nerves enter and the renal vein exits the kidney.

What is the renal sinus?

A space containing the renal pelvis and major and minor calyces; it is surrounded by renal parenchyma except where it connects with the upper ureter.

What does the renal cortex contain?

The cortex is the granular outer kidney region containing glomeruli and highly convoluted tubules.

What characterizes the renal medulla?

The medulla is the darker inner kidney region that lacks glomeruli and contains parallel tubules and small blood vessels.

How is the renal medulla organized?

It is subdivided into approximately 8–18 conical renal pyramids.

How are renal pyramids oriented?

The base faces the corticomedullary border, while the tip projects toward the renal pelvis.

What is the renal papilla?

The tip of a renal pyramid where terminal collecting ducts drain urine toward the minor calyx and renal pelvis.

What is the renal capsule?

A fibrous covering of the kidney that reflects into the renal sinus at the hilus; its inner layers line the sinus and outer layers anchor to blood vessels and the renal pelvis.

Approximately what percentage of cardiac output reaches the kidneys?

Approximately 20% of cardiac output, providing the large plasma supply needed for formation of ultrafiltrate.

What is the renal vascular sequence surrounding the glomerulus?

Afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries or vasa recta.

What is the function of the afferent arteriole?

It is a high-resistance arteriole that carries blood into the glomerulus.

What is the function of the glomerular capillary network?

It is a high-pressure capillary network specialized for filtration of plasma.

What is the function of the efferent arteriole?

It is the second high-resistance arteriole and carries blood away from the glomerulus.

What are peritubular capillaries?

A low-pressure capillary network surrounding renal tubules that arises from efferent arterioles.

What are the vasa recta?

Hairpin-shaped vessels arising from efferent arterioles of juxtamedullary nephrons that supply tubules within the renal medulla.

How is renal blood flow distributed between cortex and medulla?

About 90% perfuses superficial glomeruli and cortex, while about 10% perfuses juxtamedullary glomeruli and medulla.

What is the functional unit of the kidney?

The nephron; each kidney contains approximately 800,000–1,200,000 nephrons.

What are the two basic components of a nephron?

A glomerulus and a renal tubule.

What are the two major nephron populations?

Superficial nephrons with short loops and juxtamedullary nephrons with long loops extending deep into the medulla.

Why are juxtamedullary nephrons important?

Their long loops of Henle and associated vasa recta play a special role in producing concentrated urine.

What is the renal corpuscle?

The site where glomerular filtrate is formed, consisting of the glomerulus and Bowman's capsule.

What is Bowman's capsule?

An epithelial structure surrounding the glomerulus that contains Bowman's space and is continuous with the renal tubule.

What is Bowman's space?

The space between the parietal layer of Bowman's capsule and the glomerular capillaries into which ultrafiltrate enters.

What are podocytes?

Modified epithelial cells forming the visceral layer of Bowman's capsule that cover the glomerular capillaries with interdigitating foot processes.

Where does glomerular ultrafiltrate go after entering Bowman's space?

It flows from Bowman's space into the proximal tubule at the urinary pole of the renal corpuscle.

Where is the glomerular filtration barrier located?

Between the glomerular capillary lumen and Bowman's space.

What are the major components of the glomerular filtration barrier?

Endothelial glycocalyx and fenestrated endothelium, the glomerular basement membrane, and podocyte foot processes with filtration slit diaphragms.

What is the function of the glomerular glycocalyx?

Its negatively charged glycosaminoglycans help prevent leakage of large negatively charged macromolecules.

What is distinctive about glomerular endothelial cells?

They contain large approximately 70-nm fenestrations that permit H₂O, small solutes, and small proteins to leave the capillary.

What are the three layers of the glomerular basement membrane?

Lamina rara interna, lamina densa, and lamina rara externa.

How does the glomerular basement membrane restrict filtration?

It contains negatively charged heparan sulfate proteoglycans that restrict passage of large negatively charged solutes.

What are podocyte pedicels?

Interdigitating secondary foot processes that cover the outer surface of the glomerular basement membrane.

What are filtration slits?

Spaces between interdigitating podocyte pedicels that are connected by thin slit diaphragms approximately 4–14 nm wide.

Which proteins maintain slit diaphragm integrity?

Nephrin, NEPH1/Kirrel1, and podocin.

What happens when slit diaphragm proteins are defective?

The filtration barrier becomes abnormally permeable, allowing large proteins such as albumin to enter the urine, as can occur in nephrotic syndrome.

What are the major functions of glomerular mesangial cells?

They support glomerular capillaries, regulate GFR by altering blood flow and capillary surface area, produce extracellular matrix, phagocytose macromolecules, and secrete prostaglandins and inflammatory cytokines.

What is the correct sequence of major nephron tubular segments?

Bowman's capsule → proximal convoluted tubule → proximal straight tubule → thin descending limb → thin ascending limb → thick ascending limb → distal convoluted tubule → cortical collecting tubule → medullary collecting duct → duct of Bellini.

What structural features characterize the proximal convoluted tubule?

PCT cells have abundant apical microvilli forming a brush border and a rich mitochondrial supply, supporting extensive reabsorption.

What does the proximal tubule reabsorb?

Approximately 67% of filtered H₂O and electrolytes isosmotically, essentially all filtered glucose, proteins, and amino acids, and most filtered HCO₃⁻.

Why is the proximal tubule highly permeable to water?

AQP1 is expressed in both its apical and basolateral membranes.

What does the thin descending limb of Henle reabsorb?

Approximately 15% of filtered H₂O, mediated by AQP1.

What does the thin ascending limb of Henle reabsorb?

NaCl by a passive mechanism; it does not reabsorb H₂O.

What are the key properties of the thick ascending limb?

It is impermeable to H₂O, reabsorbs about 25% of filtered NaCl, participates in K⁺ handling, reabsorbs Mg²⁺ and Ca²⁺ paracellularly, and reabsorbs remaining HCO₃⁻.

What are the major functions of the distal tubule and collecting ducts?

They reabsorb approximately 8% of filtered NaCl, secrete variable amounts of K⁺ and H⁺, and reabsorb approximately 8–17% of filtered H₂O.

What cell types compose the cortical collecting tubule and medullary collecting duct?

Principal cells and intercalated cells.

What are the three components of the juxtaglomerular apparatus?

The macula densa, granular juxtaglomerular cells, and extraglomerular mesangial cells.

What is the macula densa?

A patch of specialized, closely packed tubular epithelial cells at the transition between the thick ascending limb and distal convoluted tubule that contacts its own glomerulus.

What are granular juxtaglomerular cells?

Specialized smooth-muscle cells of the afferent arteriole that produce, store, and release renin.

What are the major homeostatic functions of the juxtaglomerular apparatus?

It directly regulates renal blood flow and filtration rate and indirectly regulates Na⁺ balance and systemic blood pressure through tubuloglomerular feedback and renin-angiotensin-aldosterone system control.

What is tubuloglomerular feedback?

A JGA feedback mechanism in which the amount of H₂O/NaCl reaching the macula densa influences GFR.

How does the JGA participate in blood pressure regulation?

Changes in renal perfusion pressure are detected in the afferent arteriole, altering renin release from granular cells and thereby regulating the renin-angiotensin-aldosterone system.

What are the major endocrine products of the kidney?

Renin, erythropoietin, and 1,25-dihydroxyvitamin D; the kidney also produces paracrine prostaglandins and bradykinin.

What is the role of renin?

Renin is the rate-limiting enzyme of the RAAS and initiates production of angiotensin peptides; angiotensin II promotes vasoconstriction, Na⁺ reabsorption, aldosterone secretion, and ADH secretion.

What is the role of renal erythropoietin?

EPO stimulates proliferation and maturation of committed erythroid progenitors to increase RBC production; about 90% of adult EPO is produced by peritubular interstitial cells in the kidney.

How does the kidney activate vitamin D?

Proximal tubular CYP27B1/1α-hydroxylase converts 25-OH vitamin D into active 1,25-(OH)₂ vitamin D, or calcitriol, under regulation by PTH.