Urinary System SG A&P II

What are the organs of the urinary system?

two kidneys and the urinary tract (ureters, urinary bladder, urethra)

Kidneys

filter blood and form urine

Ureters

transport urine to the bladder

Bladder

stores urine

Urethra

expels urine

What are the major functions of the kidneys?

• Remove metabolic wastes from blood

• Maintain fluid/electrolyte balance

• Maintain acid-base balance

• Regulate blood pressure (via renin and blood volume)

• Regulate erythropoiesis (via erythropoietin)

• Perform metabolic functions (detoxify, activate vitamin D, gluconeogenesis)

Describe the external structure of the kidney

Location: retroperitoneal, between T12–L3 (right lower than left)

Coverings: renal fascia, adipose capsule, renal capsule

Structures: hilum (entry/exit for vessels, nerves, ureter) and renal sinus (fat-filled cavity)

Renal fascia

anchors kidney

Adipose capsule 

cushions kidney

Renal capsule

thin, dense CT covering that protects from trauma/infection

What are the internal structures of the kidney?

renal cortex, renal medulla, renal pelvis

Renal cortex

outer layer, rich in blood supply

Renal medulla

renal pyramids separated by columns

Renal pelvis

funnel-shaped chamber collecting urine from major calyces

Trace blood flow through the kidneys

Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Interlobular (cortical radiate) artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries → Interlobular vein → Arcuate vein → Interlobar vein → Renal vein

What are the main parts of a nephron?

renal corpuscle, renal tubule, collecting system

Renal corpuscle

glomerulus (leaky capillaries) + bowman’s capsule (double-layered; podocytes form filtration slits)

Renal tubule

proximal tubule, nephron loop, distal tubule

Proximal tubule

cuboidal cells with microvilli (reabsorption)

Nephron loop

descending and ascending limbs

Distal tubule

cuboidal cells, few microvilli

Collecting system

collecting ducts → papillary ducts → minor calyx

Where does filtrate begin its flow in the nephron?

glomerulus → capsular space (bowman’s capsule)

After leaving the capsular space, where does filtrate go next?

proximal convoluted tubule (PCT)

After the proximal tubule, where does filtrate travel?

nephron loop

What are the two limbs of the nephron loop that filtrate passes through?

descending limb → ascending limb

After the nephron loop, where does filtrate enter?

distal convoluted tubule (DCT)

Where does filtrate go after the distal convoluted tubule?

cortical collecting duct

What structures follow the cortical collecting duct?

medullary collecting duct → papillary duct

After leaving the papillary duct, where does urine flow next?

minor calyx → major calyx → renal pelvis

What pathway does urine follow after the renal pelvis?

ureter → urinary bladder → urethra → outside of body

Cortical nephrons

80%, mostly in cortex; short loops; less concentration ability

Juxtamedullary nephrons

long loops extending deep into medulla; surrounded by vasa recta; responsible for concentrated urine

What are the three processes in urine formation?

glomerular filtration, tubular reabsorption, tubular secretion

Glomerular filtration

fluid forced out of blood into capsular space

Tubular reabsorption

substances reclaimed from filtrate to blood

Tubular secretion

solutes moved from blood into filtrate

What is the filtration membrane?

fenestrated capillary endothelium, basal lamina, podocyte filtration slits

Fenestrated capillary endothelium

blocks cells/platelets

Basal lamina

filters large proteins

Podocyte filtration slits

allow only <6–7 nm substances

What is the GFR?

volume of filtrate formed per minute

What is GFR average value?

~125 mL/min, ~180 L/day

Glomerular hydrostatic pressure (GHP)

pushes fluid out

Capsular hydrostatic pressure (CHP)

opposes filtration

Blood colloid osmotic pressure (BCOP)

pulls fluid in

Net filtration pressure (NFP)

NFP = GHP - (CHP + BCOP)

What increases GFR?

high blood pressure or dilation of afferent arteriole

What decreases GFR?

low blood pressure, sympathetic activation, or constriction of afferent arteriole

Myogenic mechanism

smooth muscle in afferent arteriole constricts when stretched, maintaining stable GFR

Tubuloglomerular feedback

macula densa senses NaCl; signals JG cells to adjust arteriole diameter

Renin-angiotensin-aldosterone system (RAAS)

raises BP and GFR

Atrial natriuretic peptide (ANP)

lowers BP and GFR by promoting fluid loss

Sympathetic NS

decreases GFR during stress

Where does most reabsorption of water, organic compounds, and ions occur in the nephron?

proximal tubule (water, glucose, amino acids, electrolytes)

What transport mechanisms allow reabsorption in the proximal tubule?

both passive (water follows solutes by osmosis) and active (pumps solutes such as Na+) transport

How are organic molecules like glucose and amino acids reabsorbed?

actively transported back into blood via symporters in the proximal tubule cells (transcellular transport)

How is water reabsorbed in the proximal tubule?

obligatory water reabsorption (water moves by osmosis following sodium and other solutes into the bloodstream)

What type of transport occurs along the paracellular route?

passive transport of small ions and water between tubule cells

What happens in the nephron loop?

• Descending limb: permeable to water; water is reabsorbed by osmosis

• Ascending limb: impermeable to water; ions (NaCl) reabsorbed by active transport
  This helps create the medullary osmotic gradient for concentrated urine

What is reabsorbed in the distal tubule and collecting duct?

water and ions (Na⁺, Cl⁻, Ca²⁺) reabsorbed under hormonal control (ADH & aldosterone) to fine-tune balance

Which hormones control water and salt reabsorption in the DCT and collecting duct?

• ADH: Increases water reabsorption in collecting ducts

• Aldosterone: Increases Na⁺ (and water) reabsorption

• ANP: Blocks Na⁺ and water reabsorption

What is facultative water reabsorption?

water reabsorption that is controlled by ADH in the collecting ducts and varies with hydration state

Summarize how reabsorption works overall

Most reabsorption occurs in the proximal tubule (active solute transport, passive osmotic water transport). The nephron loop creates an osmotic gradient; the distal tubule and collecting system adjust water and ion reabsorption under hormonal control.

Where does tubular secretion occur?

mainly in the proximal and distal tubules; removes wastes (H⁺, K⁺, drugs) from blood to filtrate

How does ADH regulate reabsorption/secretion?

increases water reabsorption in collecting ducts

How does RAAS (aldosterone) regulate reabsorption/secretion?

increases Na⁺ and water reabsorption

How does ANP regulate reabsorption/secretion?

inhibits Na⁺ and water reabsorption

What creates concentrated urine?

differential permeability of nephron sections + countercurrent mechanism (nephron loop and vasa recta)

What causes diluted urine?

when ADH levels are low, water is not reabsorbed in the collecting ducts, so it remains in the filtrate and is excreted

What structures produce the countercurrent mechanism?

nephron loop and vasa recta

Nephron loop countercurrent mechanism

countercurrent multiplier (creates gradient)

Vasa recta countercurrent mechanism

countercurrent exchanger (maintains gradient)

What is the overall function of the urinary system related to urine formation?

to filter blood, remove metabolic wastes, and maintain fluid, electrolyte, and acid–base balance while forming urine

What is the first major step in urine formation

glomerular filtration – blood pressure forces water and solutes out of the glomerulus into the capsular space, creating filtrate

What structures participate in glomerular filtration?

the renal corpuscle, which includes the glomerulus and bowman’s (glomerular) capsule

What happens during tubular reabsorption?

useful substances such as water, electrolytes, glucose, and amino acids are returned from the filtrate to the blood through the peritubular capillaries and vasa recta

Where does most reabsorption occur?

the proximal convoluted tubule, which reabsorbs the majority of water, ions, and organic molecules

What is tubular secretion?

process in which additional wastes, toxins, and excess ions are actively transported from the blood into the filtrate

Where does secretion primarily occur?

proximal tubule and distal tubule, and to some extent in the collecting ducts

How is the filtrate concentrated or diluted as it moves through the nephron?

the nephron loop creates a medullary gradient; ADH and aldosterone regulate water and ion reabsorption in the distal tubule and collecting ducts

What is the final step in forming urine?

modified filtrate (now called urine) flows into collecting ducts → papillary ducts → minor calyx → major calyx → renal pelvis → ureter → bladder → urethra → outside of body

Summarize the overall process of urine formation

Blood is filtered at the glomerulus, filtrate is refined by reabsorption and secretion along the renal tubule and collecting system, and the final urine is concentrated or diluted based on body needs before being excreted

What is the normal color of urine and what determines it?

• pale yellow to amber, determined by pigment urochrome, produced from hemoglobin breakdown

• darker color = more concentrated; lighter = more dilute

What is the normal clarity of urine?

• Clear

• Cloudiness may indicate infection, protein, crystals, or blood cells

What is specific gravity of urine and what does it reflect?

• Density of urine compared to water; indicates solute concentration

• Normal range: 1.001–1.035

• Higher value = concentrated urine; lower = dilute urine

What is the normal pH of urine?

• 4.5 to 8.0, average around 6.0

• Acidic urine can result from high-protein diet; alkaline from vegetarian diet or UTI

What is included in normal chemical composition of urine?

• Water (~95%)

• Solutes (~5%): urea, creatinine, uric acid, ions (Na⁺, K⁺, Cl⁻, Ca²⁺, HCO₃⁻, phosphate, sulfate)

What substances are not normally found in urine?

glucose, proteins, blood cells, ketones (excess), bilirubin, bacteria

What does the presence of glucose in urine indicate?

Diabetes mellitus (glucosuria)

What does the presence of protein (proteinuria) indicate?

kidney damage due to hypertension, infection, or glomerular injury

What does blood in the urine (hematuria) suggest?

trauma, infection, kidney stones, or severe kidney disease

What can ketones in urine signal?

diabetes, starvation, or low-carbohydrate dieting

What determines the excretion rate of any solute in urine?

the balance between glomerular filtration, tubular reabsorption, and tubular secretion

What is the basic formula for calculating the excretion rate of a solute?

excretion rate = (filtration) – (reabsorption) + (secretion)

What happens to a solute that is freely filtered but 100% reabsorbed?

none is excreted (glucose in a healthy individual)

What happens to a solute that is freely filtered and not reabsorbed or secreted?

all filtered solute is excreted (creatinine)

What happens to a solute that is freely filtered and partially reabsorbed?

only a portion is excreted (sodium, chloride, and water depending on body needs)

What happens to a solute that is filtered and strongly secreted?

excretion exceeds filtration amount because additional solute is added from blood to filtrate (hydrogen ions, potassium, and many drug metabolites)

How does the body adjust excretion rate to maintain homeostasis?

by regulating how much of a substance is reabsorbed or secreted, especially in the distal tubule and collecting duct under hormonal control (ADH, aldosterone, ANP)