Complete Urinary System - BIOL 221

Main function of the urinary system

Continuously filters blood plasma to maintain homeostasis.

Structures of the urinary system

Kidneys, ureters, urinary bladder, and urethra.

Function of the kidneys

Produce urine by filtering blood.

Function of the ureters

Transport urine to the bladder.

Function of the urinary bladder

Stores urine.

Function of the urethra

Eliminates urine from the body.

Main jobs of the kidneys

Remove metabolic wastes, regulate water balance, regulate ion concentrations, and help regulate blood pressure and pH.

How much fluid do the kidneys filter each day?

About 180-200 L.

How much urine is produced each day?

About 1.5-2 L.

Why is only a small amount of filtered fluid excreted as urine?

Most filtered material is returned to the blood.

Result of kidney function

Wastes are eliminated while useful substances are conserved.

Nitrogenous wastes

Metabolic wastes removed from the blood by the kidneys.

Urea

Produced during amino acid metabolism.

Uric acid

Produced during nucleic acid metabolism.

Creatinine

Produced by muscle metabolism.

What happens if nitrogenous wastes accumulate in the blood?

Cellular function becomes disrupted and homeostasis begins to fail.

Four major processes of urine formation

Filtration, reabsorption, secretion, and excretion.

Filtration

Fluid leaves the blood and enters the nephron.

Reabsorption

Useful substances move from the nephron back into the blood.

Secretion

Additional wastes move from the blood into the nephron.

Excretion

Remaining fluid leaves the body as urine.

Location of the kidneys

Retroperitoneal (behind the parietal peritoneum).

How are the kidneys protected?

Partially protected by the ribs.

Fibrous capsule

Supports kidney tissues.

Adipose capsule

Cushions the kidneys.

Renal fascia

Anchors the kidneys.

Renal cortex

Outer region of the kidney.

Renal medulla

Inner region containing renal pyramids.

Renal pyramids

Contain nephrons and blood vessels.

Where does urine formed in the nephrons drain?

Toward the center of the kidney before entering the ureter.

Hilum

Recessed area on the medial side of the kidney where blood vessels, nerves, and the ureter enter or leave.

Blood flow to the glomerulus

Renal artery → interlobar arteries → afferent arteriole → glomerulus.

Glomerulus

Beginning of the nephron where blood pressure creates filtrate.

What creates filtrate?

Blood pressure forcing fluid out of glomerular capillaries.

Where does blood leave the glomerulus?

Through the efferent arteriole.

Blood flow after the glomerulus

Efferent arteriole → peritubular capillaries.

Reabsorption in the nephron

Substances move from filtrate to blood.

Secretion in the nephron

Substances move from blood to filtrate.

Filtrate

Fluid filtered from the blood.

Pathway of filtrate through the nephron

Bowman's capsule → proximal convoluted tubule → nephron loop → distal convoluted tubule → collecting duct.

What happens to filtrate as it moves through the nephron?

Useful substances are reabsorbed and additional wastes may be secreted.

Urine

Fluid leaving the collecting duct.

Pathway of urine out of the kidney

Renal papilla → minor calyx → major calyx → renal pelvis → ureter.

What does urine composition reflect?

Kidney function, hydration level, and blood chemistry.

Renal corpuscle

Contains the glomerulus and Bowman's capsule; site where filtration begins.

Glomerulus (renal corpuscle)

Fenestrated capillary network.

Bowman's capsule

Surrounds the glomerulus.

Why does filtration occur in the renal corpuscle?

Blood enters under unusually high pressure, forcing water and small solutes out of capillaries.

What does filtrate initially resemble?

Blood plasma.

What does filtrate contain?

Water, ions, glucose, amino acids, and nitrogenous wastes.

What remains in the bloodstream during filtration?

Blood cells and most proteins.

Why is reabsorption necessary?

Filtration is mostly nonselective, so useful substances must be returned to the blood.

Proximal convoluted tubule (PCT)

Major site of reabsorption.

What is reabsorbed in the PCT?

Most water, glucose, amino acids, and many ions.

Why do PCT cells contain many mitochondria?

To produce ATP for active transport.

Why do PCT cells have microvilli?

To increase surface area for reabsorption.

What would happen without reabsorption?

Nutrients and water would rapidly be lost in urine.

Why is reabsorption necessary?

Filtration is broad and sloppy; reabsorption selectively fixes the filtrate.

Descending limb of the nephron loop

Permeable to water.

What happens in the descending limb?

Water leaves the filtrate by osmosis, concentrating the filtrate.

Ascending limb of the nephron loop

Impermeable to water.

What happens in the ascending limb?

NaCl leaves the filtrate, making the filtrate more dilute.

What is the result of the nephron loop?

The renal medulla becomes highly concentrated ("salty").

Why is the medullary gradient important?

It allows the kidneys to conserve water later.

Distal convoluted tubule (DCT)

Fine-tunes blood composition.

What substances are secreted in the DCT?

H⁺, K⁺, drugs, and metabolic wastes.

What does secretion help regulate?

Blood pH and electrolyte balance.

Why is secretion important?

Provides additional cleanup after filtration.

Collecting duct

Passes through the concentrated renal medulla.

Why can water leave the collecting duct?

The salty medulla draws water out by osmosis.

Where does reabsorbed water go?

Into nearby capillaries and back to the bloodstream.

How does a stronger medullary gradient affect urine?

Conserves more water and produces more concentrated urine.

Role of the nephron loop

Creates the environment that allows later water reabsorption.

Antidiuretic hormone (ADH)

Conserves water during dehydration.

What triggers ADH release?

Increased blood osmolarity during dehydration.

Where are osmoreceptors located?

Hypothalamus.

Where is ADH released from?

Posterior pituitary.

Where does ADH act?

Collecting duct cells.

How does ADH increase water reabsorption?

Inserts aquaporin channels into collecting duct cell membranes.

Effect of ADH on water permeability

Water permeability increases.

Effect of ADH on urine volume

Urine volume decreases.

Effect of ADH on urine concentration

Urine becomes more concentrated.

What happens without ADH?

Collecting ducts remain relatively impermeable to water.

Effect of no ADH on urine

Large volumes of dilute urine are produced.

Diabetes insipidus

Impaired ADH signaling prevents effective water reabsorption.

Effects of diabetes insipidus

Large volumes of dilute urine and dehydration.

Cortical nephrons

Have short nephron loops that extend only slightly into the medulla.

Juxtamedullary nephrons

Have long nephron loops that extend deep into the medulla.

Why are juxtamedullary nephrons important?

Long loops create a stronger medullary gradient, conserving more water.

Normal urine output

About 1.5-2 L/day.

Characteristics of healthy urine

Pale yellow, clear, and slightly acidic.

What gives urine its yellow color?

Pigments from RBC breakdown.

Specific gravity

Reflects urine concentration.

How does concentrated urine affect specific gravity?

Specific gravity increases.

Healthy urine should NOT contain

Glucose, proteins, blood cells, or hemoglobin.

What may abnormal urine composition indicate?

Kidney disease or damage.

Why does glucose appear in urine during diabetes mellitus?

The PCT cannot reabsorb all filtered glucose.

What can glomerular damage cause?

Proteinuria or hematuria.

Why does dehydration make urine darker?

More water is reabsorbed, making urine more concentrated.

Foamy urine

May indicate glomerular damage causing protein to leak into urine.