Urinary System Pt. 2

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.

How does glucose in urine increase urination?

Glucose remains in the filtrate and pulls water with it by osmosis.

Functions of the ureters

Transport urine from the kidneys to the bladder by peristalsis.

Urinary bladder

Temporarily stores urine and expands as urine volume increases.

Transitional epithelium

Allows the bladder to stretch.

Detrusor muscle

Relaxes during filling and contracts during urination.

Internal urethral sphincter

Smooth muscle under involuntary control.

External urethral sphincter

Skeletal muscle under voluntary control.

What happens as the bladder fills?

Stretch receptors activate and parasympathetic neurons stimulate detrusor contraction.

Micturition

Occurs when the external urethral sphincter relaxes.

How can urination be delayed?

Higher brain centers temporarily suppress the micturition reflex.