Chapter 25 - The Urinary System

Nephron anatomy

Glomerular capsule, Proximal Convoluted Tubule (PCT), Nephron Loop, Distal
Convoluted Tubule (DCT), and Collecting Duct

Nephron 3 functions

Filtration, reabsorption, and secretion

Glomerulus

capillary network associated with Glomerular capsule

Vasa Recta

capillary network associated with Nephron Loop

Glomerular Filtration

1. Net filtration pressure forces fluid from the glomerulus to enter the Glomerular capsule

2. Increase in net filtration pressure increases Glomerular filtration rate (GFR; filtrate produced/min.

3. Filtered: water, ions, nutrients such as glucose, some wastes.

Formation of urine

1. Glomerular filtration

2. Tubular Reabsorption

3. Tubular Secretion

Tubular reabsorption

return of water and solutes from the filtrate to the blood through PCT, descending limb, ascending limb, DCT, and collecting duct

PCT tubular reabsorption

Na+ (Na+/L+ ATPase pump; active)

Cl- (leakage channels; passive)

K+ (initially secreted through pump, but passively reabsorbed via leakage channels; passive)

HCO3 (co-transport with Na+; secondary active)

H2O (osmosis; passive)

Glucose and amino acids (reabsorbed by facilitated diffusion or co-transport; if concentration exceeds transport maximum, not all will be reabsorbed and will instead remain in filtrate)

Descending limb tubular reabsorption

impermeable to solutes, water is reabsorbed via osmosis

Ascending limb tubular reabsorption

impermeable to water, Na+/K+/Cl- reabsorbed actively using 2 transporters

DCT and collecting duct tubular reabsorption

Na+ (active)

Cl- (passive)

H2O (osmosis, if ADH is present)

Tubular secretion

Secretion of substances from the vasa recta into the tubules:

H+ (active into PCT and collecting duct using protein transporter, reabsorption of bicarbonate and secretion of H+ helps to maintain blood pH)

K+ (secreted via Na+/K+ ATPase pump in collecting duct)

metabolic wastes such as ammonia and creatinine are also secreted

ADH impact on nephron

If ADH is present, collecting ducts are permeable to water; water is reabsorbed from the collecting duct and the urine is concentrated

If ADH is not present, collecting duct is impermeable to water; water will not be reabsorbed from the collecting duct and urine is very dilute

ADH is inhibited by

excess water intake, alcohol, ANH (atrial natriuretic hormone)

ADH is stimulated by

dehydration, nicotine

Aldosterone

secreted from adrenal cortex in response to angiotensin II, stimulated the Na+/K+ pump in the DCT and collecting duct (Na+ reabsorbed and K+ secreted)

Urine moves by

peristalsis

Urination

Urge to urinate occurs when bladder has 200 mL of urine, as detected by stretch receptors and transmitted to PNS, which triggers relaxation of urethral
sphincter and contraction of smooth muscle of urinary bladder

Forceful opening of urethra sphincter

occurs when 500mL of urine in bladder

Diabetes insipidus

develops because the posterior pituitary gland secretes insufficient ADH, symptoms are polyuria and excessive thirst

Treatment of diabetes insipidus

desmopressin, synthetic form of ADH

Renal calculi (stones)

due to calcium, magnesium, or crystals of uric acid; may cause a urinary obstruction in the ureters which may impair glomerular filtration due to elevated pressure in the nephron; main symptom is intense pain

Renal calculi treatment

can be surgically removed or shattered with sound waves

Renal failure

acute renal failure happens suddenly due to renal ischemia, obstruction, drugs, or trauma; chronic renal failure occurs gradually

Renal failure treatment

hemodialysis or kidney transplant

Rhabdomyolysis

Rare muscle injury in which myoglobin released into urine, caused by high intensity exercise, dehydration, or over-heating; symptom is dark brown urination