AN OVERVIEW OF THE URINARY SYSTEM

FUNCTIONS OF THE URINARY SYSTEM

► Remove wastes

► Regulate levels of ions (K+, Na+, Cl -, Ca++, HPO4-2), glucose, amino acids, calcitriol (active Vitamin D), and erythropoietin, creatinine

► Regulate blood osmolarity

► Regulate blood volume and pressure

► Regulate blood pH - NH3 (ammonia, a base) and H+ (acid)

Physiological Mechanisms of the Nephron

► Glomerular filtration

► Reabsorption

► Secretion

GLOMERULAR FILTRATION

► Occurs in glomerulus of renal corpuscle - end product is called ''filtrate"

► Passive process

► H2O (osmosis)

► Urea (diffusion)

► All particles smaller than large proteins and blood cells (RBC, WBC platelets) are filtered (return to blood)

REABSORPTION (in the-P.C.T.)

► 65% of all filtrate is reabsorbed here (H2O) - passive

► 50% of the urea filtered is re-absorbed (because it's small) – passive

► Na⁺ (and K+, SO₄^-3, Cl^-, Ca⁺⁺, PO₄^{-3 )}

►Na⁺ & glucose (symport), and vitamins enter P.C.T cell by active transport

REABSORPTION (in the descending Loop of Henle)

► Impermeable to ions
► Increase in osmolarity due to H₂O reabsorption

REABSORPTION (in the ascending Loop of Henle) - Na+!!!

REABSQRPTION (in the D.C.T.)

► More Na⁺/Cl^- reabsorption occurs here

► Ca⁺⁺ reabsorption, regulated by PTH (secreted from parathyroid glands) - PTH increases blood Ca⁺⁺

REABSORPTION (in the Collecting Duct)

► More H₂O reabsorption regulated by vasopressin (ADH), secreted by posterior pituitary gland

► More Na⁺ and H₂O reabsorption regulated by aldosterone (secreted from adrenal glands)

SECRETION (in the P.C.T.)

► Deamination of glutamine in the proximal convoluted tubule cells generates ammonia:

NH₃ + H⁺ → NH₄⁺

► As more H⁺ is secreted, HCO3^- is reabsorbed.

► These reactions increase during acidosis to return homeostatic balance.

SECRETION (in the D.C.T.)

► K⁺ secreted in exchange for Na⁺ reabsorption

SECRETION (in the Collecting Duct)

► H⁺ secreted as a by-product of our primary buffer system:

CO₂ + H₂O → H₂CO₃ →H⁺ + HCO₃^-

► Resulting HCO₃^- can buffer blood pH.

► Amino acids are deaminated in the PCT cells.

► The resulting ammonia binds H⁺ to form NH₄⁺, buffering urine.

REGULATION

Glomerular filtration rate (GFR)

► Controlled by constriction/dilation of afferent arterioles

► Increased blood volume / blood pressure causes stretching of the heart, which releases ANP - this causes
relaxation of afferent and efferent arterioles and thereby increases GFR

(↑ GFR causes less Na⁺ / H₂O reabsorption) thus ↓ ing blood volume and blood pressure to normal

► Sympathetic stimulation (NorEPI via a₁ receptors) can cause vasoconstriction of afferent arterioles (and therefore
lower GFR) - this increases blood flow to other areas.

► Decreased blood volume / blood pressure (as might be seen in diuresis / natriuresis) causes renin release by JG
cells:

Renin releases Angiotensin I from liver precursor

ACE (lungs) converts AG I to AG II

AG II stimulates constriction of afferent and efferent arterioles and thereby decreases GFR

(↓ GFR causes more Na⁺ / H₂O reabsorption) thus ↑-ing blood volume and blood pressure to normal

RENTN-ANGIOTENSIN PATHWAY

► AG II stimulates adrenal cortex to secrete aldosterone

► Aldosterone causes increased Na⁺ / H₂O reabsorption

► This causes increased K ⁺ excretion and an increase in blood volume / pressure.

► AG II also stimulates ADH release from pituitary gland

► ADH acts on collecting duct to increase H₂O reabsorption which also increases blood volume / pressure.

SUMMARY

₪ The kidney filters approximately 180 L of H₂O from plasma and forms 1-2 L of urine every day.

₪ Through reabsorption of these filtered products and secretion of wastes, the kidney is responsible for:

Regulating pH of blood and body fluids

Maintaining blood glucose levels

Maintaining Ca⁺⁺ homeostasis

Maintaining electrolyte balance of ICF/ECF

Regulating blood volume and blood pressure

Excretion of toxins and wastes: urea, ammonia, creatinine, uric acid, drugs

₪ The kidney is also responsible for:

Vitamin D formation (calcitriol)

RBC formation (erythropoietin)

₪ Regulation occurs neuronally via sympathetic stimulation and hormonally via renin-angiotensin pathway, aldosterone, ADH, and ANP.