Urinary System Review

Animals must eliminate metabolic waste products:

• Carbohydrates and fat end as water and CO2 and are easily excreted.
• Proteins and nucleic acids contain nitrogen and also produce nitrogenous waste.

Ammonia (NH3) is the most common and toxic nitrogenous waste.

• Ammonia is soluble in water; aquatic animals who secrete NH3 through gills are ammonotelic.
• Terrestrial and other aquatic animals must convert NH3 to urea or uric acid.

Ureotelic animals mostly excrete urea.

• Urea is water-soluble but results in a larger water loss.
• This is how we roll.

Uricotelic animals mostly excrete uric acid.

• It is insoluble in water and precipitates out of the urine with little water loss.
• This is the ‘white’ part of bird and reptile droppings

How Do Vertebrates Maintain Salt and Water Balance?

The kidney:

• Maintains salt and water balance
• Removes ammonia from the blood
• Regulates blood pH
• Produces bicarbonate.

How Does the Mammalian Kidney Produce Concentrated Urine?

The ureter, renal artery, and renal vein enter the kidney on the concave side.

• The ureter branches and envelops the renal pyramids.
• Renal pyramids make up the medulla, or internal core.
• The cortex is the outer layer of the medulla.

Ureter:

• A duct from the kidney that leads to the urinary bladder.

Urethra:

• A tube for urine excretion.

Nephron: the main functional unit of the kidney

The renal tubules

• Begins with Bowman’s capsule, which encloses the glomerulus.
• Then the proximal convoluted tubules—the initial, twisted segments of the renal tubule

Loop of Henle (Descending and ascending branches)

• The ascending limb of the loop of Henle becomes the distal convoluted tubule
• The distal convoluted tubules join the collecting ducts.
• Collecting ducts descend through renal pyramids and empty into the pelvis.
• Pelvic divisions join and leave the kidney as the ureter.

The blood

Afferent arterioles carry blood to the glomeruli.

• Glomeruli are drained by efferent arterioles that become the peritubular capillaries.
• The vasa recta is a network of peritubular capillaries parallel to the loops of Henle and the collecting duct.

All peritubular capillaries from a nephron will drain into venules that lead to the renal vein.

What Mechanisms Regulate Kidney Function?

A constant glomerular filtration rate (GFR) needs blood supplied to the kidneys under adequate pressure.

• Autoregulatory mechanisms ensure blood supply and blood pressure:
• Dilation of afferent renal arterioles— maintains glomerular blood pressure
• This maintains adequate GFR

Kidney releases hormone renin if GFR still falls, this activates angiotensin:

Angiotensin:

• Constricts efferent renal arterioles and peripheral blood vessels to raise blood pressure.

The hypothalamus can stimulate release of antidiuretic hormone (ADH) if pressure is too low.

• ADH increases the permeability of membranes to water.
• More water = more pressure

Stretch receptors in the aorta and carotid arteries will inhibit ADH if blood pressure increases.

• Less circulating ADH means less water reabsorbed leading to lower blood volume and pressure.

Atrial muscle fibers release atrial natriuretic peptide (ANP) when blood volume in the atria increases.

• ANP decreases the reabsorption of Na+ in the kidney.
• Increased loss of Na+ and water decreases blood volume and pressure.