Mammalian Kidneys and Osmoregulation

What do mammals rely on for osmoregulation?

kidneys

What nitrogen product do mammals excrete?

urea

What U:P ratio do mammals have?

greater than 1, humans 4

How can mammals make water?

metabolic water, have a high metabolis

Describe kangaroo rats and their osmoregulatory adaptations, icnlduing

• H20 loss/gain

• adaptations

• heat exchange

• gain water via food and metabolic H20

• lose water via respiration, feces and urine (urea)

• produce highly concentrated urine (U:P = 12)

• use nasal passage heat exchange to reduce water loss

Describe kangaroo rats nasal passage heat exchange

• heat exchange in nasal passages to reduce respiratory water loss

• inhale= hot, dry air picks up heat and water from nasal lining, which cools lining

• exhale = the now warm, humid air passes over the cool nasal lining, the exhaled air cools, causing water vapor to condense back into liquid on the nasal surfaces

• retains more water

Describe camel osmoregulatory adaptations

• use heat exchange in nasal passages to retain some water

• At night, cool nasal passages cool the warm, humid air exhaled from the lungs.

• Water vapor in that exhaled air condenses onto the nasal lining and can be absorbed.

• so they exhale desaturated air (75% relative humidity)

What do marine mammals salt intake depend on and what is common prey’s osmolarity?

• salt intake varies with prey

• bony fish = 300mOsm, low salt, high protein/Nh3 excretion

• invertebrates = 1000mOsm, high salta

• plants = 1000mOsm, high salt

Describe marine mammal water and salt gain/loss

• gain water via food/drink, metabolic H20

• lose water via REWL, cutaneously when surfacing, via urine/feces, via lactation

• gain salt via food/drink

• lose salt via urine/feces and lactation

What unique osmoregulatory adaptation to marine mammals have?

reniculate kidneys that can produce concentrated urine and eliminate excess salts

What are the main excretory organs and what are their functions?

• liver - processes hydrophobic waste (large waste, carbs, fats)

• colon - processes waste, excretes/reabsorbs H20

• kidney - processes hydrophilic waste, primary organ for osmoregulation, blood pressure, and blood pH regulation via blood filtration

What are the parts of the urinary system?

• kidneys drain into ureters

• ureters travel to bladder

• urine exits bladder into urethra via internal smooth muscle sphincter

• urine exits urertha via external skeletal muscle sphincter

What are the 6 functions of the kidney?

1. ion balance (esp. Na+ and K+)

2. osmotic balance - by determining volume and conc of urine/blood

3. blood pressure - by controlling blood/ECF volume (influenced by hormones)

4. blood pH - by retaining/excreting H+ or HCO3-

5. Excretion - nitrogenous and hydrophilic waste

6. Endocrine - produces renin that starts blood pressure regulation cascade and erythropoietin to refulate RBC synthesis

What is renin and what organ produces it?

hormone produced by kidney that begins blood pressure regulation cascade

What is erythropoietin and what organ produces it

hormone produced by kidney that regulates red blood cell synthesis

How does osmolarity change across the kidney?

increasing osmolarity as you go deeper from cortex to medulla

What three key steps occur in the nephron?

• filtration

• reabsorption

• secretion

What is the Bowman’s Capsule and glomerulus ?

• structure at the start of nephron that surrounds the glomerulus, a bundle of capillaries

• blood enters the glomerulus via afferent arteriole and exits via efferent arteriole to the vasa recta

• diameter of arterioles regulate glomerular blood pressure and amount of fluid filtered into Bowman’s Capsule

• The afferent arteriole is usually wider than the efferent arteriole, helping maintain high pressure in the glomerulus for filtration.

If blood pressure is low, what happens to the afferent and efferent arterioles?

dilate to allow more blood enter glomerulus to maintain glomerular pressure

Describe filtration in the kidney

• movement of plasma across capillary membranes using blood pressure in Bowman’s capsule

• creates filtrate of water, ions, glucose, amino acids, and small waste inside nephron

• blood exits Bowman’s capsule via efferent arteriole and forms vasa recta

Describe kidney reabsorption, including what is reabsorbed

• body takes back stuff it wants from filtrate to blood, mainly through vasa recta

• lots of H20 from plasma ends up in filtrtae, but 99% of filtered water is reabsorbed/restored

• always reabsorbed: glucose, amino acids, H20

• sometimes reabsorbed: ions, additional H20

• occurs mostly in proximal tubule, but also in Loop of Henle, Distal tubule, and collecting duct

Describe kidney secretion, including what is secreted

• adding something from blood to filtrate

• always secreted: creatinine (measure of kidney function)

• sometimes secreted: ions

• occurs in proximal tubule, distal tubule, and collecting duct

How does filtrate move through the nephron?

• bowman’s capsule/glomerulus

• proximal convoluted tubule

• Loop of Henle

• distal convoluted tubule

• collecting duct

Describe the Proximal convoluted tubule

• most reabsorption and secretion occurs here; reabsorbs as much as pssible

• reabsorbs ALL glucose via active cotransporters with Na+

• reaborbs all amino acids

• reaborbs lots of H20, some sodium

• Water follows the reabsorbed solutes by osmosis into the surrounding vasa recta

How does the vasa recta create a strong osmotic gradient for water?

• Low hydrostatic pressure: blood has passed through the high-resistance efferent arteriole, so there is less pressure pushing water out of the capillary.

• High osmotic pressure: some water was filtered out at the glomerulus, but proteins stayed in the blood. This makes the blood protein concentration higher, which pulls water into the capillaries.

Describe the loop of henle, including the descending vs ascending limb

• estabolishes and maintains conc gradient in medulla for later via facilitated diffusion

• descending limb - permeable to H20 (aquaporins), not permeable to ions

• ascending limb - not permeable to H20, permeable to ions (channels)

How does the countercurrent multiplier in the loop of Henle concentrate the fluid surrounding the medulla?

• The descending limb lets water leave but retains salt, so filtrate becomes concentrated.

• the ascending limb does not let water leave but moves NaCl into the medulla.

• this builds a high salt concentration in the medulla, allowing water reabsorption into medulla and concentrated filtrate

• the vasa recta have blood flowing in the opposite direction and act as a countercurrent exchanger

Describe the distal convoluted tube

• site of specialized secretion and absorption based on body’s needs

• reabsorbs Na+ in response to aldosterone

• secretes excess K+ in response to aldosterone

What does aldosterone cause to happen in the kidneys?

reabsorption of sodium and secretion of potassium in distal convoluted tubule

Descibe the collecting duct

• site of regulated H20 reabsorption in response to ADH

• if ADH is present → permeable to H20 (aquaporins present), water reabsorb passively, more conc urine

• if ADH not present → impermeable to H20 (no aquaporings), less conc urine

what does the strenght of the medullary osmotic gradient depend on?

loop of henle length

What stays in the blood during filtration in Bowman’s Capsule?

blood cells and large proteins; too large to pas through filtration barrier

What happens in the ascending limb of the loop of Henle?

NaCl leaves the filtrate, but water cannot, making the filtrate more dilute and the medulla saltier.

Why are long loops of Henle useful in desert mammals?

They create a stronger medullary osmotic gradient, allowing more water reabsorption and reducing water loss in urine.

What is the external urethral sphincter made of?

Skeletal muscle, so it is under voluntary control.

What causes water to move out of the collecting duct when ADH is present?

The high solute concentration of the medulla pulls water out by osmosis once aquaporins are inserted.

What stimulates renin release from the kidney?

Low blood pressure, low blood volume, or low NaCl delivery to the kidney.

How do kidneys help regulate blood pH?

They secrete H⁺ into urine and reabsorb or produce HCO₃⁻, helping prevent the blood from becoming too acidic.

What are major routes of water loss in marine mammals?

Respiration, urine, feces, skin loss while at the surface, and lactation.