Water and Ionic Balance: Kidney Function

Lecture 6

osmotic pressure

• total concentration of solutes

• determines direction of water movement

Water moves from ___ osmotic pressure to regions of ____ osmotic pressure

from LOW to HIGH

Hypoosmotic

soln with lower osmotic pressure

Hyperosmotic

soln with higher osmotic pressure

Isosmotic

two soln have same osmotic pressure

Ionic composition

two soln have same osmotic pressure, different ion concentrations

Freshwater animals are ____osmotic

• hyperosmotic

• water constantly diffuses in while ions constantly diffuse out

How do freshwater fish compensate for their osmoregulation?

• very dilute urine

• actively transport Na+ and Cl- into their body via the gills

Saltwater animals are ____osmotic

• hypoosmotic

• water constantly diffuses out (osmosis), ions constantly diffuse in (diffusion)

• fish drink seawater to gain water

• ions are pumped out

In oceans, nearly all invertebrates are…

isotonic with the environment (osmoconformers)

Freshwater animals are also known as…

hyperosmotic regulators

Saltwater fish are also known as…

hypoosmotic regulators

How do seabirds, ocean lizards, and sea turtles do osmoregulation?

excess salt is secreted by extrarenal organs called salt glands

What happens to fish that migrate between rivers and oceans?

• they switch between being hyperosmotic regulators (fresh water) and hypoosmotic regulators (sea water)

What directs the shift to hypoosmotic regulation?

cortisol → chloride cells in gills that pumps salt out

What directs the shift to hyperosmotic regulation?

prolactin → freshwater hormone → chloride cells reverse to pump salts in

What are osmotic conformers?

osmotic pressure of body fluids is always the same as the water

Example of an osmotic regulator?

blue crabs

What do nitrogenous wastes result from?

metabolism of proteins and nucleic acids

What does the type of nitrogenous waste excretion depend on?

water availability and survival strats

Describe ammonia as a form of nitrogenous waste

• most aquatic animals that breathe water

• most toxic but highly water soluble

• easily produced and diluted

Describe urea as a form of nitrogenous waste

• seen in most terrestrial animals

• energy expensive

• less toxic

Describe uric acid as a form of nitrogenous waste

• insects, reptiles, and birds

• most energy expensive

• highly INsoluble in water → excreted as a paste

How many steps do renal tubules perform? List them

1. filtration

2. secretion

3. reabsorption

4. osmoconcentration

Filtration within the kidneys

water and small solutes pass through barrier; cells/macromolecules stay behind

Secretion within the kidneys

transport epithelia move specific solutes from blood into tubule lumen for excretion

Reabsorption within the kidneys

transport epithelia move specific solutes/water back into the body; active or passive

Osmoconcentration within the kidneys

water is removed from tubule lumen, leaving solutes behind → produce more concentrated excretory fluid (urine)

What are the types of renal organs?

1. protonephridia

2. mesonephridia

3. malpighian

Protonephridia

seen in simple invertebrates (flat worms) → blind-end ducts; ultrafiltration driven by cilia

Mesonephridia and metanephridia

• ultrafiltration driven by fluid/BP

• requires circulatory system

• Mesonephridic tubules: aquatic vertebrates

• metanephric tubules: terrestrial animals

Malpighian tubules

filtration driven by active secretion of ions; seen in anthropods

What is the urine forming organ?

the kidneys

What are kidenys?

paired organs on dorsal side of abdominal cavity

How does blood enter and exit the kidney?

supplied by renal artery, exits via renal vein

Where does urine drain?

into ureters

Urine pathway

kidneys → ureters → urinary bladder → urethra

Nephron

smallest functional unit of the kidney

What does a nephron consist of?

a tubule and associated vascular component

Afferent arteriole

part of the vascular component; carries blood to glomerulus

Glomerulus

part of the vascular component; tuft of capillaries that filters a protein-free plasma into the tubular component

Efferent arteriole

part of the vascular component; carries blood from glomerulus

Peritubular capillaries

part of the vascular component; supply the renal tissue; involved in exchanges with the fluid in the tubular lumen

Bowman’s capsule

part of the tubular component; collects glomerular filtrate

Proximal tubule

part of the tubular component; uncontrolled reabsorption and secretion of selected substances

Loop of Henle

part of the tubular component; establishes an osmotic gradient in the renal medulla; important in kidney’s ability to produce urine of varying concentration

Distal tubule and collecting duct

part of the tubular component; controlled reabsorption of Na+ and H₂O and secretion of K+ and H+

Juxtaglomerular apparatus

combined vascular/tubular component; produces substances involved in the control of kidney function → secretes renin into blood in response to low BP

Tubular absorption is…

highly selective

Mammalian tubules reabsorb…

• 99% filtered salt and water

• 100% filtered glucose and amino acids

Reabsorption of most substances occurs in the…

proximal tube

To be reabsorbed, a substance must…

pass through the renal tubular cell and the capillary wall

Water is _____ by osmosis

passively

Na+ reabsorption is…

active

What does the descending limb of the Loop of Henle do?

allows for water reabsorption but not Na+

What does the ascending limb of the Loop of Henle do?

allows for Na+ absorption but not water

The Loop of Henle acts as a…

countercurrent multiplier → allows urine to be concentrated n

Water reabsorption is under ____ control

hormonal

What hormone increases water reabsorption?

vasopressin

Vasa recta

• blood vessels closely associated with each other and the Loop of Henle

• help conserve vertical osmotic gradient in renal medulla

• countercurrent exchange occurs when it forms a hairpin loop

Urea recycling contributes to…

medullary hypertonicity → helps regulate BP

Renin-angiotensin-aldosterone system

1. JGA secretes renin in blood in response to low BP

2. a stimulus causes aldosterone to be secreted

3. increase in Na+ reabsorption and secretion of vasopressin (promotes water retention)