Lecture 6 - Water + Ionic Balance, Kidney Function

Osmotic pressure

Measure of the total concentration of solutes; determines direction of water movement

Water moves from regions of __ osmotic pressure to regions of __ osmotic pressure.

Low to high

Isosmotic

Two solutions have the same osmotic pressure.

Hyperosmotic

Solution with higher osmotic pressure

Hyposmotic

Solution with lower osmotic pressure

Ionic composition

Two solutions of the same osmotic pressure can have different ion concentrations.

→ Each type of ion diffuses in response to its concentration differences.

Explain how ionic composition of fluids on

different sides of a membrane affect osmotic pressures.

The greater the difference in ionic composition across a semipermeable membrane, the steeper the concentration gradient, resulting in a higher osmotic pressure that aggressively drives water toward the side with more ions to try and achieve balance.

Hyperosmotic regulator

-Animals that use energy to keep themselves hyperosmotic to the environment

-Ex. freshwater animals

-Excrete dilute urine and actively transport Na+ and Cl- back into bodies

Hyposmotic regulator

-Animals that expend energy to expel excess ions

-Ex. bony fishes

Marine osmoconformer

-Isotonic with the environment

-Ex. nearly all invertebrates in the ocean

-Many cannot survive a wide range of salinities

How do seabirds and marine reptiles get rid of excess salts in their bodies?

Extrarenal organs called salt glands → use energy from ATP

Give an example of a fish that migrates between rivers and oceans

Salmon

If a fish moved from fresh water to salt water, which hormone would help it to compensate

for the saltier environment?

Cortisol → Saltwater hormone; chloride cells in gills pump salts out

How about if a fish moved from salt water to fresh water?

Prolactin → Freshwater hormone; chloride cells pump salts in

In fish, where are chloride cells located, and what do they do in fresh water and salt water?

In the gills

In freshwater, they pump salts in.

In saltwater, they pump salts out.

On this graph, which is an osconformer and which is an osmoregulator?

Blue crab is an osmoregulator. Mussel is an osmoconformer.

What is the primary form of nitrogenous waste that a freshwater fish would be expected

to excrete?

Ammonia (most toxic but highly water soluble)

What is the primary form of nitrogenous waste that a dog would be expected

to excrete?

Urea (more expensive metabolically but less toxic)

What is the primary form of nitrogenous waste that a lizard would be expected

to excrete?

Uric acid (most expensive metabolically, highly insoluble, conserves water)

What animals secrete ammonia?

Most aquatic animals that breathe water

What animals secrete urea?

Most terrestrial animals

What animals secrete uric acid?

Insects, reptiles, birds

The 4 basic processes that take place in the renal tubules

1. Filtration

2. Secretion

3. Reabsorption

4. Osmoconcentration

Filtration

Water and small solutes pass through a barrier while cells and large molecules remain behind.

Secretion

Transport epithelia move specific solutes into the tubule lumen for excretion.

Reabsorption

Transport epithelia move specific solutes and water back into the body from the lumen.

Osmoconcentration

Water is removed from the lumen, leaving solutes behind and producing a more concentrated excretory fluid.

The three types of renal organs

1. Protonephridia

2. Mesonephridia and metanephridia

3. Malpighian tubules

Protonephridia

-Simple invertebrates

-Blind end ducts project into body cavity

-Ultrafiltration driven by cilia moving fluid outward

Mesonephridia and metanephridia

-Ultrafiltration driven by fluid pressure

-Requires a circulatory system

→ Mesonephric = aquatic vertebrates

→ Metanephric = terrestrial animals

Malpighian tubules

-Filtration driven by active secretion of ions

-Arthropods

Kidneys

Urine-forming organ

Where does blood enter and exit the kidneys?

Renal artery; renal vein

Where do ureters empty in reptiles and birds?

The hindgut

Nephron

Smallest functional unit of the kidney; 1 million in human kidney; consists of tubule and vascular component

Vascular component of nephron

Afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries

Tubular component of nephron

Bowman’s capsule, proximal tubule, Loop of Henle, distal tubule, collecting duct

Combined vascular/tubular component of nephron

Juxtaglomerular apparatus

Afferent arteriole

Carries blood to the glomerulus

Glomerulus

A tuft of capillaries that filters a protein-free plasma into the tubular component

Efferent arteriole

Carries blood from the glomerulus

Peritubular capillaries

Supply the renal tissue; involved in exchanges with the fluid in the tubular lumen

Bowman’s capsule

Collects the glomerular filtrate

Proximal tubule

Uncontrolled reabsorption and secretion of selected substances occur here

Loop of Henle

Establishes an osmotic gradient in the renal medulla that is important in the kidney’s ability to produce urine of varying concentration

Distal tubule and collecting duct

Variable, controlled reabsorption of Na+ and H2O and secretion of K+ and H+ occur here; fluid leaving the collecting duct is urine, which enters the renal pelvis

Juxtaglomerular apparatus

Produces substances involved in the control of kidney function

Tubular reabsorption is ___.

highly selective

The descending limb of the loop of Henle allows reabsorption of _______ but not ________.

water but not Na+

The ascending limb of the loop of Henle allows reabsorption of _______ but not ________.

Na+ (active process) but not water

What is meant by the “countercurrent multiplier” of the kidney?

A mechanism within the loop of Henle that creates a high salt concentration deep inside the medulla of the kidney that allows highly concentrated urine (up to 1200 mosm)

What causes aquaporin 2 channels to be added to the kidney, where are they added, and

what do they accomplish?

Why? Due to vasopressin (antidiuretic hormone)

Where? The distal tubule and collecting duct

What do they do? Increase water reabsorption

Water reabsorption is under ___.

hormonal control

What do the vasa recta accomplish in the kidney and, very briefly, how do they do this?

Help conserve vertical osmotic gradient in the renal medulla through countercurrent exchange from a hairpin loop; supplies blood to the renal medulla

What is meant by “urea recycling” in the kidney, and how does this affect water

reabsorption?

-Contributes to medullary hypertonicity

-The lowest section of the collecting duct is permeable to urea. Urea diffuses out.

-Some urea is recycled into the loop of Henle and medulla to increase osmolarity

What are the stimuli for renin secretion from the juxtaglomerular apparatus, and how does

this ultimately result in aldosterone secretion?

Stimuli = low blood volume, low blood pressure

Renin catalyzes angiotensinogen → angiotensin I. Angiotensin I → Angiotensin II via angiotensin-converting enzyme. Angiotensin II is the primary stimulus for aldosterone secretion.

What are the primary functions of aldosterone?

-Increases Na+ absorption

-Stimulates secretion of vasopressin (ADH), which promotes water retention by kidneys

-Potent constrictor of systemic arterioles, directly increasing blood pressure