Animal Physiology: Kidneys

Mammalian blood osmolarity

~300

Where does our body’s water come from?

• External environment (in aquatic animals)

• Dietary Water

• Water produced from digestion of foods

Why is it important to maintain balance between internal and external solute composition?

It impacts how enzymes work

Skin as an osmotic barrier

• Mucus covering in some animals assists to prevent permeability

• Minimizes water loss

Why don’t we like excreting straight ammonia?

It’s highly toxic and smells bad so attracts predators

What animals secrete uric acid?

Molluscs, Reptiles, Birds

What animals secrete urea?

Mammals

Is production of uric acid or urea more efficient?

Production of uric acid

Nephridia

Very basic “kidney” structures

Malphighian tubules

“Kidneys” for animals that use hemolymph instead of blood.

Functions of the Kidney

• Regulation of extracellular fluid volume (blood pressure)

• Regulation of osmolarity at 290 mOsm

• Maintenance of ion balance

• Regulation of pH

• Excretion of wastes

• Production of hormones

How much water/food flows in and out of us each day?

2 Liters

What food produces the most H2O when digested?

Lipids

What percent of total cardiac output goes to the kidneys?

20-25%

Nephrons

The base functional unit of a kidney

Renal Cortex

The outer layer of kidney

Outer Medulla

The middle layer of kidney

Inner Medulla

The inner layer of kidney

Filtrate

The filtered blood that travels through the renal tubule

Podocytes

Finger-like cells in the inner Bowman’s capsule that cling to capillaries to increase surface area for diffusion

Glomerular Filtration Highlights

• 15-25% of water and solutes are removed from plasma

• Glomerular filtration rate is 120 ml/min

• Filters 180 liters daily

Changes in osmolarity as filtrate moves along the nephron

• 300 in Bowman’s Capsule

• 300 at end of proximal tubule

• 100 at end of Loop of Henle

• 50-1200 at end of Collecting Duct

Regulation of GFR (Glomerular Filtration Rate)

• Renal blood flow relatively constant

• Myogenic response— smooth muscles in arterioles contract when stretched

• Sympathetic neurons! Poor emotional state = more pee

Juxtaglomerular Apparatus (JGA)

The little hormonal system built into a nephron

Micropuncture Experiments

Measure osmolarity of solution at different points in the nephron

What does the proximal tubule filter?

• Lots of Na+ removed from filtrate and returned to capillaries

• Solutes like Cl- filter out passively

• When NaCl in tubule decreases, water transport decreases

Proximal Tubule Characteristics

• Brush Border (very folded) on apical side

• Tight Junctions

• Lots of mitochondria

How is Na+ reabsorbed into the bloodstream?

Small osmotic gradients power flow, and it flows throw Na+ channels that are always open

Why is Na+ reabsorption important?

Because it powers the filtration of literally everything else in the kidneys, such as glucose and Cl- and H2O

Early proximal convoluted tubule characteristics

Has all sorts of transporters, generates Na+ gradient and uses that to move other things

What type of sugar is removed along the proximal tubule?

Inulin

Normal glucose concentration in urine

Less than 1mg/ml

What does it mean if there is glucose in the urine?

Something is wrong with the kidneys OR we got diabeetus (blood glucose so high kidneys can’t keep up)

What does the Loop of Henle act as?

A counter current multiplier

Where is the bottom of the Loop of Henle located?

Inner medulla :)))

What is the filtrate osmolarity at the bottom of the loop of henle?

1200 mOsm/L

Descending Loop of Henle characteristics (there are 5)

• Thin Cells

• Few mitochondria

• No brush borders

• Low permeability to NaCl and Urea

• Water-permeable

Thin Ascending Loop of Henle Characteristics (there are 3)

• Permeable to NaCl

• Low permeability to Urea

• Very low water permeability

Thick Ascending Loop of Henle Characteristics (there are 2)

• Active transport of Na and Cl

• Very low permeability to water

The Single Effect (Loop of Henle)

Because the ascending loop of henle is impermeable to water, osmolarity of interstitial fluid increases and allows for H2O to flow out of the descending loop of henle (and for Na and Cl to come in)

NKCC (Ion Channel)

Moves two Cl- ions with an Na+ and K+

Lives in thick ascending loop of Henle

Vasa Recta

Blood vessels that are in between the ascending and descending Loop of Henle

What does the Vasa Recta do?

Provides a counter current to the Loop of Henle

What impacts medullary thickness?

Thickness increases in species that live where water is scarce

Collecting Duct

• Permeability to water controlled by ADH

• Very permeable to Urea

• Controls how dilute urine is

Renal Clearance: what is this shit?

The volume of plasma completely cleared of a substance per unit time (Inulin, for our ppurposes)

Renal Clearance Equation

GFR = (V * U)/P

U is Inulin Concentration in Urine (mg/ml), V is rate of urine production (l/min), P is Inulin Concentration in Plasma (mg/l), GFR is glomerular filtration rate (l/min)

What happens is Clearance is smaller than GFR?

Substance is being reabsorbed into bloodstream

What happens if Clearance is greater than GFR?

Substance is being secreted in tubules

2 ways that Kidneys influence fluid volume and osmolarity

• Amount of water excreted

• Amount of Na excreted

What triggers release of ADH?

Increase in osmolarity

What triggers decreased ADH release?

Atrial stretch receptors, increased blood volume and pressure

What does ADH do?

Signals the body to make a smaller volume of urine

What is another name for ADH?

Vasopressin

What receptor does ADH bind to?

A good ol’ GPCR

Renin

Enzyme produced by kidneys that is released in response to low blood pressure and signals the body to raise it

Aldosterone

Steroid that prompts the kidneys to reabsorb sodium and water

What does Angiotensin II do?

Increases ADH secretion and stimulates thirst