Excretory System (Exam 3)

nephron

• the functional unit of the kidney

• the basic functional unit

Filtrate is produced then

modified by secreting or extracting molecules in/out before excretion

The 3 steps of urine formation in the kidneys

1. Glomerular Filtration

2. Tubular Reabsorption

3. Tubular Secretion

The renal corpuscle is made up of:

• the Glomerulus* (‘ball of yarn’ - small fenestrated capillaries)

• Bowman’s Capsule (container surrounding glomerulus to collect filtrate)

the Glomerulus

• ‘ball of yarn’ - small fenestrated capillaries

• Glomus in cyclostomes

Bowman’s Capsule

container surrounding glomerulus to collect filtrate

In the renal corpuscle, the filtrate fluid and solutes…

leave the blood and enter Bowman’s capsule

What type of capillaries are in the glomerulus?

Fenestrated

Podocytes

• (‘foot’- ‘cells’) make up the inner lining of the Bowman’s capsule

• They surround the glomerular vessels and limit the flow out of the vessels but allow water, glucose, and salts to pass

1. Glomerular Filtration

• Filtrate is produced and drained by the Bowman’s capsule

• It should be free of proteins and cells (strained by the podocytes) and full of waste and salts

  • This includes molecules that we don’t want to urinate out!

• We pull everything out, then put back in the things we want to keep

During glomerular filtration, We pull everything out, then put back in the things we want to keep, which is why….

This is why it’s filtrate and not called urine yet

In addition to podocytes, we control how leaky the glomerulus is by

altering the Hydrostatic and Osmotic Pressures

The flow is almost always into…

the lumen of the glomerulus, but the volume is what changes based on the net of applied forces

Osmotic flow =

Net Hydrostatic Pressure + Net osmotic pressure

Lower Glomerular Filtration Rate

Lower GFR (<60) indicates chronic kidney disease (CKD) or failure

Higher Glomerular Filtration Rate

high GFR (>120-130) often indicates glomerular hyperfiltration, a state of over-filtration commonly seen in early diabetes or obesity

GFR

glomerular filtration rate measures how well kidneys filter blood

Steps of Excretion

1. Glomerular Filtration

2. Tubular Reabsorption

2. Tubular Reabsorption

• The filtrate is drained into the proximal convoluted tubule

• It still contains a lot of salt, amino acids, glucose, etc. that we want to hold on to so we Reabsorb them back out of the nephron

We reabsorb important ions back….

into the bloodstream throughout the length of the nephron tube (PCT, LoH, + DCT)

Not all sections reabsorb the same things however

Through both active and passive transport, we pull a huge amount back out of the filtrate (humans produce 180 L of filtrate per day, only have 5 liters of blood)

Tubular reabsorption relies on

active transport creating a concentration gradient that then allows other solutes to move

Tubular Reabsorption Steps

1. Active pumps push sodium out of the cell into areas around tubules, creating a low sodium concentration within the walls

2. This concentration gradient is used to co-transport sodium with other solutes we don’t want to excrete

3. Water passively diffuses through aquaporins (H20 channels) following the strong osmotic gradient produced by the active transport

4. Some other solutes (Cl-, Ca+2, and urea) and lipid soluble

   molecules passively diffuse following their own concentration gradients

Tubular Reabsorption First Step

Tubular Reabsorption Second Step

Tubular Reabsorption Third Step

Tubular Reabsorption Fourth Step

THC (active ingredient in marijuana) and many other recreational drugs are

lipid soluble, explaining why urine tests will be positive long after last ingestion

Urea and Glucose (Tubular Reabsorption)

What does the kidneys start as? (Embryology)

Starts as Intermediate Mesoderm

• Differentiates into the pronephros (Head Kidney) before

  changing in derived groups

All three major structures of the kidney arise from

the nephrogenic cord/ridge that forms in early development

The evolutionary and embryonic development of the vertebrae kidney

Pronephron —> Mesonephron —> Metanephron

Pronephros Region

• In all groups, the anterior pronephros develops but is retained in only cyclostomes (and some fish larva)

• Mostly transient in derived groups

Mesonephros Region

The longer kidney tissue develops many more numerous and complex renal corpuscles

Metanephros Region

• Amniote kidney develops as outpocketing of nephric duct

• Mesonephric structures sometimes link with gonads

Kidney Types

• Mesonephric Kidney

• Opisthonephric Kidney

• Metanephric Kidney

Mesonephric Kidney

Formed from both the pronephros (non-functional for excretion) and mesonephros tissue

Opisthonephric Kidney

• ‘Mesonephric but longer’

• Pronephros mostly gone

• Mesonephros significantly expands posteriorly (more excretory capacity)

Opisthonephric Kidney (males)

• Males dump sperm into mesonephric ducts (aka Wolffian ducts) which also drain urinary waste

• A true ‘urogenital system’

Opisthonephric Kidney (females)

In females the Mullerian ducts carry egg gametes

Metanephric Kidney

• All of the mesonephric tissue loses excretory function and disappears

• The superficial cortex and deep medulla differ in:

  • salt content

  • Structures present

• How far the intermediate segment of the nephron descends into the saltier medulla of kidney is entirely influenced by salt concentration requirements from environment

The superficial cortex and deep medulla differ in:

• salt content

• Structures present

How far the intermediate segment of the nephron descends into the saltier medulla of kidney is entirely influenced by

salt concentration requirements from environment

Metanephric Kidney Drawling

Loop of Henle Structure

The Loop of Henle is the intermediate portion which produces concentrated urine and is only present in Birds and Mammals (convergently derived)

ADH

• Anti-Diuretic Hormone

  • It increases urine saltiness

  • It decreases urine volume

Sharks

• Chondrichthyes use rectal glands to excrete excess salt

• Can modulate blood flow to organ to move into more freshwater systems

• They also use trimethylamine-N-oxide (TMAO) to increase body salinity and avoid water loss

trimethylamine-N-oxide (TMAO)

Sharks use trimethylamine-N-oxide (TMAO) to increase body salinity and avoid water loss

Birds

• Some animals, like the Red Lark, are so good at conserving water that they don’t ever drink

• Some drink saltwater and use orbital salt glands to remove excess salinity

Mammals

Kangaroo rats also don’t drink, but also use respiratory system

modifications to conserve water