Excretory System: Kidneys and Nephrons

Excretion

Process in which metabolic wastes are filtered out by the blood and removed from the body as urine.

Kidneys

• Purpose

• Distinctions in the parts

The kidneys are responsible for the aforementioned filtration of the blood and the preparation of a urine product for excretion

Key anatomical distinctions between the kidney cortex and the kidney medulla.

3 main Kidney Functions

• Regulation of blood pressure

• Regulation of pH

• Stimulate the generation of red blood cells.

(liver breaks them down, and the epiphyseal are of bones makes them [erythrocytes])

What is a nephron

Functional Unit of the kidney

Nephron structures

Main structures

• Glomerulus

• Proximal convoluted tubule

• Loop of Henle

• Distal convoluted tubule

• Collecting duct

Structures in the Renal Corpuscle

• Glomerulus

• Bowman’s capsule

Bolded are the most important ones

• Afferent Arteriole

• Podocytes

• Efferent Arteriole

Renal Corpuscle

Renal Corpuscle

• The first structure of the nephron that contains the glomerulus and the bowman’s capsule.

  • Site for filtration within the renal cortex.

RENAL CORTEX

Bowman’s Capsule

First step of filtration, filters blodo

Glomerulus

site inside the renal corpuscle that makes up the filtering network of arterioles. BLOOD COMES IN (afferent arteriole), and we see the secretion of filtrate into the bowman’s capsule and into the system followed then BLOOD LEAVING (through the efferent arteriole)

IN THE RENAL CORTEX

Afferent Arteriole

Blood vasculature that enters the renal corpuscle

• Provides nutrient-rich blood for filtration

Podocytes

Foot Like Processes that form a fenestrated filtration barrier

• Used for filtration specificity.

• Makes sure the right things come in (no rbc’s etc)

Efferent Arteriole

Blood vasculature that exits the renal corpuscle

• Can form the peritubular capillaries or the vasa recta

Peritubular capillaries

Blood vessels surrounding the convoluted tubules

They are more toward the renal cortex

Vasa recta

Blood vessels surrounding the loop of Henle

More toward the renal medulla.

Proximal Convoluted Tubule

• What is secreted?

• What is absorbed?

• What makes it special?

Site of the majority of reabsorption

There is reabsorption via active transport.

Reabsorbed:

• Na+, Cl-

• 100% of reabsorbed glucose and amino acids

Secreted:

● H+

Cotransport occurs here

Loop of Henle

• Def

• Nephron tube that descends into the kidney medulla and exists between the convoluted tubules

Descending Loop of Henle

Descends into the medulla from the proximal convoluted tubule.

• Permeable to water

• Water flows out of the urine, into the blood

• Urine concentration increases

INCREASES CONCENTRATION

Ascending Limb

Ascending limb: Ascends out of the medulla, connecting to the DCT

• Impermeable to water

• Ions and salts flow out of the urine, into the blood

• Urine concentration decreases

DESCREASES CONCENTRATION

(water cannot leave)

Distal Convoluted Tubule

• Reabsorption of Na+ and Cl-

• Secretion of K+ and H+

  • Water passively follows reabsorbed salts

  • (water leaves the tubule)

Collecting Duct

Bulk passive reabsorption of water, secretion of K+

• Urine concentration increases

Has a lot of aquaporins

Pathways of the urine AFTER nephron

Collecting Duct → renal pelvis → Ureter → Bladder → Urethra

Excretory Hormones

• Def

Excretory hormones are capable of manipulating blood volume and blood

pressure as well as the relative concentrations of molecules and substrates within the blood and the filtrate

Renin - Angiotensin - Aldosterone system (RAS)

A system of hormones, tissues, and organs which together are capable of increasing blood pressure in response to detected low blood pressure

Renin - Angiotensin - Aldosterone System

• Key Players

Angiotensinogen

Renin

Angiotensin I

Angiotensin II

Aldosterone

Juxtaglomerular cells

• Function

• What do they release

Juxtaglomerular cells (in the liver): Modified smooth muscle cells associated with the afferent arteriole

• Capable of detecting changes in blood pressure and blood volume

• Release renin in times of low blood pressure

Angiotensinogen

First step in the Renin - Angiotensin - Aldosterone System

Released from the liver

• Gets activated by renin

Renin

An enzyme capable of converting angiotensinogen into its activated form, angiotensin I

• Can also be triggered by the fight or flight response

• Can be released by low bp or blood volume

Angiotensin Converting enzyme (ACE)

An enzyme that is capable of converting angiotensin I → angiotensin II

• Released from the walls of blood vessels in the lungs..

Angiotensin II

Increases blood pressure and volume (is created when the lungs release ACE)

1. Stimulate additional aldosterone release

2. Increase Na+ reabsorption at the proximal tubules

3. Systemic vasoconstriction

4. Increase thirst, increasing liquid intake

Aldosterone

• Where is it from

• How does it act

• How does it affect them?

Mineralocorticoid released from the adrenal gland (cortex) that affects the kidney

Specifically, it acts on the distal tubules and collecting ducts of nephrons

Increases water and sodium reabsorption

Increases potassium secretion

Renin-Angiotensin-Aldosterone System (RAS)

• ALL STEPS

Juxtaglomerular cells sense changes in blood pressure and blood volume.

• Angiotensinogen is released from the liver. It floats around in the blood stream.

• Renin is released from kidneys due to a fight or flight response or low blood pressure (juxtaglomerular cell in kidney). Makes Angiotensinogen into angiotensin I

• Angiotensin converting enzyme (ACE): An enzyme that is capable of converting angiotensin I into angiotensin II

• Angiotesnin II increases blood pressure and volume

  • Stimulates additional aldosterone release

  • Increases Na+ reabsorption at the PROXIMAL tubules (more pressure)

  • Systemic Vasoconstriction

  • Increase thirst, increasing liquid intake.

Aldosterone is released, which increases water and sodium reabsorption and increases potassium secretion

Antidiuretic Hormone (ADH or Vasopresin)

Produced in the hypothalamus

Stored and released by the posterior pituitary gland

Capable of increasing blood volume and pressure

Effect:

• Increases aquaporin channel presence within the collecting ducts

• Increases water absorption

What inhibits ADH

Alcohol: Inhibitor of ADH

• Less water is reabsorbed, causing increased urination

• Why it is easy to get dehydrated when drinking.

Atrial Natriuretic Peptide (ANP)

Produced by the cells of the heart atrial cells in response to atrial distension caused by increased blood volume and pressure

Means of reducing blood volume and pressure:

• Increasing the glomerular filtration rate (GFR)

• Decreasing sodium reabsorption

• Increasing sodium excretion

• Inhibiting renin and the renin-angiotensin-aldosterone system

Osmoregulation

The maintenance of internal water balance by an organism

• Achieved via absorption and secretion of water and solutes

• Affected by water and solute concentrations of the surrounding environment

Hypotonic Solution

Swollen Cell

Isotonic Solution

Normal Cell

Hypertonic Solution

Shriveled Cell

Marine (saltwater fish) and how they undergo osmoregulation

• Hypotonic to their environment

  • Environment has higher solute concentration

• Passive water loss to environment

Maintenance adaptations:

1. Constant drinking

2. Rare urination

3. Secretion of salt through the gills

Freshwater Fish

• Hypertonic to their environment

  • Environment has a lower solute concentration

• Passive water gain from environment

Maintenance adaptations:

1. Rarely drink water

2. Constant urination

3. Absorption of salt through the gills

Small Substanse filtration in nephrons

Afferent Arteriole → Glomerulus → Bowmans capsule.

What is fluid and substances filtered from the blood and bowmans capsule called?

Filtrate

Four main processes in nephron

Filtratio

Reabsorption

Secretion

Excretion

What passes the bowman’s capsule and what is stopped

Passes:

• Urea

• Sodium

• Water

• Calcium

Stops:

• Protein

• RBC’s

What kind of pressure causes bowman’s capsules to move filtrate in?

Hydrostatic Pressure.

Aldosterone increases potassium secretion