Regulating The Internal Environment

Ch. 44

The Excretory System

Osmoregulation: management of the body’s water content & solute composition Controlled by movements of solutes between internal

fluids and the environment

Examples: Saltwater fish: Hypoosmotic to seawater, so it loses water

by osmosis. Balances water loss by constantly drinking water & rarely urinating. Gills/skin give off salt.

Freshwater fish: Hyperosmotic to its environment, so constantly gains water. Balances water gain by rarely drinking & excreting lots of urine. Lost salt is replenished by food and salt intake in gills.

Overview

Filtration: Water & solutes forced by blood pressure across selectively permeable membranes of a capillary cluster into excretory tubule

Reabsorption: epithelium reclaims valuable substances from filtrate

Secretion: other substances like toxins and excess ions are extracted from body fluids and added to excretory tubule

Examples

Excretory systems help maintain HOMEOSTASIS!!!!! by regulating water balance & removing waste

Different organisms have various excretory mechanisms

Contractile vacuoles: Found in protists. The vacuoles obtain water, merge with the plasma membrane, release water to the environment

Flame bulbs: Found in flatworms (planaria). Flame bulbs found along branching tube system in flatworm. Body fluid filters across membrane, cilia sends fluid through tube system. Wastes excreted from nephridiopores.

Metanephridia:

tubular excretory system found in most annelids, including earthworms.

Fluid enters the nephrostome, passes through the collecting tubule (which includes a bladder that opens to outside by nephridiopore)

Wastes remain in the fluid and are excreted Certain salts are pumped back into the blood Earthworm’s urine is very dilute, balancing the

osmotic uptake of water through the skin

Malpighian tubules:

Found in insects and other arthropods. Outfoldings of the digestive tract collect body fluids

from hemolymph that bathes the cells Fluid enters the midgut and passes down to the

hindgut with digested food Valuable materials are passed back out through the

walls of the digestive tract and are reabsorbed Waste continues down and exits through the anus This excretory system is highly effective in conserving

water and is a key adaptation for survival on land

The Mammalian Kidney

Mammals have a pair of kidneys Supplied with blood by renal artery & renal vein

Urine exits each kidney through a duct called the ureter The ureters drain into the urinary bladder and is

expelled through the urethra during urination

Kidney: consists of about a million nephrons Typically functions in both osmoregulation & excretion

Nephron: individual filtering tubes found in kidney The tube travels from the cortex down into the medulla,

back up to the cortex, then back down through the medulla, into the renal pelvis.

Filtration

Glomerulus: ball of capillaries where filtration begins Bowman’s capsule: cup-shaped swelling that surrounds

the glomerulus.

Filtration starts as blood pressure forces fluid from the blood in the glomerulus into the Bowman’s capsule.

The filtrate then passes through 3 regions Proximal tubule Loop of Henle (has a descending & ascending limb) Distal tubule

The distal tubule empties into collecting duct, which empties into the renal pelvis, which is drained by ureter

Hormones

Antidiuretic Hormone (ADH) Important in regulation water balance Increases reabsorption of water in the body Increases concentration of salts in urine Increases permeability of collecting duct to water

More water diffuses out of the collecting duct

Aldosterone Acts on nephrons’ distal tubules, causing more

reabsorption of sodium & water Increases permeability of distal tubules to sodium

More sodium diffuses out, water passively follows (why?)

Pg. 950

Adaptations

Marine animals expel ammonia (NH3) directly into water

Birds, insects, reptiles convert urea uric acid Uric acid is nearly insoluble in water Allows water conservation since nitrogen waste is given

off as solid

Mammals convert ammonia urea Conversion occurs in liver Urea is less toxic than ammonia

This makes it require less water to expel in urineThus water conservation