The Excretory

System Biology 20

Introduction

Follow along on page 376

What dangers exist if your body is unable to regulate

the fluid balance of your tissues?

What challenged would the body have to respond to if

the kidneys failed to work?

Explain how the circulatory system and excretory

system interact during exercise.

The Excretory System

The excretory system regulates the chemical composition of

body fluids by removing metabolic wastes and retaining the

proper amounts of water, salts, and nutrients.

The primary functions of the excretory system is to

Get rid of wastes

Eliminate useless by-products excreted from cells

Eradicate harmful chemical build-ups

Maintain a steady, balanced chemical concentration

The human excretory system includes the lungs, skin,

liver and kidneys.

The lungs are responsible for excreting the waste

product of cellular respiration – carbon dioxide.

The liver converts many toxic substances into less

harmful substances to be eliminated by the kidneys.

What happens when we eat too

much meat?

Nitrogenous wastes are formed in the body when amino

groups (NH2) are removed from amino acids during

protein metabolism in a process called deamination.

Amino groups (NH2) combine with hydrogen ions to form

ammonia (NH3), which is very toxic.

Ammonia is combined with carbon dioxide in the liver to

create urea, which the body can tolerate in higher

concentrations.

Urea is then eliminated from the body via the kidneys

(and a small amount from the skin).

The breakdown of nucleic acids, such as DNA and RNA,

also produces nitrogenous waste, which is converted by

the liver into uric acid.

Uric acid is also eliminated by the kidneys.

The Urinary System Urinary System - kidneys filter the blood to form urine,

which is excess water, salt, urea and uric acid.

The Nephron

The functional unit of the kidneys is the nephron.

Nephrons are long, slender tubes where urea and uric

acid is filtered out of the blood, along with excess salts,

glucose and water.

Afferent arterioles from the renal artery supply the nephrons with the blood to be filtered.

These arterioles form a special kind of capillary bed, called a glomerulus, where the blood is filtered.

Blood does not leave this capillary bed via venules, but through other arterioles, called efferent arterioles.

These arterioles form a second capillary bed, called peritubular capillaries, that wrap around the nephron.

This time blood leaves the capillaries via venules and leave the kidney through the renal vein.

The glomerulus is surrounded by part of the nephron,

called the Bowman’s capsule, in the cortex of the

kidney.

Fluid containing urea, uric acid, salts, glucose and

water leaves the capillaries of the glomerulus and

enters the Bowman’s capsule.

This process is called filtration.

This fluid leaves the Bowman’s capsule via the proximal

tubule where nutrients are reabsorbed from the filtrate

back into the blood.

Salt is actively transported out of the filtrate into the cortex of the kidney and water follows due to osmotic pressure.

Ammonia in the blood diffuses into the filtrate in the proximal tubule in a process called secretion. Excess hydrogen ions and potassium ions are secreted here as well.

Bicarbonate ions are reabsorbed to help maintain blood pH.

The filtrate then travels down the loop of Henle into the medulla of the kidney.

Loop of Henle

Filtrate from the glomerulus passes down the

descending arm of the loop of Henle.

Because the surrounding tissue in the outer medulla has

a high concentration of salt ions, water passes out of

the filtrate by osmosis.

This keeps you from losing too much water; a process

called osmoregulation.

The filtrate is now very concentrated.

As the concentrated filtrate passes up the ascending

arm of the loop of Henle, salts diffuse out of the tubule

into the medulla.

The filtrate then travels through the distal tubule in

the cortex of the kidney.

More salt and water is reabsorbed and excess hydrogen

and potassium ions are secreted.

By the end of the distal tubule, the filtrate is now

called urine, which empties into a collecting duct.

The collecting duct passes through the medulla of the

kidney and more water is reabsorbed by osmosis.

All of the collecting ducts merge in the renal pelvis of

the kidney and the urine leaves the kidney via a ureter

and is transported to the bladder.

Stretch receptors in the bladder cause urinary

sphincters to relax and urine passes out of the body via

a urethra.

Anatomy of the Urinary System

pH Balance

Kidneys maintain the pH balance in the body. ( 7.3 to

7.5)

Carbonic Acid is produced in cellular respiration which

ionized to produce H+ ions in the blood.

H+ ions are buffered by bicarbonate ions, preventing a

change in pH. This reaction produces carbon dioxide and

water.

The kidneys then restore the buffer and release

bicarbonate back into the blood stream, and excrete

the H+ ions.

Hormonal Control of Urine

Production

Aldosterone increases the reabsorption of Na+ ions in

the kidneys, thereby increasing water reabsorption via

osmosis. This increases blood volume, and therefore,

increases blood pressure.

Antidiuretic hormone (ADH), also called vasopressin,

increases permeability of the distal tubule and

collecting duct to water thereby increasing water

reabsorption. It is released when the body is dehydrated

and causes the kidneys to conserve water, thus

concentrating the urine, and reducing urine volume.