NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 42

THE INTERNAL ENVIRONMENT

Remember our definition of homeostasis

Homeostasis - tendency of life to try and maintain the structure and regulate the                                        internal environment.

One of the biggest and most frequent homeostatic challenges an animal faces comes from the digestive process.

The digestive system is unselective in what crosses the intestinal wall and enters the cells.

Because of this unselective absorption animals are often presented with qualitative and quantitative differences of materials entering the bloodstream.

         Excess (or to little): Mineral, especially salt

                                                 Water

                                                 Sugar

                                                 Protein

                                                 Fat

                                                 Vitamins

                                                 Alcohol and other drugs

The urinary or excretory system restores and maintains the proper internal regulation of these materials in the body despite differences in diet.

The major functions of the urinary system in mammals.

         Excretion of cellular waste products

         Regulation and maintenance of body fluid composition

                   Ions, water, nutrients

         Secretion of hormones

These functions occur simultaneously as the kidneys selectively filter the blood.

Water and nutrients are return to the blood.

Wastes (urea), toxins, excessive water and nutrients are collected and eliminated from the body as urine.

Urea is a compound that removes ammonia from the blood.

         Protein >> amino acids >>> simpler molecules + ammonia

         -Since ammonia is toxic it must be removed from the system.

         -The liver (detoxifies) converts the ammonia to urea.

         -The kidneys filter out the urea with other wastes.

Simple excretory systems

The flame cells in flatworms are the simplest excretory system.

         -network of tubes branching throughout the body

The tubes end at hollow bulbs called flame cells.

Water and dissolved wastes are filtered into the bulbs. The cilia of the flame cell direct the fluid through tubes where water and nutrients are absorbed from the solution and more wastes are added from body cells.

Eventually, the solution of concentrated wastes reach one of numerous pores in the skin and is released to the outside

The nephridia in earthworms and mollusks function as simple kidneys.

In the earthworm nearly every segment contains a pair of nephridia.

Wastes and nutrients collected from the blood and tissues fills the body cavity (coelom).

This coelomic fluid enters the nephridia through funnel shaped openings called nephrostomes, and passes through a narrow twisted tube where useful substances are reabsorbed into the blood stream.

Urine with the waste is stored in bladder-like portions of nephridia and excreted through excretory pores in the body wall.

Excretion in human and other vertebrates is accomplished by the kidneys and the urinary system (See fig 42.3)

The kidneys are complex organs that resemble a dense collection of nephridia.

         -paired, bean-shaped organs

         -about 5 x 3 x 1 inch

Unfiltered blood enters through renal arteries and filtered blood leaves through renal veins (See fig 42.4)

The ureter passes urine (by peristalsis) to the hollow muscular bladder where it is collected and stored until the bladder is emptied through the urethra.

“I’ve really got to go bad”

The retention of urine in the bladder is controlled by two sphincter muscles located at the base of the bladder just above the junction with the urethra.

Receptors in the walls of a distended bladder trigger reflexive contractions and the sphincter nearest the bladder opens involuntarily. The lower sphincter is under voluntary control.

The average adult bladder can hold about a pint of urine, but the need to go can be triggered by much smaller amounts. Stretched receptors sense bladder fullness.

Urination

         -both sphincter muscles at the bladder's base relax

         -the bladder contracts, forcing urine down the urethra

Human kidney structure.

The kidney has a solid outer layer, which contains over one million individual nephrons where urine forms.

This outer layer has two parts:

         -the cortex

         -the medulla

Inside is a hollow inner chamber called the renal pelvis that funnels the urine into the ureter.

The nephrons have 3 major parts: (See fig 42.4)

         -Glomerulus - a filter for blood

         -Bowman's capsule - collects the filtrate (fluid filtered from the blood)

         -A long, twisted tubule that is subdivided into three parts

                   1. proximal tubule

2. loop of Henle

3. distal tubule that leads to the collecting duct

What’s the idea behind this design?

Large surface area where nutrients are selectively reabsorbed from the filtrate back into the blood, while wastes and some water are left behind to form urine.

How this works: (See fig. 42.5)

1) Arterioles conduct blood to each nephron.

2) Within Bowman's capsule the arteriole subdivides into a capillary network called the glomerulus

3) The glomerulus capillary walls are permeable to water and dissolved substances

4) The watery filtrate, resembling blood plasma minus the proteins, is collected in Bowman's capsule for transport through the nephron

5) The blood leaving the glomerulus in the arteriole is much more concentrated, containing blood cells, proteins and fat droplets too big to be filtered out

6) These arteriole then branch into smaller highly porous capillaries that surround

          the tubule.

7) Tubular reabsorption occurs as the blood reabsorbs water and nutrients from the filtrate.

                   a. Reabsorption of salts and other nutrients is by active transport                    b. Water is reabsorbed by osmosis.

                   c. Wastes (urea) remain in the tubule and become concentrated                                                  as water leaves

8) Any waste materials remaining in the blood are actively secreted into the                              tubule

9) During its passage through the collecting duct, additional water leaves the          filtrate by osmosis until the urine reaches equilibrium with the highly

                    concentrated surrounding fluid

Kidneys as organs of homeostasis.

The kidneys filter a human's blood 350 times daily, fine-tuning blood composition. Kidney failure rapidly leads to death.

The kidneys also regulate the water balance:

Over 45 gallons of water daily enter Bowman's capsule. Most is reabsorbed.

This reabsorption is by osmosis (the water follows salt and nutrient reabsorption).

The amount of water reabsorbed and therefore how concentrated the urine becomes is regulated by antidiuretic hormone (ADH)

ADH (also called vasopressin) levels in the blood increases the permeability of the distal tubule and collecting duct to water. More water is reabsorbed.

ADH is made by the hypothalamus in response to receptor cells there (that monitor the concentration of the blood) and in the heart (monitor blood volume).

When the concentration of blood rises or blood volume falls, more ADH is released.

Drinking beer causes dilution of blood and increase in blood volume. Less ADH is released, less water is reabsorbed from the distal tubules and collecting ducts, the urine retains more water. The bladder fills faster and the urine is much less concentrated.