The Excretory SystemCh. 44

By:

Theresa Aller, Stephanie Byun, Rachel Erickson, Gi-youn Lee

What is Homeostasis?

Homeostasis is the steady-state physiological condition of the body or internal balance.  homeostasis is a dynamic state, an interplay between outside factors that tend to change the internal environment and internal control mechanisms that oppose such changes.

Regulators & Internal Conditions

Regulators use internal control mechanisms to moderate internal change in the face of external fluctuations.  An example is the freshwater fish who are able to maintain a stable internal concentration of solutes in blood even though the concentration is different from the water it lives in.

Conformers & Changing Conditions

Conformers adapt to changing conditions by allowing its internal condition to vary with certain external changes.

Heat Exchange

All organisms, including ectotherms and endotherms exchange heat through four processes: conduction, convection, radiation, and evaporation.

Note: Heat is always transferred from a higher temperature object to one of lower temperature.

Conduction

The direct transfer of thermal motion (heat) between molecules of

objects in direct contact with each

other (ex: lizard on hot rock).

Conduction

Convection

The transfer of heat by movement of air or liquid past a surface

(breeze contributes to heat loss from a

lizard’s skin).

Convection

Radiation

The emission of electromagnetic waves by all objects warmer than absolute zero.  Radiation can transfer heat between objects that are not in direct contact (lizard absorbing heat from sun).

Radiation

Evaporation

The removal of heat from the surface of a liquid that is losing some of its molecules as gas.  Evaporation of water from a lizard's moist surfaces that are exposed to the environment has a strong cooling effect.

Evaporation

Circulation & Heat Exchange

Circulation aids in heat exchange through two different ways: vasodilation and vasoconstriction.

Organisms can also trap heat within their body arranging their blood vessels into what is called a circulatory heat exchanger.

Vasodilation

An increase in the diameter of superficial blood vessels (near body surface) triggered by nerve cells that relax the muscles of the vessel wall.  It warms the skin, increasing the transfer of body heat to a cool environment by radiation, convection, conduction, and evaporation.

Vasoconstriction

The reverse process of vasodilation, it reduces blood flow and heat transfer by decreasing the diameter of superficial vessels.

Countercurrent Heat Exchangers

A countercurrent heat exchanger traps heat in the body core, thus reducing the heat loss from the outer parts of the body. These animals are often submerged in cold water or in contact with ice or snow. Heat in the arterial blood coming form the body’s core is transferred directly to the venous blood instead of being lost in the environment.

How Ectotherms Maintain High Body Temperatures

Ectotherms generate relatively little metabolic heat and conform to the temperature of their surroundings. Conduction, which is the direct transfer of heat between molecules in direct contact with the organism, also helps. Convection, the transfer of heat by the movement of air or liquid past a surface, and radiation, transfer of heat between objects not in direct contact, helps as well.

The Body’s Feedback Mechanism

There is a set temperature for your body and a control center. If it is too hot, then the heating mechanism is turned off, which produces no heat and lowers your body temperature. In turn, if it is too cold, then your heating mechanism is turned on, which raises your body temperature up to the designated set temperature.

What is Osmoregulation?

Osmoregulation is the process in which animals regulate solute concentrations and balance the gain and loss of water.

For example, seagulls can deal with high salt in their diet by having salt excreting glands that help them get rid of excess salt inside their bodies. Secretory cells actively transport salt from the blood into the excretory tubules.

Influence of Nitrogen Wastes

Different nitrogenous wastes vary in their toxicity and energy costs. Ammonia is only tolerable in low concentrations, so the organism must have access to a lot of water. Urea requires energy to expend it from ammonia, which is 100,000 times more toxic. Uric acid is relatively non-toxic, but requires even more.

What is Excretion?

Excretion is the process of animals getting rid of the nitrogen-containing waste products of metabolism.

Key Functions of the Excretory Process

The excretory process is composed of 4 functions: filtration, reabsorption, secretion, and excretion.

Filtration

Filtration is the extraction of water and small solutes, including metabolic wastes from the body fluid into the excretory system.

Reabsorption

Reabsorption is the process of taking up sugar, vitamins, organic nutrients and water that have been filtered.

Secretion

Secretion is the discharge of wastes from the body fluid into the filtrate.

Excretion

Excretion is the disposal of nitrogen containing waste products of metabolism. 

Kidney & The Circulatory System

The nephron of the kidney is supplied with blood from the afferent arteriole that subdivides into the capillaries of the glomerulus, these capillaries then reform into the efferent arteriole which subdivides again, forming the peritubular capillaries which surround the loop of Henle.  The peritubular capillaries then feed back into the renal vein.  

Feedback Mechanisms

One of the main aspect of the mammalian kidney is to adjust the volume and osmolartiy of urine depending on the animal’s water and salt balance and the rate of urea production. Thus feedback mechanisms help the kidney to carry out these processes with a combination of nervous and hormonal controls.

ADH (Antidiuretic hormone)

ADH is released when the osmolarity of the blood rises above over a set point which increases the permeability of the epithelium to water.  The amount of ADH released is greatly reduced when large amounts of water are consumed in order to increase the osmolarity of the blood.

RAAS (Renin-angiotensin-aldosterone system)

A drop in blood pressure triggers renin release from the JGA (juxtaglomerular apparatus).  In turn, the rise in blood pressure and volume resulting from the actions of angiotensin II and aldosterone reduce the release of renin.

NAF (Atrial natriuretic factor)

The NAF counteracts with the RAAS system.  NAF released from the heart inhibits release of renin from the JGA, inhibits NaCl reabsorbtion by collecting ducts and reduces aldosterone release from adrenal glands, and all of these actions lower blood pressure and volume.  

Kidney Function

A filtrate composed of water, salts (including NaCl and others), HCO3

-, H+, urea, glucose, amino acids, and some drugs goes through the mammalian nephron and collecting duct through 5 steps. Through this process the filtrate becomes urine.

Proximal Tubule

In the proximal tubule, secretion of H+ and reabsorption of HCO3

-

occurs. Nutrients, such as glucose, amino acids, and K+ are transported and the volume and composition of the filtrate is altered.

Reabsorption of most of the salt from the filtrate occurs here. The salt diffuses into the cells of transfer epithelium during this process.

Descending Limb of Loop of Henle

Reabsorption of water occurs here as water is also lost to the interstitial fluid.

In the descending limb of the loop of Henle, the transport epithelium is not permeable to salt.

Ascending Limb of Loop of Henle

As the filtrate moves up the ascending limb, it becomes diluted because salt is lost without the loss of water through the increased osmolarity of the interstitial fluid of the medulla.

NaCl was concentrated in the descending limb, but as the filtrate moves up the ascending limb, the epithelium actively transports NaCl into the interstitial fluid.

Distal Tubule

The concentrations of K+ and NaCl is regulated in the distal tubules. pH regulation also occurs.

NaCl concentration is regulated by varying the amount of NaCl reabsorped from the filtrate.

Collecting Duct

Finally, NaCl is actively reabsorped as the collecting duct determines how much NaCl will be excreted in the urine.

NaCl is diffused out of the collecting duct along with urea.

Filtrate Concentration of Water

As water descends the tubules, it moves out of the duct through osmosis, which concentrates the solutes that are left behind in the filtrate.

Filtrate Concentration of NaCl

As NaCl ascends the tubules, it is diffused out allowing the interstitial fluid of the renal medulla to maintain high osmolarity.

Other Organs That Aid in Excretion

The organs  (in general) that aid in excretion are: posterior vena cava, renal artery/vein, aorta, kidney, ureter, urinary bladder, and urethra.

The End