V. Fluid Balance and Disposal of Metabolic Wastes

A. Functions of the Excretory System

1. Excrete Metabolic Wastes

2. Osmoregulation -maintaining fluid and ion balance

3. Reabsorption of important body fluid constituents

B. Metabolic Wastes -H2O, CO2, Nitrogenous Wastes

1. Ammonia -highly toxica) Least energy to produceb) Requires the most water to releasec) Excreted by freshwater organisms

2. Uric Acid -least toxica) Most energy to produceb) Least water to excretec) Excrete by insects, birds, snails & reptiles

3. Urea -less toxic than ammoniaa) Produced in the liver from ammonia in the urea cycleb) Excreted by amphibians and mammals

C. Osmoregulation in Vertebrates

1. Aquatic Marine (hypertonic environment) lose water and gain salt

a) Bony fish -drink water, gills excrete salt, low urine volume

B) Sharks -concentrate urea and salt in tissue. Hypertonic to ocean. Water enters gills. Excess Na+ excreted by rectal gland

C) Birds & Reptiles -salt glands in head. Excretes salt from water consumed

D) Whales & dolphins -diet high in protein and salt. Very concentrated urine

2. Aquatic, Freshwater -(hypotonic) gain water lose salt

a) Fish -mucus cover scales. Gills bring in salt. Large urine volume

b) Amphibians -skin brings in salt. Large urine volume

D. Human Excretory System1. Lungs - H20 CO2

2. Liver -produces urea

3. Sweat glands -5 to 10% of all metabolic wastes (urea, salt, & water)

4. Kidney -maintains chemical balance

E. Structure of the Kidney

1. Cortex -outer layer

2. Medulla -inner layer

3. Renal pelvis -collects urine on the medial concave side

4. Ureter -carries urine to the bladder

Structure of the Kidney

5. Urinary Bladder -storage of urine until micturation (urination)

6. Urethra -short tube in which urine travels from the bladder to the outside of the body

7. Nephron -more than a million in each kidney. Functional unit

a) Renal Corpuscle: arterioles and glomerulus

b) Bowman’s capsule, proximal convoluted tubule, Henle’s loop, distal convoluted tubule, and the collecting tubules

F. Urine Formation (96% H2O, 2.5% Urea, 1.5% Salts)

• Urine formation consists of 3 steps:

– Filtration

– Reabsorption

– Secretion

1. Filtration -the removal of materials from the blood into the Bowman’s Capsule

a) The renal artery carries blood to the afferent arteriole which branches into a group of capillaries called the glomerulus

b) All materials leave blood except proteins and blood cells. The filtrate enters the Bowman’s capsule

c) The efferent arteriole carries blood away from the glomerulus and is very narrow, thus increasing pressure

d) Filtrate contains glucose, amino acids, Na+, K+, Cl-urea, HCO3-

e) Filtration rate 45 gallons per 24 hrs

2. Reabsorption -selective return of

materials from filtrate back into the blood. a) Glucoses, amino acids and water are

absorbed back into the blood through capillaries covering the proximal convoluted tubule and Henle’s loop.

b) Also: vitamins, Na+, Cl-, HCO3- and K+

c) Some are recovered by diffusion and active transport. The rate is controlled by the amount of each in the blood.

d) 65% of filtrate is recovered from the PCT. Eventually, 99% of the filtrate is recovered

3. Secretion -release of materials from the blood into the filtrate selectively

a) Occurs in the Distal convoluted tubule where potassium ions, hydrogen ions, ammonia and some drugs are put back into filtrate

b) Hydrogen ion secretion adjusts blood pH.

G. Regulation of Urine Volume

1. When fluid intake is low, higher salt concentrations are found in the tissues

a) This increases the osmotic pressure in the tissues

b) The pressure is sensed by larger blood vessels and the hypothalamus

c) The hypothalamus will then produce ADH- antidiuretic hormone

2. Antidiuretic Hormone -increases the rate at which water is reabsorbed

H. Regulation of Body Temperature

1. Heat production can be increaseda) Shivering -increased contracting of skeletal muscle

generates heat through increased metabolismb) Nonshivering Thermogenesis -hormonal triggering of

increased metabolic rate

2. Adjusting the rate of heat exchange with the environment

a) Vasodilation -blood vessels near skin dilate increasing blood flow and allowing more heat exchange

b) Vasoconstriction -blood vessels near skin contract to reduce blood flow to prevent heat loss

c) Contraction of skin raises fur creating a layer of insulating air near the skin

3. Evaporative heat lossa) Sweat glands -secrete perspiration which

is vaporized thus cooling the body

b) Mammals lacking sweat glands may pant or use saliva and urine to cool themselves

4. Behavioral Responses: changing location or body position

a)Basking in the sun or moving to shade

b)Standing on one foot (minimize contact with hot sand)

c)Raising ears or gills to cool body

5. The Thermostat

a) The body’s thermostat is a group of nerve cells concentrated in the hypothalamus and are they are located in two areas

1) Heating center -controls vasoconstriction, erection of fur, shivering and nonshivering thermogenesis.

2) Cooling center -controls vasodilation and sweating

b) Nerve cells that sense temperature are in the skin, hypothalamus and other parts of the nervous system

1) Ruffini organs (warm temperature receptors) excite cooling center and inhibit the heating center

2) Bulbs of Krause (cold receptors) inhibits cooling center and excites heating center