• Homeostasis is the process of maintaining a constant internal environment despite changing external conditions
• Ensures that the organs of the body operate within their normal limits thereby working efficiently
• Based on a feedback system
• Information is sent to a control center which directs the organ to react accordingly

Two major control centers:

1. Brain of the nervous system
2. Glands of the endocrine system

Several factors must be regulated including:

1. Temperature
2. Heart rate
3. Respiratory rate
4. Blood pressure
5. Water balance
6. Blood sugar levels
7. Detoxification

4.2   Homeostasis, the Excretory System

• Excretion removes harmful wastes from the organism

Three systems important in excretion:

1. Respiratory
• Lungs remove CO₂ and water
2. Integumentary
• Blood vessels in the skin dilate to allow more heat to escape
• Sweat glands produce sweat (urea, salts and water) that draws heat from the body to evaporate it
3. Urinary
• Kidneys remove urine that is a combination of urea, salts, and water

Two major functions:

1. Elimination
• Removal of harmful wastes from the body
2. Regulation
• Control and balance of several substances in the blood including water, acids and bases (pH), ions and salts

The major organs of the system include:

• 2 kidneys Ü filters blood, producing urine
• 2 ureters Ü carry urine to urinary bladder
• 1 urinary bladder Ü stores urine
• 1 urethra Ü carries urine to the outside of the body

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Associated blood vessels:

• renal artery Ü carries contaminated blood into the kidneys
• renal vein Ü carries purified blood from the kidney, placing the blood back into circulation

• main excretory organs of the body
• found on the back wall of the abdomen on either side of the spine just below the ribs
• basic filtering unit inside the kidney is the nephron unit
• each kidney contains 1 – 1.25 million nephron units
• parts of the nephron unit are found in both the cortex and medulla of the kidney

Three major sections of the kidney:

1. Cortex
• Outer part of the kidney
• Contains the glomerulus, Bowman’s capsule and blood supply of the nephron unit
2. Medulla
• Middle of the kidney
• Contains the loop of Henle and collecting duct of the nephron unit
3. Pelvis
• Inner cavity of the kidney
• Area where the collecting ducts of all the nephron units terminate



 

Nephron Function

The nephron unit carries out its filtering of the blood in two stages:

1. Filtration
• Blood leaves the renal artery and enters the glomerulus
• Blood pressure forces the plasma of the blood, containing both waste material and useful material, into the Bowman’s capsule
• This material is called filtrate
• Filtrate contains such things as:

water	glucose	sodium
urea	amino acids	potassium
salt	vitamins	uric acid

2. Reabsorption
• Useful materials contained within the filtrate are reabsorbed by the capillary network that surrounds the renal tubule
• Reabsorption of water occurs by osmosis
• Reabsorption of glucose, sodium, amino acids, etc occurs by active transport
• Fluid that remains in the renal tubule after reabsorbtion is urine
• Path of urine: collecting duct Ü pelvis Ü ureter Ü urinary bladder Ü urethra



The Kidney and Homeostasis

• The kidney maintains homeostasis by regulating the following mechanisms:

1. Water balance
• Hypothalamus of the brain directs the pituitary gland of the endocrine system to control levels of the hormone vasopressin or andtidiuretic hormone (ADH) in the blood
• This hormone travels through the blood to the kidneys where it directs the rate of water reabsorption
• Increased vasopressin Ü increased water reabsorption
• Decreased vasopressin Ü decreased water reabsorption
2. Blood pressure
• Blood volume is regulated by the hormone aldosterone
• Aldosterone affects the rate of sodium ion reabsorption which in turn affects the rate of water reabsorption
• Increased aldosterone Ü increased water reabsorption Ü higher blood pressure
• Decreased aldosterone Ü decreased water reabsorption Ü lower blood pressure
3. Blood composition
• Useful materials contained in the filtrate (glucose, amino acids, etc) are reabsorbed until the threshold level of the given material is reached
• Defined as the maximum amount of material that can be moved across the nephron back into the blood
• When the threshold is reached, the remaining material is removed as waste e.g. glucose in diabetes
4. Blood pH
• Should be close to 7
• The kidney removes excessively acidic or basic substances from the blood
5. Waste removal
• Urea is the major waste product removed from the kidneys
• Placed in the blood by the liver through the detoxification process
• Excess amino acids are broken down in the liver
• Converted to ammonia and then to urea

• Largest internal organ in the body
• Located high and to the right in the abdomen
• Gall bladder and common bile duct are associated with the liver
• Carries out three main functions associated with homeostasis:
1. Warehouse
• Stores excess glucose in the form of glycogen and vitamins A, B, and D
2. Factory
• Produces valuable substances including:
1. Bile for digestion (stored in the gall bladder)
2. Fibrinogen for blood clotting
3. Plasma proteins
4. Protein synthesis (making proteins)
5. Fat and carbohydrate metabolism
3. Detoxification center
• Destroys worn out RBCs, parts of which are used for bile production
• Breaks down excess amino acids
• Removes bacterium, certain drugs, and hormones from the blood and inactivates or changes them to a less poisonous form (e.g. alcohol).

1. Cirrhosis
   • Caused by overloading the liver with toxins such as alcohol
   • Liver cells die and are replaced by fat and connective tissue which are not able to carry out normal liver functions

2. Jaundice
   • Bile is not excreted properly so that it is reabsorbed by the blood
   • Skin and whites of the eyes turn yellow

3. Hepatitis
• Produced by a viral infection. Two major forms:
1. Hepatitis A or infectious hepatitis
• Spread in unsanitary food or water
2. Hepatitis B or serum hepatitis
• Spread by contaminated blood supplies, unsterile needles or contaminated objects that puncture the skin

• Causes moderate liver damage, fever, and jaundice