Excretion

Excretion: It is the removal of metabolic waste products from the body. (Note: not to be confused with secretion)

Excretion in Plants:

Homeostatic role in excretory system- temperature regulation.

Ectotherms: depend on external environment for temperature regulation.

Endotherms: generate their own heat from metabolic reactions.

The Skin:

Structure:

1. Epidermis: Surface layer composed of dead cells with a melanin pigment.
2. Dermis tissue: Connective tissue to contain and support skin structures e.g. sweat glands, nerve and thermal receptors.

Functions:

1. Protection:

Epidermis: protects from UV rays (due to melanin pigment) and general barrier to pathogens.

Sebum oil: acts as a surface disinfectant.

2. Makes Vitamin D
3. Energy Storage in fat tissue
4. Excretion by sweating
5. Temperature regulation:

In cold climate:

• Hair stands on skin due to erector muscle contraction- Piloerection. Holds warm air around skin.
• Blood vessel contraction- Vasoconstriction.
• Shivering.

In hot climate:

• Hair lies flat.
• Sweating.
• Blood vessels come close to surface. This causes blushing.

Organs of Excretion:

The excretory system:

The kidneys:

Functions:

• Excretion
• Osmoregulation (water and salt regulation)
• pH control

Urine Production in the Nephron: (This is a basic analysis of the nephron system)

1. Blood enters the afferent arteriole into the glomerulus.
2. Filtration due to blood pressure forces water and waste products out of blood. Filtration occurs in the Cortex of Kidney.
3. Useful substances are reabsorbed and waste is transported through Bowman’s capsule.
4. Most water in filtrate is diffused out of convoluted tubule into capillaries. Glucose and other minerals are absorbed by active transport.
5. The ascending limb of the loop of Henle absorbs most salt. Reabsorption of materials occurs in the Medulla and the Cortex.
6. The distal convoluted tubule adjusts the filtrate’s pH level before it is expelled into the collecting duct.

Kidney control of urea concentration:

• If salt level are too high or cells are dehydrated the hormone ADH (Anti-diuretic hormone) triggers more absorption of water in the distal convoluted tubule and collecting ducts. This increases urea concentration in urine but the overall urine concentration is reduced.
•  If low salt levels exist then no ADH is made and volume of urine remains constant or increased.

The Bladder:

Urine is the carried from both kidneys by the ureters to the bladder.

Function: It is used in the storage of urine from the Kidneys. Urine is then transported from the bladder by a tube called the urethra.