Excretion

Excretion

By Naomi Keddy

Homeostasis: is the maintenance of a constant external environment in an organism. Excretion helps homeostasis Example of excretion helping homeostasis is temperature regulation in animals. The control of temperature in the body is needed because temperature regulates the chemical reactions.

Ectotherms: gain or lose heat to their external environment.

Endotherms: generate their own heat from metabolic reactions.

Function of the skin:

Protection:

• Epidermis protects against damage.

• Melanin protects against ultraviolet radiations.

• Sebum helps keep the epidermis intact.

• Vitamin D is made in the skin

• Fats are store in the skin for heat and energy.

• Skin is used as sense organ for touch, pain or temperature

• Excretion sweat removes water and salt from the body.

Temperature regulation:

• Cold conditions cause:

1. Hairs to stand up to keep the skin warm.

2. Blood vessels become narrow to retain heat.

3. Shivering is movement to keep the body warm.

• Warm conditions cause:

1. Sweating, which cools the body due evaporating.

2. Blood vessels widen to lose heat.

 The role of the excretory system:

• Regulating body temperature.

• Controlling osmosis

• Controlling the concentration of body fluids

• Removing waste products.

Organs of Excretion:

• Lungs: excrete water and carbon dioxide.

• Skin: excrete water and salt.

• Kidneys: excrete water, salt and urea in the form of urine.

The Human excretory system:

The Kidneys:

The urinary system consists of two kidneys, two ureters, the bladder and the urethra.

The Kidneys are located just below the diaphragm.

The mains processes of the kidneys:

• Filtration: The incoming of the blood into the kidneys is filtered. This takes place in the outer cortex.

• Reabsorption: Some useful materials are taken back into the blood and this is called reabsorption. It occurs in the cortex and the medulla.

• Secretion: Some substances are secreted from the blood into the cortex such as potassium and hydrogen ions. 

The Functions of the kidneys:

1. Excretion: The kidneys remove waste products from the bloodstream and convert them to urine.
2. Water content: The kidneys control the water content by varying the water content of urine.
3. Salt concentration: The kidneys control the salt concentration of body fluids by varying the amount of salt released in the urine.
4. pH Control: The kidneys control the pH of the body fluids by producing urine that is either more or less acidic. 

The Nephron:

 The nephron carries out the function of the kidneys and it is located in the cortex and the medulla of the kidneys.

The processes of the nephron:

• Filtration means that water and small molecules pass from the blood to the nephron.

1. Blood entering the nephron in the afferent arteriole contains waster products.

2. Filtration takes place in the glomerulus. Small molecules such as glucose, amino acids, vitamins and minerals are forced out of the plasma into bowman’s capsule. 

• Reabsorption means that molecules pass from the nephron back into the blood. 

1. Most of the water is reabsorded by osmosis. Useful molecules such as glucose, amino acids and vitamins are reabsorbed by diffusion and active transport ( means it requires energy in the form of ATP)

2. The descending limb of the loop of Henle is permeable to water so small amount of water is reabsorbed by osmosis.

3. The ascending limp of the loop of Henle is permeable to salts. 

The hormone ADH:

• Is released from the pituitary gland.

• Controls the volume of urine formed.

• Is released when plasma has too little water or too much salt.

ADH Causes:

• Increased reabsorption of water in the distal tubule and the collecting ducts.

• A low volume of urine.

• ADH is not released when we drink sufficient water or consume a low salt diet.