Viewed under the light microscope it has a striped appearance made up of multinucleate cells known as muscle fibres
Surrounding the muscle fibre is a plasma membrane called the sarcolemma
Each fibre is made up of many myofibrils running parallel to one another
Surrounding each myofibril is a system of membranes called the sarcoplasmic reticulum and between the closely packed myofibrils are many mitochondria
Myofibrils are made up of repeating subunits called sarcomeres, which produce the striped appearance of a muscle fibre and are responsible for muscle contraction
The ends of a sarcomere are called the Z lines
There are two types of filament that form the striped pattern of a muscle - actin and myosin
Although the actin and myosin filaments do not change in length when a muscle contracts, the appearance of the banding patterns in the sarcomere is changed
Role of loop of Henle, medulla, and collecting duct
1. The descending limb of the loop is permeable to water but impermeable to salts, while the ascending limb is impermeable to water but allows salt to be passed out
2. This creates a hypertonic environment in the medulla, causing water to leave the filtrate by osmosis and enter the surrounding blood capillaries
3. The length of the loop of Henle is related to an animal's need to conserve water, with terrestrial animals having a longer loop
1. When filtrate enters the distal convoluted tubule from the loop of Henle, it is hypotonic
2. The wall of the distal convoluted tubule and collecting duct can have low permeability to water, allowing less water to be reabsorbed if the blood solute concentration is too low
3. ADH increases the permeability of these regions, allowing more water to be reabsorbed and the blood solute concentration to be maintained
When filtrate enters the distal convoluted tubule from the loop of Henle, it's solute concentration is lower than that of normal body fluids - it is hypotonic
If the solute concentration of the blood is too low, relatively little water is reabsorbed as the filtrate passes on through the distal convoluted tubule and the collecting duct
If the solute concentration of the blood is too high, the hypothalamus of the brain detects this and causes the pituitary gland to secrete a hormone - antidiuretic hormone or ADH
ADH causes the walls of the distal convoluted tubule and collecting duct to become much more permeable to water, and most of the water in the filtrate is reabsorbed