Section 2 i) Summary

In plants, excretion happens primarily through gas exchange. Oxygen and carbon dioxide are excreted from photosynthesis and respiration respectively. Toxins within the plant will collect within a dying leaf so it can be removed from the plant.

In humans however, it is more complicated.
We excrete through our skin: sweat contains water, salts and urea. This is also important for osmoregulation and homeostasis.
We excrete through our lungs: we breathe out carbon dioxide and water vapour, which again aids osmoregulation.
And finally, we excrete through urination. Urine contains water, salts and urea.

Urine is excreted through this system. It is important to not get confused between the urethra and the ureter.

Urea, the key part of urine, is made in the liver. Excess amino acids are broken down in a process called deamination. The urea then enters the blood, which must now be cleaned as urea is a toxic chemical containing nitrogen. The blood is cleaned in the kidneys

The most important part is the nephron, the filtration unit. There are about 1.25 million units in each kidney. In the nephron, blood is passed through the glomerulus at a high pressure, forcing out smaller molecules: Urea, water, salt, and glucose.

The solution created is known as glomerular filtrate. It enters the Bowman's capsule, then the proximal convoluted tubule. This is where glucose is selectively reabsorbed using active transport. Water and mineral salts are also reabsorbed. The filtrate then enters the loop of Henle. In the first hald, just water is reabsorbed, and in the second just mineral salts. The distal convoluted tubule continues to reabsorb water and mineral salts. This is now urine, which is about 95% water and 2% urea, the rest being excess salts and nutrients. The urine travels down the collecting duct and ureter into the bladder, where it is stored until excreted.

Osmoregulation
The amount of water in the body is regulated by the hypothalamus which causes the pituitary gland to release ADH, anti-diuretic hormone.

Excess of water detected by hypothalamus > Pituitary gland stimulated to produce less ADH > less ADH travels through the blood into the nephron > Tubules become less permeable > Less water is reabsorbed > Normal water content level achieved

Deficit of water detected by hypothalamus > Pituitary gland stimulated to produce more ADH > more ADH travels through the blood to the nephron > Tubules become more permeable > More water is reabsorbed > Normal water content level achieved