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

Section 2 i) Key Words

ADH: Anti-diuretic hormone causes the tubules of the nephron to become more permeable and thus reabsorb more water.

Bladder: The organ where urine is stored before being expelled from the body through the urethra.

Bowman's capsule: The capsule encasing the glomerulus, receives the glomerular filtrate through ultrafiltration.

Convoluted tubule: The two convoluted tubules (proximal being the closer one to the glomerulus, distal being the further one) are where water, glucose, and salts are reabsorbed into the blood.

Glomerular filtrate: The substance created by ultrfiltration. Made of glucose, urea, water and salts.

Glomerulus: A series of capillaries under high pressure that forces small molecules through the walls.

Homeostasis: Maintaining constant internal conditions, e.g. temperature, water content, etc.

Hypothalamus: The part of the brain that detects water content and stimulates the pituitary gland.

Loop of Henle: A loop of tubule within the nephron where salts and water are reabsorbed. Separates the two convoluted tubules.

Nephron: The filtration unit within the kidney that removes waste products and cleans the blood.

Osmoregulation: Regulation of water content. The skin sweats, the kidneys remove and reabsorb water, and the lungs exhale water vapour. These all regulate how much water is in our bodies.

Pituitary gland: The gland that releases ADH.

Selective reabsorption: The reabsorption of some chemicals but not others, e.g. all glucose is reabsorbed, but only a certain amount of water and salt is, and no urea.

Ultrafiltration: The process of filtering the blood to separate large particles such as cells from small molecules such as glucose. Produces glomerular filtrate.

Urea: Substance created by breaking down amino acids. Toxic to the body, and excreted in urine.

Ureter: The tube that transports urine from the kidneys to the bladder.

Urethra: The tube that excretes urine from the bladder.

Section 2 i) Specification

Flowering plants:
2.67 understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf

Plants excrete carbon dioxide as it is produced as a waste product of respiration, as well as oxygen as it is a waste product of photosynthesis. They both leave the leaf through the stomata by diffusion in gas exchange.


Humans:
2.68 recall that the lungs, kidneys and skin are organs of excretion

The lungs excrete carbon dioxide, the kidneys excrete urea and other waste products in the blood to be passed through the urinary tract, and the skin excretes urea and salts in sweat.

2.69 understand how the kidney carries out its roles of excretion and osmoregulation

Excretion
Urea is a chemical produced in the liver from excess amino acids (deamination), and is toxic to the body. It is released from the liver into the blood, and the blood then flows through the renal artery into the kidneys. Inside the kidneys, there are many nephrons (filtration units within the kidney). The blood flows through capillaries into the glomerulus, where high pressure forces water, urea, salts and glucose into the bowmans capsule and through the proximal convoluted tubule. Here, glucose is selectively reabsorbed into the bloodstream from the glomerular filtrate by active transport. Salts are reabsorbed, but only just enough, no excess. Once the glomerular filtrate has travelled through the proixmal convoluted tubule, the loop of Henle, and the distal convoluted tubule, it enters the collecting duct as urine and is transported to the bladder, where it is held until it is excreted.

Osmoregulation
ADH (anti-diuretic hormone) is released from the pituitary gland, as stimulated by the hypothalamus in the brain. If the hypothalamus detects a rise in water content, the pituitary gland will release less ADH, but if it detects a fall in water content, the pituitary gland will release more ADH. More ADH makes the tubules more permeable, so more water is reabsorbed, and the opposite is also true.

2.70 describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra

Blood is filtered to remove waste products in the kidneys, and leaves through the collecting duct into the ureter, which transports urine to the bladder. Urine is expelled through the urethra.

2.71 describe the structure of a nephron, to include Bowman’s capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct

The nephron is a series of tubules and blood vessels. The glomerulus is inside the bowman's capsule, and this is where ultrafiltration occurs. The filtrate travels through the proximal convoluted tubule, then the loop of Henle, then the collecting duct.

2.72 describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate

Blood is pushed through the glomerulus at a high pressure, causing smaller molecules to be forced through into the bowman's capsule. Larger particles (cells etc.) remain in the blood as they are too large to fit through. Glomerular filtrate is therefore made up of water, glucose, urea and salts.

2.73 understand that water is reabsorbed into the blood from the collecting duct

Water is reabsorbed from the glomerular filtrate throughout the whole series of tubules, including the collecting duct, except for the second half of the loop of Henle, where only salts are reabsorbed.

2.74 understand that selective reabsorption of glucose occurs at the proximal convoluted tubule

Glucose is a very valuable nutrient required for respiration, so is selectively reabsorbed in the proximal convoluted tubule. This happens through active transport, because while this requires respiration to occur, the glucose saved means that it is worth it.

2.75 describe the role of ADH in regulating the water content of the blood

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

2.76 understand that urine contains water, urea and salts.

Urine is the substance excreted by the kidneys. It contains urea, a toxic substance from broken down amino acids, excess salts, and excess water. It should not contain glucose or proteins, which are indications of illness.