Carbon dioxide, $CO_{2}$, is excreted through the lungs. Carbon dioxide is made in body tissues during respiration, and here, it diffuses out of the blood into the air in the alveoli, then exhaled out.
Urea, excess water and ions are excreted through the kidneys. Urea is carried to the kidneys in the plasma where it is filtered out and leaves the body dissolved in the form of urine. Urea is formed in the liver from excess amino acids.
Blood enters the kidney under high pressure. It reaches the glomerulus, a tightly coiled knot of capillaries.
Water, glucose, and urea are filtered out.
Large molecules like blood cells and plasma proteins are too big to pass through, so they remain in the blood.
Selective reabsorption
As the filtered fluid flows along the nephron tubule, the body reclaims the substances it still needs. The blood vessels wrapped around the nephron pull these substances back into the bloodstream.
All of the glucose is reabsorbed back into the blood for respiration.
Some of the ions are reabsorbed depending on body requirements.
Most of the water is reabsorbed to prevent dehydration.
Urine formation
Whatever is not reabsorbed passes to the end of the nephron and collects as urine. Urine flows into the ureter, then down to the bladder.
Urine contains: urea, excess water, and excess ions.
Excretion is important because metabolic wastes like urea are highly toxic. If urea builds up in the blood, it damages cells, disrupts cellular chemistry, and can lead to organ failure.
Amino acid metabolism in the liver:
Assimilation is when the food molecules that have been absorbed now become part of the cells or are used by the cells. The liver converts absorbed amino acids into useful plasma proteins, such as fibrinogen for blood clotting.
Deamination is the removal of the nitrogen-containing part of amino acids to form urea. This process takes place in the liver to safely break down excess amino acids that the body cannot store.
