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13 Excretion in Humans
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13.1 Excretion in Humans
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1. Excretion of Carbon Dioxide
Carbon dioxide (CO₂) is a waste product of aerobic respiration.
It is excreted through the lungs.
Blood carries carbon dioxide to the lungs, where it is exhaled during breathing.
Removing carbon dioxide is important because it can make the blood too acidic if it
accumulates.
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2. Excretion by the Kidneys
The kidneys are responsible for the excretion of:
Urea (waste from excess amino acids)
Excess water
Excess ions (like sodium and potassium)
The kidneys filter the blood, remove waste substances, and help maintain a healthy
balance of water and salts.
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3. Identification of Excretory Organs in Diagrams
You should be able to identify the following in diagrams and images:
Kidneys — two bean-shaped organs on either side of the spine.
Ureters — tubes that carry urine from each kidney to the bladder.
Bladder — a muscular sac that stores urine.
Urethra — the tube through which urine leaves the body.
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4. Kidney Structure (in Diagrams)
In kidney diagrams, you should recognize two main parts:
Cortex — the outer layer of the kidney where initial filtration occurs.
Medulla — the inner part of the kidney containing structures that collect and
concentrate urine.
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Supplement Content
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5. Structure and Function of a Nephron
The nephron is the basic functional unit of the kidney. It performs filtration,
reabsorption, and urine formation.
Structure and Roles:
(a) Role of the Glomerulus
The glomerulus is a network of capillaries inside the nephron.
It performs filtration:
Filters water, glucose, urea, and ions from the blood.
Blood pressure forces these substances out of the capillaries into the Bowman's
capsule.
Larger molecules like proteins and blood cells stay in the blood.
(b) Role of the Nephron in Reabsorption
After filtration, the nephron reabsorbs important substances back into the blood:
All of the glucose is reabsorbed for energy.
Some of the ions (like sodium) are reabsorbed depending on the body's needs.
Most of the water is reabsorbed to maintain water balance.
This selective reabsorption occurs mainly in the proximal convoluted tubule, loop of
Henle, and distal convoluted tubule.
(c) Formation of Urine
After reabsorption:
Urea, excess water, and excess ions remain.
These form urine, which passes through the collecting duct into the ureter, then to
the bladder for excretion.
(Details of exactly how filtration and reabsorption happen are not required.)
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6. Role of the Liver in Assimilation of Amino Acids
The liver plays an important role in assimilation:
It converts amino acids into proteins, which are then used by the body for building
cells, enzymes, hormones, and more.
Assimilation means taking digested products and using them for body functions.
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7. Formation of Urea in the Liver
When there are excess amino acids (more than needed for making proteins):
The liver breaks them down.
The nitrogen-containing part is removed and converted into urea.
Urea is then transported in the blood to the kidneys for excretion in urine.
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8. What is Deamination?
Deamination is the process by which the nitrogen-containing part of an amino acid is
removed.
The removed part is converted into urea.
The remaining part of the amino acid can be used for energy or converted into
carbohydrates or fats.
Deamination occurs in the liver.
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9. Importance of Excretion
Excretion is vital because waste products can be toxic if they accumulate.
Urea, in particular, is poisonous at high concentrations.
If waste substances are not removed:
They can damage cells.
They can disrupt body functions.
They can lead to death.
Therefore, organs like the kidneys, lungs, and liver are essential for maintaining
internal balance (homeostasis) by removing waste.
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Summary in Short:
Carbon dioxide is excreted by the lungs.
Kidneys excrete urea, excess water, and ions.
Know the diagrams: kidneys, ureters, bladder, urethra, cortex, medulla.
Nephron structure:
Glomerulus filters blood.
Nephron reabsorbs useful substances.
Urine is made of waste materials.
Liver:
Converts amino acids to proteins.
Forms urea from excess amino acids via deamination.
Excretion is important to prevent toxicity, especially from urea.
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