CIE iGCSE Biology- 13.1 Excretion in humans- Study Notes- New Syllabus
CIE iGCSE Biology-13.1 Excretion in humans- Study Notes – New syllabus
CIE iGCSE Biology-13.1 Excretion in humans- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- State that carbon dioxide is excreted through the lungs
- State that the kidneys excrete urea and excess water and ions
- Identify in diagrams and images the kidneys, ureters, bladder and urethra
Supplement
- Identify in diagrams and images the structure of the kidney, limited to the cortex and medulla
- Outline the structure and function of a nephron and its associated blood vessels, limited to:
(a) the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions
(b) the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood
(c) the formation of urine containing urea, excess water and excess ions
(details of these processes are not required) - Describe the role of the liver in the assimilation of amino acids by converting them to proteins
- State that urea is formed in the liver from excess amino acids
- Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea
- Explain the importance of excretion, limited to toxicity of urea
Excretion of Carbon Dioxide Through the Lungs
Key Statement:
Carbon dioxide is excreted through the lungs.
Carbon dioxide is excreted through the lungs.
🧠 What Is Excretion?
- Excretion is the process of removing waste products of metabolism from the body.
- Carbon dioxide (CO₂) is a waste product of aerobic respiration.
⚙️ How Carbon Dioxide Is Removed:
- Produced in Cells
During aerobic respiration:
Glucose + Oxygen → Carbon dioxide + Water + Energy - Transported in Blood
CO₂ diffuses from cells into the bloodstream.
It is transported to the lungs, mainly dissolved in plasma or as bicarbonate ions. - Exhaled from the Lungs
In the lungs, CO₂ diffuses into the alveoli.
It is excreted from the body when we breathe out (exhalation).
📌 Why It’s Important:
If carbon dioxide builds up, it can:
- Lower blood pH (make it too acidic)
- Interfere with enzyme function
Therefore, excreting CO₂ is vital for homeostasis.
🧾 Summary:
Carbon dioxide, a waste product of respiration, is transported to the lungs by the blood and excreted from the body through exhalation.
Carbon dioxide, a waste product of respiration, is transported to the lungs by the blood and excreted from the body through exhalation.
Excretion by the Kidneys
Key Statement:
The kidneys excrete urea, excess water, and excess ions.
The kidneys excrete urea, excess water, and excess ions.
🧠 What Are These Substances?
| Substance | Source |
|---|---|
| Urea | Formed in the liver from the breakdown of proteins/amino acids |
| Excess water | Comes from diet, metabolism, or fluid imbalance |
| Excess ions | Extra salts (like sodium, potassium) from food |
⚙️ How the Kidneys Work:
- Blood enters the kidney through the renal artery.
- Filtration occurs in small units called nephrons.
- Useful substances are reabsorbed (like glucose and needed water).
- Waste products (urea, extra water, and ions) form urine.
- Urine is passed to the bladder and then excreted from the body.
📌 Why This Is Important:
- Helps maintain water balance and ion concentration
- Removes toxic waste (urea) from the body
- Keeps internal environment stable – part of homeostasis
🧾 Summary:
The kidneys filter the blood and excrete urea, excess water, and excess ions in the form of urine.
The kidneys filter the blood and excrete urea, excess water, and excess ions in the form of urine.
Urinary System: Organs to Identify
1. Kidneys
- Location: Two bean-shaped organs located on either side of the spine, just above the waist.
- Function: Filter waste, excess water, and salts from the blood to produce urine.
2. Ureters
- Location: Two thin muscular tubes, one from each kidney.
- Function: Carry urine from the kidneys to the bladder using peristaltic movements.
3. Bladder
- Location: A sac-like organ located in the lower abdomen (pelvic area).
- Function: Stores urine until it is released.
4. Urethra
- Location: A tube extending from the bottom of the bladder to the outside of the body.
- Function: Carries urine out of the body during urination.
🧠 Revision Tip:
| Structure | Function | Clue to Identify in Diagrams |
|---|---|---|
| Kidneys | Filter blood to form urine | Bean-shaped, high in abdomen |
| Ureters | Carry urine to bladder | Thin tubes from kidneys to bladder |
| Bladder | Stores urine | Balloon-like structure in pelvis |
| Urethra | Releases urine out | Tube from bladder to outside |
Structure of the Kidney: Cortex and Medulla
Overview of Kidney Anatomy
Each kidney is bean-shaped and has two main internal regions:
- Cortex (outer layer)
- Medulla (inner layer)
1. Cortex
Location: The outer region of the kidney, just beneath the thin capsule.
Function:
- Contains most of the glomeruli and Bowman’s capsules.
- Plays a key role in filtration of blood.
2. Medulla
Location: The inner region, found beneath the cortex.
Function:
- Involved in reabsorption and concentration of urine.
- Contains loops of Henle and collecting ducts.
Structure and Function of a Nephron
The nephron is the functional unit of the kidney – it filters blood, reabsorbs useful substances, and helps form urine.
🧪 (a) Role of the Glomerulus – Filtration
- The glomerulus is a network of capillaries inside the Bowman’s capsule.
- High pressure in the glomerulus forces small molecules out of the blood:
- Water
- Glucose
- Urea
- Ions (salts)
- This process is called ultrafiltration.
- Blood cells and proteins remain in the blood as they are too large.
🔄 (b) Role of the Nephron – Reabsorption
- The filtered fluid passes through the nephron tubule.
- The nephron selectively reabsorbs:
- All of the glucose
- Some of the ions
- Most of the water
- These substances return to the blood via surrounding capillaries.
- This ensures important materials are not lost in the urine.
🚽 (c) Formation of Urine
- After reabsorption, the remaining fluid contains:
- Urea (waste from protein breakdown)
- Excess water
- Excess ions
- This fluid is now called urine.
- Urine flows to the collecting duct, then to the ureter, bladder, and out via the urethra.
📌 Summary Table:
| Part of Nephron | Function |
|---|---|
| Glomerulus | Filters small molecules (water, glucose, urea, ions) from blood |
| Tubule (nephron) | Reabsorbs all glucose, some ions, and most water |
| Collecting duct → Bladder | Remaining fluid becomes urine (urea, excess water and ions) |
Role of the Liver in Assimilation of Amino Acids
Key Concept: The liver plays a central role in the assimilation (use and conversion) of amino acids by converting them into proteins needed by the body.
🧠 What Are Amino Acids?
- Amino acids are the building blocks of proteins.
- They come from the digestion of dietary proteins in the small intestine.
- Absorbed into the blood and transported to the liver.
🧬 Role of the Liver:
1. Protein Synthesis:
- The liver uses amino acids to make important proteins such as:
- Plasma proteins (e.g., albumin, fibrinogen)
- Enzymes
- Clotting factors
- These proteins are either released into the bloodstream or sent to other body parts.
2. Amino Acid Sorting:
- The liver maintains balanced levels of amino acids.
- Ensures a steady supply for growth, repair, and metabolism.
- Excess amino acids are not stored – they undergo breakdown.
3. Deamination:
- Deamination is the removal of the amino group (–NH₂) from excess amino acids.
- The amino group is converted to urea, which is transported to the kidneys for excretion.
- The remaining part of the amino acid may be used for energy or converted to fat/glucose.
📌 Summary: The liver assimilates amino acids by converting them into essential proteins needed for the body, while managing excess through deamination.
Urea Formation in the Liver
Key Statement: Urea is formed in the liver from excess amino acids.
📌 Why This Happens:
- The body cannot store extra amino acids.
- After a protein-rich meal, amino acid levels may rise above what is needed.
- The liver safely processes the surplus to prevent harmful build-up.
🔄 How It Works:
- Excess amino acids travel to the liver via the bloodstream.
- In the liver, deamination occurs:
- The amino group (-NH₂) is removed from the amino acid.
- This forms ammonia (NH₃), which is toxic.
- Ammonia is quickly converted into urea, which is less harmful.
- Urea is then:
- Released into the bloodstream
- Transported to the kidneys
- Excreted in urine
🧾 Summary: The liver converts excess amino acids into urea through deamination, and the urea is excreted by the kidneys in urine.
Deamination
Key Statement: Deamination is the removal of the nitrogen-containing part of amino acids to form urea.
📌 Where It Happens: In the liver
🧬 Why Deamination Is Necessary:
- Amino acids are needed to build proteins.
- Excess amino acids cannot be stored in the body.
- Their nitrogen-containing amino group can be toxic if left in the body.
- The liver ensures safe removal and processing.
🔄 What Happens During Deamination:
- The amino group (-NH₂) is removed from the amino acid.
- This is converted into ammonia (NH₃).
- Ammonia is toxic, so it is immediately converted into urea.
- Urea is transported in the blood to the kidneys.
- It is then excreted in urine.
🧾 Summary: Deamination in the liver removes the nitrogen part of amino acids, forming urea, which is excreted by the kidneys.
Importance of Excretion
Key Statement: Excretion is essential because urea is toxic and must be removed from the body.
🧬 What Is Urea?
- Urea is a waste product formed in the liver.
- It is made during deamination of excess amino acids.
- Urea contains nitrogen, which can be harmful if it builds up.
⚠️ Why Is Urea Dangerous?
- Urea can poison cells if it accumulates.
- It may disrupt metabolic processes.
- Can lead to organ failure or death if not excreted.
🧪 Role of Excretion:
- The kidneys filter urea from the bloodstream.
- Urea is removed from the body in urine.
- This helps maintain a safe and stable internal environment.
🧾 Summary: Excretion of urea is vital because its toxic buildup in the body can damage cells and disturb normal functions.