Changes in Composition of the Transport Medium (HSC SSCE Biology): Revision Notes
Changes in Composition of the Transport Medium
Introduction to blood composition changes
The circulatory system plays a vital role in maintaining life by transporting essential materials throughout your body. Blood serves as the transport medium, carrying nutrients and gases to cells while collecting and removing waste products. As blood circulates through different organs, its basic components (blood cells and plasma) remain constant, but the concentration of dissolved substances and gases changes depending on which organ it passes through.
Each organ has a specific function that affects blood composition. Some organs add substances that the body needs, while others remove waste products that must be eliminated. Understanding these changes helps us appreciate how different organs work together to maintain the body's internal environment.
Understanding the Difference
It's important to distinguish between blood components (cells and plasma) which remain constant, and the concentrations of dissolved substances which vary as blood passes through different organs. Think of it like a delivery truck that stays the same, but its cargo changes at each stop.
General patterns of blood composition change
Oxygen and carbon dioxide exchange
In most organs and tissues throughout the body (with the important exception of the lungs), a consistent pattern occurs:
- Oxygen concentration decreases as blood passes through tissues
- Carbon dioxide concentration increases as blood passes through tissues
This pattern results from cellular respiration, the process by which cells produce energy. During cellular respiration, cells consume oxygen from the blood and release carbon dioxide as a waste product. This means that blood leaving most organs has less oxygen and more carbon dioxide than when it entered.
Universal Pattern in Body Tissues
Remember: In ALL organs except the lungs, blood loses oxygen and gains carbon dioxide. This is due to cellular respiration occurring in every living cell throughout your body. The lungs are the only exception where this pattern reverses.
Nutrient and waste exchange
Another universal pattern occurs across all organs and tissues:
- Nutrients move out of the blood and into cells (e.g., glucose, amino acids)
- Waste products move from cells into the blood
This exchange ensures that cells receive the fuel they need for metabolism while waste products are collected for eventual removal from the body.
Changes in specific organs
The lungs
The lungs represent a unique exception to the general pattern described above. Here, the opposite occurs:
- Oxygen concentration increases as oxygen diffuses from the air sacs (alveoli) into the blood
- Carbon dioxide concentration decreases as this waste gas diffuses from the blood into the alveoli to be exhaled
This gas exchange is essential for maintaining adequate oxygen levels in the blood and removing the carbon dioxide produced by cellular respiration throughout the body.
The Lungs: The Only Exception
The lungs are unique because they are the ONLY organ where:
- Oxygen concentration increases (instead of decreasing)
- Carbon dioxide concentration decreases (instead of increasing)
This is a common exam question, so make sure you understand why this exception occurs!
The small intestine
The small intestine is the primary site where digested food enters the bloodstream:
- Digestive end products increase significantly, particularly:
- Glucose (from carbohydrate digestion)
- Amino acids (from protein digestion)
These nutrients travel directly from the small intestine to the liver through the bloodstream, where they undergo further processing. This is why blood leaving the small intestine has much higher concentrations of these substances compared to blood entering it.
Direct Route to the Liver
Blood from the small intestine goes directly to the liver via the hepatic portal vein. This allows the liver to process and regulate nutrient levels before they reach the rest of the body. Think of the liver as the body's "quality control centre" for nutrients.
The stomach
While the stomach is primarily known for breaking down food, it also contributes to blood composition:
- Water diffuses into the blood from the stomach contents
- Certain substances are absorbed, including alcohol
This explains why alcohol consumed on an empty stomach enters the bloodstream rapidly, as some absorption begins in the stomach itself.
Why Alcohol Acts Quickly
Unlike most nutrients that are primarily absorbed in the small intestine, alcohol can be absorbed directly through the stomach lining. This is why drinking on an empty stomach leads to faster intoxication - there's nothing to slow down the absorption process.
The liver
The liver acts as the body's central metabolic processing centre and performs numerous functions that alter blood composition:
Nutrient processing:
- Digestive end products decrease, including glucose, fatty acids, and amino acids
- Glucose levels may increase or decrease depending on the body's needs (the liver stores excess glucose as glycogen and releases it when needed)
Waste production:
- Urea is added to the blood when the liver breaks down proteins and removes nitrogen
Detoxification:
- Toxins and harmful substances are removed, including alcohol and drugs
Nutrient storage and release:
- Some vitamins and iron are removed from circulation for storage
The Liver's Multiple Functions
The liver is one of the most metabolically active organs in your body. It can both:
- Add substances to the blood (like urea from protein breakdown)
- Remove substances from the blood (like toxins, excess nutrients)
- Regulate levels of glucose depending on body needs
This dual role makes the liver essential for maintaining homeostasis.
An important note: fatty acids from digestion enter the bloodstream through the lymphatic system rather than directly from the intestine. Blood entering the heart therefore has high lipid content, which is then metabolised when it passes through the liver.
The kidneys
The kidneys are the body's primary filtration system, responsible for removing nitrogenous wastes:
- Urea concentration decreases significantly as the kidneys filter this waste product out of the blood
- Blood leaving the kidneys has the lowest percentage of nitrogenous wastes compared to any other organ
- Excess water is removed from the blood
- Salt concentrations are regulated according to the body's needs
This careful regulation of water and salt levels is crucial for maintaining the body's fluid balance and blood pressure.
Kidneys: Lowest Waste Concentration
A key exam point: Blood leaving the kidneys has the lowest concentration of nitrogenous wastes (especially urea) in the entire body. This is because the kidneys actively filter these waste products out for excretion in urine.
The large intestine
The large intestine contributes to blood composition through absorption:
- Water is absorbed into the blood
- Salts (electrolytes) are absorbed back into circulation
- Vitamins are absorbed, particularly vitamin K produced by intestinal bacteria
This reabsorption of water is essential for maintaining proper hydration and preventing excessive fluid loss.
Bacterial Contribution
The large intestine houses trillions of beneficial bacteria that produce vitamin K, which is then absorbed into the bloodstream. This is one example of how the gut microbiome contributes to our nutrition.
Endocrine glands
Endocrine glands release chemical messengers directly into the bloodstream:
- Hormones are added to the blood as it passes through these glands
Examples include insulin from the pancreas, thyroid hormones from the thyroid gland, and adrenaline from the adrenal glands. These hormones then travel throughout the body to reach their target organs and regulate various bodily functions.
Summary table of blood composition changes
| Tissue/Organ | Change in Blood Composition |
|---|---|
| Most organs and general body tissue | • Oxygen decreases • Carbon dioxide increases • Nutrients (glucose, amino acids) decrease • Wastes increase |
| Lungs | • Oxygen increases • Carbon dioxide decreases |
| Small intestine | • Glucose increases • Amino acids increase |
| Stomach | • Water increases • Alcohol increases (if consumed) |
| Liver | • Digestive end products (glucose, fatty acids, amino acids) decrease • Urea increases • Toxins and alcohol decrease • Some vitamins and iron decrease |
| Kidneys | • Urea decreases (lowest level) • Excess water decreases • Salt concentration regulated |
| Large intestine | • Water increases • Salts increase • Vitamins increase |
| Endocrine glands | • Hormones increase |
Exam Success Tips
- Remember the exception: The lungs are the only organ where oxygen increases and carbon dioxide decreases in the blood
- Think about function: Understanding what each organ does helps you remember how it affects blood composition
- Practice drawing flow diagrams: Trace blood through different organs and note the changes at each step
- Use the table: Creating and memorising a summary table like the one above can be very helpful for exam revision
- Link structure to function: Each organ's role in the body directly explains the changes it makes to blood composition
Key Points to Remember
- Blood composition changes as it circulates through different organs, even though blood cells and plasma remain the same
- In all organs except the lungs, blood loses oxygen and gains carbon dioxide due to cellular respiration
- The lungs are unique: blood gains oxygen and loses carbon dioxide through diffusion
- The small intestine adds digestive nutrients (glucose and amino acids) to the blood
- The liver is the metabolic centre, processing nutrients, producing urea, and removing toxins
- The kidneys filter nitrogenous wastes (especially urea) and regulate water and salt balance, resulting in blood with the lowest waste concentration
- Different organs either add needed substances (like nutrients and hormones) or remove waste products from the blood