17. What is a stimulus? What is the function of the liver? How does blood travel through the circulatory system? respiration? What vessels carry deoxygenated blood? What is fertilization? What is the difference between a tendon and a ligament? How many chambers does the heart have? 25. What is cartilage? Describe the first, second and third line of defense (immune system). What is a gland?

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Let's break down each of your questions one by one:

1. **What is a stimulus?**
   - A stimulus is any event or situation that evokes a response from an organism. It can be internal (like hunger) or external (like light, sound, or temperature) and is crucial for triggering reactions in living organisms.

2. **What is the function of the liver?**
   - The liver has several vital functions, including:
     - Metabolism of carbohydrates, proteins, and fats.
     - Detoxification of harmful substances.
     - Production of bile, which aids in digestion.
     - Storage of vitamins and minerals.
     - Regulation of blood clotting.

3. **How does blood travel through the circulatory system?**
   - Blood travels through the circulatory system in a loop:
     - Oxygen-poor blood returns to the heart via veins.
     - The right atrium receives this blood and sends it to the right ventricle.
     - The right ventricle pumps the blood to the lungs via the pulmonary arteries for oxygenation.
     - Oxygen-rich blood returns to the left atrium from the lungs.
     - The left atrium sends the blood to the left ventricle.
     - The left ventricle pumps oxygenated blood out to the body through the aorta.

4. **What is respiration?**
   - Respiration is the biochemical process in which cells convert glucose and oxygen into energy, carbon dioxide, and water. It can be aerobic (with oxygen) or anaerobic (without oxygen).

5. **What vessels carry deoxygenated blood?**
   - The vessels that carry deoxygenated blood are primarily the veins, specifically:
     - The superior and inferior vena cavae, which return blood to the heart from the body.
     - The pulmonary arteries, which carry blood from the heart to the lungs.

6. **What is fertilization?**
   - Fertilization is the process by which a sperm cell from a male merges with an egg cell from a female, resulting in the formation of a zygote. This marks the beginning of the development of a new organism.

7. **What is the difference between a tendon and a ligament?**
   - A tendon is a fibrous connective tissue that connects muscles to bones, allowing for movement. A ligament, on the other hand, connects bones to other bones at joints, providing stability and support.

8. **How many chambers does the heart have?**
   - The human heart has four chambers: two atria (the right atrium and left atrium) and two ventricles (the right ventricle and left ventricle).

9. **What is cartilage?**
   - Cartilage is a flexible connective tissue found in various parts of the body, including joints, the rib cage, ear, nose, and between vertebrae. It provides support and cushioning.

10. **Describe the first, second, and third line of defense (immune system).**
    - **First line of defense:** Physical and chemical barriers such as skin, mucous membranes, and secretions (like saliva and tears) that prevent pathogens from entering the body.
    - **Second line of defense:** The innate immune response, which includes phagocytic cells (like macrophages), inflammation, and fever, that respond to pathogens that breach the first line.
    - **Third line of defense:** The adaptive immune response, which involves specific recognition of pathogens by lymphocytes (B cells and T cells) and the production of antibodies.

11. **What is a gland?**
    - A gland is an organ that produces and secretes substances such as hormones, enzymes, or other fluids. Glands can be classified as endocrine (releasing hormones directly into the bloodstream) or exocrine (secreting substances through ducts to the outside of the body or into the digestive system).

If you have any further questions or need more details on any of these topics, feel free to ask!
**Simplified Answers:** 1. **Stimulus:** A stimulus is something that causes a response in an organism, like light or hunger. 2. **Liver Function:** The liver helps in digestion, cleaning the blood, storing nutrients, and making bile. 3. **Blood Circulation:** Blood moves from the body to the heart, then to the lungs for oxygen, back to the heart, and out to the body. 4. **Respiration:** It's how cells use oxygen to make energy. 5. **Deoxygenated Blood Vessels:** Veins carry blood without oxygen back to the heart. 6. **Fertilization:** When a sperm and egg join to start a new life. 7. **Tendon vs. Ligament:** Tendons connect muscles to bones, while ligaments connect bones to bones at joints. 8. **Heart Chambers:** The heart has four parts: two upper chambers (atria) and two lower chambers (ventricles). 9. **Cartilage:** A flexible tissue in joints and other parts that supports and cushions. 10. **Immune System Lines:** - **First:** Skin and other barriers keep out germs. - **Second:** Cells and inflammation fight off invaders. - **Third:** Special cells make antibodies to target specific germs. 11. **Gland:** An organ that makes and releases substances like hormones or enzymes into the body. If you need more details on any of these topics, let me know!

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