Lesson Notes By Weeks and Term v4 - SHS 2
MAMMALIAN SYSTEMS
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MAMMALIAN SYSTEMS
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Subject: Biology
Class: SHS 2
Term: 2nd Term
Week: 16
Grade code: 2.4.1.LI.2
Strand code: 4
Sub-strand code: 1
Content standard code: 2.4.1.CS.1
Indicator code: 2.4.1.LI.2
Theme: SYSTEMS OF LIFE
Subtheme: MAMMALIAN SYSTEMS
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This lesson explores the body's essential "waste management" system, known as the excretory system. Just as we must dispose of 'borla' (rubbish) from our homes and communities to stay healthy, our bodies must constantly remove harmful waste products from our cells. Failure to do so can lead to serious illness and death. We will learn about the key organs responsible for this process—the kidneys, liver, lungs, and skin—and understand how their work is vital for maintaining a stable, healthy internal environment, a concept called homeostasis.
A. Fundamental Definitions Excretion: This is the process by which metabolic wastes (waste products from chemical reactions inside body cells) are removed from the body. These wastes are often toxic if allowed to accumulate. *Example:* When your cells break down proteins, they produce a toxic substance called ammonia, which is converted to a less harmful substance, urea. Excretion is the removal of this urea. Metabolic Wastes: These are the by-products of metabolism. The main metabolic wastes in humans are: Carbon Dioxide (CO₂): From cellular respiration. Urea: From the breakdown of excess amino acids in the liver. Excess Water and Mineral Salts: From various metabolic processes and diet. Bile Pigments (Bilirubin): From the breakdown of old red blood cells in the liver. Homeostasis: This is the maintenance of a constant or stable internal environment in the body, despite changes in the external environment. Think of it as the body's ability to keep everything "just right." *Analogy:* An air conditioner keeps a room at a constant temperature whether it's hot or cool outside. Your body does the same for its temperature, water levels, salt concentration, etc. Excretion is a key process for achieving homeostasis. Excretion vs. Egestion: This is a common point of confusion. Excretion is the removal of *metabolic* waste (from cells). Egestion is the removal of undigested and unabsorbed food materials from the gut as faeces. Faeces are not metabolic waste because they were never inside the body's cells. B. The Organs of Excretion and Their Roles in Homeostasis
Let's examine the four main excretory organs. The Lungs Structure: Spongy, air-filled organs located in the chest cavity. The functional units are tiny air sacs called alveoli. Excretory Product: Carbon Dioxide (CO₂). Process: During cellular respiration, our cells produce CO₂ as a waste product. This CO₂ diffuses from the cells into the bloodstream, is transported to the lungs, and is removed from the body when we exhale (breathe out). Link to Homeostasis: Blood pH Regulation: CO₂ dissolves in blood to form a weak acid (carbonic acid). If too much CO₂ accumulates, the blood becomes too acidic. By removing CO₂, the lungs help keep the blood pH within a very narrow, safe range (around 7.35-7.45). The Skin Structure: The largest organ of the body, consisting of the epidermis (outer layer) and dermis (inner layer). The dermis contains sweat glands. Excretory Product: Sweat, which is mostly water, but also contains dissolved mineral salts and a small amount of urea. Process: Sweat glands produce sweat, which is released onto the skin surface through pores. Link to Homeostasis: Thermoregulation (Temperature Control): This is the skin's *main* homeostatic function. When we are hot, we sweat. As the sweat evaporates from our skin, it takes heat with it, cooling the body down. This is very important in the hot Ghanaian weather. Excretion (Minor Role): The removal of urea and salts via sweat is a secondary, minor function compared to the kidneys. The Liver Structure: A large, reddish-brown organ located in the upper right part of the abdomen. It has a very rich blood supply. Excretory Products (it produces them, but doesn't expel them from the body): Urea: The liver is the site of deamination, the process of breaking down excess amino acids. The amino group (-NH₂) is removed, forming toxic ammonia. The liver quickly converts this ammonia into a less toxic compound called urea through the ornithine cycle. This urea is then released into the blood to be filtered and removed by the kidneys. Bilirubin (Bile Pigments): The liver breaks down old, worn-out red blood cells. The haemoglobin from these cells is broken down into products, including a yellowish pigment called bilirubin. This is excreted as part of bile into the small intestine and gives faeces its characteristic colour. Link to Homeostasis: Detoxification: The liver removes toxins, drugs, and alcohol from the blood, preventing them from harming the body. This is a critical homeostatic function. By converting toxic ammonia to less toxic urea, it maintains the chemical balance of the blood. The Kidneys (The Urinary System) This is the principal excretory organ in mammals. Structure: Macroscopic (Visible): Humans have a pair of bean-shaped kidneys located in the back of the abdominal cavity. A slice through the kidney reveals three main regions: the outer cortex, the inner medulla, and the central collecting area called the pelvis. The pelvis leads to the ureter. Microscopic (The Nephron): The functional unit of the kidney is the nephron. There are over a million nephrons in each kidney. Each nephron consists of: Bowman's Capsule: A cup-shaped structure surrounding a ball of capillaries called the glomerulus. A Long Tubule: This tubule has several parts: the proximal convoluted tubule, the Loop of Henle, and the distal convoluted tubule. Collecting Duct: Several nephrons drain into a single collecting duct. Excretory Product: Urine, which consists of water, urea, salts (like sodium chloride), and other waste products like creatinine and uric acid. Process: Urine Formation (A Three-Step Process) Step 1: Ultrafiltration (at the Glomerulus & Bowman's Capsule) Blood enters the glomerulus under high pressure. This pressure forces water, glucose, amino acids, mineral salts, urea, and other small molecules out of the blood and into the Bowman's capsule. This filtered fluid is called the glomerular filtrate. Large components like red blood cells, white blood cells, platelets, and large proteins are too big to pass through and remain in the blood. *Analogy:* It's like sieving coconut milk. The milk (filtrate) passes through the sieve, while the chaff (blood cells, proteins) stays behind. Step 2: Selective Reabsorption (in the Tubules) The glomerular filtrate contains useful substances that the body cannot afford to lose (glucose, some water, some salts). As the filtrate flows through the proximal tubule, Loop of Henle, and distal tubule, these useful substances are actively and passively reabsorbed back into the blood capillaries surrounding the tubule. Crucially, all glucose is reabsorbed. This is why there should be no glucose in the urine of a healthy person. The amount of water and salts reabsorbed is carefully controlled based on the body's needs. This step is "selective" because the body chooses what to take back. Waste products like urea are left behind in the tubule. Step 3: Tubular Secretion (in the Tubules) Some waste products (like excess potassium ions, hydrogen ions, and some drugs) are actively transported from the blood into the fluid in the distal tubule. This is another way the body gets rid of waste and helps to fine-tune the composition and pH of the blood. Final Product: The remaining fluid in the collecting duct is now called urine. It flows from the collecting ducts into the renal pelvis, down the ureter to the urinary bladder for temporary storage, and is finally expelled from the body through the urethra. Link to Homeostasis: Osmoregulation (Water Balance): The kidneys are the primary organs for controlling the water potential of the blood. If you drink a lot of water, the kidneys reabsorb less water and produce a large volume of dilute (pale) urine. If you are dehydrated (e.g., after playing football in the sun), the hormone ADH (Anti-diuretic Hormone) is released, causing the kidneys to reabsorb *more* water, producing a small volume of concentrated (dark yellow) urine. Blood Pressure and Salt Balance: The kidneys regulate the amount of salt (e.g., sodium) in the blood, which is crucial for maintaining blood pressure. Blood pH Regulation: By secreting hydrogen ions (H⁺) and reabsorbing bicarbonate ions (HCO₃⁻), the kidneys help maintain the blood's pH balance, complementing the work of the lungs. Summary Table of Excretory Organs
| Organ | Structure | Excretory Product(s) | Main Homeostatic Function | | :--- | :--- | :--- | :--- | | Lungs | Spongy sacs (alveoli) | Carbon Dioxide, Water Vapour | Regulates blood pH | | Skin | Glandular (sweat glands) | Sweat (Water, Salts, Urea) | Thermoregulation (body cooling) | | Liver | Large, glandular organ | Produces Urea & Bilirubin | Detoxification; converts ammonia to urea | | Kidneys | Bean-shaped; made of nephrons | Urine (Water, Urea, Salts) | Osmoregulation; filters blood; regulates blood pH & pressure |
Guided Practice (With Solutions)