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: 18
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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Welcome, students! Today, we are going to explore our body's amazing waste management system, known as the excretory system. Think about a busy kitchen in your home or at a chop bar. After cooking jollof rice or fufu, there are leftovers, peels, and smoke. If this waste isn't cleared, the kitchen becomes dirty and unusable. Our body's cells are like tiny kitchens; they are constantly working (metabolism), and this work produces waste products. The excretory system is responsible for removing these potentially toxic wastes. This process is crucial for maintaining a stable, healthy internal environment, a concept we call homeostasis.
A. Fundamental Definitions Excretion: This is the process by which metabolic wastes (waste products from chemical reactions inside body cells) and other non-useful substances are eliminated from an organism. *Example:* Urea produced from protein breakdown is a metabolic waste. Egestion: This is the process of discharging undigested or waste material (faeces) from the digestive tract. It is NOT excretion because the faeces consist of material that was never absorbed into the body's cells. Homeostasis: This is the maintenance of a constant and stable internal environment in the body, despite changes in the external environment. This includes regulating temperature, water balance, pH, and chemical concentrations. The excretory system is a key player in homeostasis. B. The Major Excretory Organs and Their Functions
Let's examine the four main organs involved in excretion in humans. The Kidneys (The Master Filters)
The kidneys are the primary excretory organs. Humans have a pair of reddish-brown, bean-shaped organs located in the back of the abdomen. Structure: External: Each kidney is supplied with blood by a renal artery and drained by a renal vein. A tube called the ureter carries urine from each kidney to the urinary bladder for temporary storage. Urine is finally expelled through the urethra. Internal: A kidney has three main regions: the outer cortex, the inner medulla, and the central collecting region called the renal pelvis. The Nephron: The functional unit of the kidney is the nephron. There are over a million nephrons in each kidney! A nephron has several parts: Bowman's Capsule: A cup-shaped structure surrounding a ball of capillaries called the glomerulus. Renal Tubule: A long, coiled tube consisting of the Proximal Convoluted Tubule (PCT), the Loop of Henle, and the Distal Convoluted Tubule (DCT). Collecting Duct: The DCTs of several nephrons empty into a collecting duct, which carries urine to the renal pelvis. Function: Urine Formation This occurs in three main steps in the nephron: Step 1: Ultrafiltration (at the Glomerulus) Blood enters the glomerulus under high pressure. This pressure forces water, glucose, amino acids, mineral salts, urea, hormones, and other small molecules out of the blood and into the Bowman's capsule. Large molecules like proteins and blood cells are too big to pass through, so they remain in the blood. The filtered fluid is called glomerular filtrate. Step 2: Selective Reabsorption (at the Renal Tubule) The glomerular filtrate contains useful substances that the body needs back. As the filtrate flows through the PCT, Loop of Henle, and DCT, these useful substances are reabsorbed back into the blood. What is reabsorbed? Most of the water, all of the glucose, and some mineral salts and amino acids. This process is "selective" because the body only takes back what it needs. For example, if you are well-hydrated, less water is reabsorbed. Step 3: Tubular Secretion (at the DCT) Some waste products, like excess potassium ions and drugs (e.g., penicillin), are actively transported from the blood into the filtrate in the distal convoluted tubule.
The remaining fluid, which is now highly concentrated with waste products like urea, excess salts, and water, is called urine. Role in Homeostasis (Osmoregulation): The kidneys are vital for regulating the water potential of the blood. If you drink a lot of water (like several sachets of pure water on a hot day), your blood becomes dilute. The pituitary gland releases less Antidiuretic Hormone (ADH). This makes the collecting ducts less permeable to water, so less water is reabsorbed, and you produce a large volume of dilute (pale) urine. If you are dehydrated (e.g., during Harmattan), your blood becomes concentrated. The pituitary gland releases more ADH. This makes the collecting ducts more permeable, more water is reabsorbed back into the blood, and you produce a small volume of concentrated (dark yellow) urine. The Lungs Structure: The main organs of the respiratory system, composed of millions of tiny air sacs called alveoli. Excretory Function: The lungs excrete carbon dioxide (CO₂) and water vapour, which are waste products of aerobic cellular respiration in all body cells. This happens during exhalation. The Skin Structure: The largest organ of the body. The relevant structures for excretion are the sweat glands, which are coiled tubes in the dermis that lead to pores on the skin surface. Excretory Function: The skin excretes sweat. Sweat is primarily composed of water, but it also contains mineral salts (like sodium chloride, which makes it taste salty) and a small amount of urea. Role in Homeostasis (Thermoregulation): While it does excrete wastes, the primary homeostatic role of sweating is to cool the body down. When sweat evaporates from the skin, it takes heat with it. The Liver