Chapter 32 Excretion 3.4 Learning Objectives 3.4.6 The Excretory System in Humans 1. Role of the excretory system in homeostasis. 2. Importance of temperature regulation within the body. 3. Outline the different methods of temperature regulation in animals 4. Function, location and excretory products of the lungs, skin, and urinary system. 5. Structure and basic function of the urinary excretory system in humans. 6. Role of the kidney in regulating body fluids. 7. Identification of the site of filtration. 8. Explain the process of reabsorption in the cortex, in the medulla and renal pelvis. 9. Describe the pathway of urine from the kidney to the urethra. 10. The nephron structure and its associated blood supply. Explain the stepwise formation of urine. Excretory system consists of: 1. The Skin (Sweat salt & water) 2. The Lungs (CO 2 and Water Vapour) 3. The Kidneys (Urine salt, water & urea) Excretion is the removal of waste products of metabolism from the body. Homeostasis is the maintenance of a constant internal environment. Homeostasis and excretion are closely linked because they involve; Removing waste products of metabolism from the body. Regulation of body temperature. Control of osmosis and regulating concentration of body fluids Temperature Regulation Two types of animals; Endotherms: gain or lose heat to or from their external environment (coldblooded such as snakes frogs, fish, lizards). Ectotherms: generate their own heat from their metabolic reactions (warmblooded such as cats, dogs, humans).
All metabolic reactions are controlled by enzymes and so they are affected by both temperature and ph. The main organs responsible for controlling body temperature are the liver and the skin. The liver generates the heat while the skin helps to control heat loss. The Skin Epidermis: Protective layer, prevents entry of pathogens Malpighian Layer: continuously dividing cells which form new epidermis Granular Layer: contain waterproof material called keratin (also found in hair and nails) Cornified Layer: dead cells from granular layer Dermis: Protects internal organs from damage, involved in temperature control and contains collagen, hair follicles, capillaries, sweat glands and nerve receptors. Functions of the skin Protection Melanin protects from UV damage Sebum oil produced to keep hair and skin moist Dermis & adipose tissue protect internal organs Epidermis prevent water loss and pathogen entry Sense Organ Nerve receptors detect temperature and touch Food Storage Stores excess fat in the adipose tissue for warmth and protection Excretion Sweat glands produce sweat to eliminate excess salt and water Vitamin Production
Uses sunlight to produce vitamin D to help absorb calcium Temperature Regulation Occurs in 3 key ways; If cold, erector muscles cause the hairs to stand (piloerection), trapping a layer of warm air around the skin If hot, sweat is produced and evaporates from the skins surface cooling it Vasodilation & vasoconstriction The Urinary System FUNCTIONS Urethra: Releases urine from the body Bladder: Stores Urine. We control the release using 2 sphincter muscles located at the bottom Ureters: Carry urine from kidneys to bladder Aorta: Carries waste-rich blood into the renal artery and kidneys Vena Cava: Carries filtered blood away from the renal vein and kidneys Kidneys are located in the abdominal cavity in the small of the back. 3 Functions; Excretion: Formation of Urine Osmoregulation: Kidneys control water & salt content in the body by reducing urine production on hot days and increasing it if water intake is high. ph control: The ph of the blood should be close to 7.4. The kidneys can increase or decrease the blood ph by absorbing ions. Vertical Section of a kidney
Excretion occurs in stages; Filtration, Reabsorption & Secretion FILTRATION Occurs in the glomerulus (in the cortex). The incoming blood is filtered to remove small particles (waste & useful). REABSORPTION Occurs in the cortex and medulla. Useful materials such as glucose, vitamins, water and amino acids are reabsorbed back into the blood. SECRETION Occurs in the cortex. Harmful substances such as potassium and hydrogen ions are secreted (pushed) from the blood into the kidneys. Once urine is formed it leaves the kidney through the renal pelvis and ureters and enters the bladder, before being released through the urethra. H.L. Only The Nephron Each kidney contains 1 million nephrons, each one is 3cm long.
Waste Pathway Blood Pathway The nephron forms urine in 3 process; 1. Filtration, 2. Reabsorption, 3. Secretion FILTRATION Occurs in the glomerulus. High pressure forces small molecules such as; salt urea, water, glucose and amino acids to be forced out of the glomerulus into Bowman s capsule. (Forming Glomerular Filtrate). ADAPTATIONS OF GLOMERULUS FOR FILTRATION 1. Higher pressure due to narrowness of afferent arteriole. 2. Large surface area of capillaries in glomerulus. 3. Glomerulus have more pores than other capillaries. 4. Bowman's capsule is only 1 cell thick. REABSORPTION On exiting the Bowman s Capsule ~99% of the glomerular filtrate is reabsorbed back into the blood. In the proximal convoluted tubule most water is reabsorbed by osmosis. Glucose, amino acids, salts and vitamins pass back into the blood by active transport and diffusion. Active transport involves the movement of molecules against a concentration gradient using energy. ADAPTATIONS OF PCT FOR REABSORPTION Thin-walled (one cell) Long with numerous infolding's (large surface area) Large number of mitochondria to make energy for active transport
In the descending loop of Henle water is reabsorbed by osmosis. In the ascending loop of Henle salt passes into the surrounding medulla by diffusion and active transport. This alters the concentration causing water to be pulled out (reabsorbed) by osmosis. Function of loop of Henle: reabsorb water. In the distal convoluted tubule some water and salts may be absorbed, it controls the ph and water and salt content of the blood. Any remaining water may be reabsorbed in the collecting duct by osmosis. Any liquid remaining in the collecting duct is now urine and is sent to the pelvis of the kidney, into the ureters and then the bladder for storage. SECRETION Involves some substances moving from the blood into the nephron. Glomerular Filtrate Vs Urine Glomerular Filtrate: 1. Much more water than urine (180 litres vs 1.5 litres) 2. Many useful molecules which have to be reabsorbed before urine is formed
Control of Volume of Urine Produced ADH = Anti-diuretic Hormone is produced by the Pituitary Gland. ADH causes the distal convoluted tubules and the collecting ducts to allow increased reabsorption of water. This reduces the salt concentration in the blood and the volume of urine produced. If we have large amounts of water in our blood, the salt concentration is low, this means ADH is not produced. As a result less water is reabsorbed into the blood, so more urine is produced.