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Bio-113University of Bridgeport Lab 5 Endocrine & Urinary Systems - Online Learning Objectives: · Understand how feedback mechanisms maintain hormone homeostasis · Describe examples of hormonal regulatory pathways · Describe the steps in urine production · Explain how micturition is controlled Task 1: The Endocrine System The endocrine system, vital to homeostasis, plays an important role in regulating the activity of body cells. By acting through blood-borne chemical messengers, called hormones, the endocrine system organs orchestrate cellular changes that lead to growth and development, reproductive capability, and the physiological homeostasis of body systems. Overview The endocrine system consists of many glands, which work by secreting hormones into the bloodstream to be carried to a target cell (Figure 1).Target cells are cells that contain receptors that bind to a specific hormone. Endocrine system hormones work even if the target cells are distant from the endocrine glands. Through these actions, the endocrine system regulates nearly every metabolic activity of the body to produce an integrated response. The endocrine system can release hormones to induce the stress response, regulate the heartbeat or blood pressure, and generally directs how your cells grow and develop. Figure 1. Hormones circulate through the blood and act only on target cells, which may be located far away from the secreting cell. Endocrine glands are usually heavily vascularized, containing a dense network of blood vessels. Cells within these organs produce and contain hormones in intracellular granules or vesicles that fuse with the plasma membrane in response to the appropriate signal. This action releases the hormones into the extracellular space, or into the bloodstream. The endocrine system can be activated by many different inputs, allowing for responses to many different internal and external stimuli. Endocrine System Function The endocrine system, along with the nervous system, integrates the signals from different parts of the body and the environment. In addition, the endocrine system produces effector molecules in the form of hormones that can elicit an appropriate response from the body in order to maintain homeostasis. The nervous system produces immediate effects. The endocrine system is designed to be relatively slow to initiate, but it has a prolonged effect. As an example, the long-term secretion of growth hormone in the body influences the development of bones and muscles to increase height and also induces the growth of every internal organ. This happens over the course of many years. Hormones like cortisol, produced during times of stress, can change appetite, and metabolic pathways in skeletal and smooth muscle for hours or weeks. The endocrine system is involved in every process of the human body. Starting from the motility of the digestive system, to the absorption and metabolism of glucose and other minerals, hormones can affect a variety of organs in different ways. Some hormones affect the retention of calcium in bones or their usage to power muscle contraction. In addition, they are involved in the development and maturation of the adaptive immune system, and the reproductive system. Crucially, they can affect overall growth and metabolism, changing the way every cell assimilates and utilizes key nutrients. During your Practical lab, you will review the location of the different endocrine glands and the hormones that they produce. Here, we will focus on how hormone levels are regulated and the physical consequences of their imbalance. Regulation of the Endocrine System The key aspect of homeostasis is maintaining the appropriate levels of different hormones circulating throughout the body. This is best explained by the analogy of the thermostat in your home. If the room is too cold, the thermostat detects this and turns on the heating. The room then warms up. The temperature is prevented from rising too high because the thermostat detects when the optimum temperature has been exceeded and turns off the heating. This is negative feedback, and keeps the temperature constant in the house (Figure 2). https://biologydictionary.net/endocrine-system/ Figure 2. Example of a system regulated by negative feedback. Diseases of the Endocrine System Endocrine system diseases primarily arise from two causes – either a change in the level of hormone secreted by a gland, or a change in the sensitivity of the receptors in various cells of the body. Therefore, the body fails to respond in an appropriate manner to messenger signals. Among the most common endocrine diseases is diabetes, which hampers the metabolism of glucose. This has an enormous impact on the quality of life since adequate glucose is not only important for fueling the body, but it is also important in maintaining glucose at an appropriate level to discourages the growth of microorganisms or cancerous cells. Diabetes Diabetes, or diabetes mellitus, refers to a metabolic disease where the blood consistently carries a high concentration of glucose. This is traced back to the lack of effective insulin hormone, produced by the pancreas, or a lack of functioning hormone receptors. Diabetes mellitus could either arise from a low level of insulin production from the pancreas (Type I) or an insensitivity of insulin receptors among the cells of the body (Type II). Insulin is an anabolic hormone that encourages the transport of glucose from the blood into muscle cells or adipose tissue. Here, it can be stored as long chains of glycogen, or be converted into fat. Concurrently it also inhibits the process of glucose synthesis within cells, by interrupting gluconeogenesis, as well as the breakdown of glycogen. A spike in blood sugar levels causes the release of insulin. Its release protects cells from the long-term damage of excess glucose, while also allowing the precious nutrient to be stored and utilized later. Glucagon, another hormone secreted by the pancreas (alpha cells), acts in an antagonistic manner to insulin and is secreted when blood sugar levels drop (Figure 3). Figure 3. Negative feedback mechanism is used to maintain adequate blood glucose levels. Two hormones secreted by the pancreas have antagonistic (opposite) effects on blood glucose levels. Hypothyroidism The hypothalamus and the pituitary gland are the main controls that regulate the actual thyroid’s activity. When T3 or T4 hormone levels become too low, the hypothalamus will respond by secreting TSH-releasing hormone (or TRH). TRH signals the pituitary gland to create more thyroid stimulating hormone (or TSH). The thyroid gland, in turn, will respond by making more thyroid hormone in a feedback loop. The levels will be finely tuned to maintain a balance of T3 to T4 hormones (Figure 4). TSH release will have a direct impact on the hormones’ respective levels. The role of TSH can be summed up as a stimulus that will lead the thyroid gland to release more hormone. Abnormally high levels of TSH can indicate an underactive thyroid. This is a condition called hypothyroidism. The related symptoms of having T3 and T4 in excess are listed below: · Anxiety · Hair loss · Irritability · Hyperactivity · Hand trembling Hyperthyroidism On the other hand, when the T3 and T4 levels fall under the functional amounts, the body will undergo changes in the opposite direction. Chronically high thyroid hormone levels will lead to hyperthyroidism. The high T3 and T4 levels will signal the pituitary gland to release less TSH in the system. Common symptoms may include the following: · Insomnia · Fatigue and tiredness · Depression · Difficulty concentration · Muscle pain Figure 4. Homeostasis of thyroid hormone levels. Secretion of thyroid hormone is controlled by the hypothalamus via the anterior pituitary. High levels of thyroid hormones will inhibit both the hypothalamus from secreting TRH and the anterior pituitary from secreting TSH. This is an example of negative feedback loop. Task 2: The Urinary System Overview How the urinary system works is relatively simple, although the supplementary roles of the kidneys can be complex. Blood is transported to the kidneys via the renal artery. A system of filtration units within the kidney regulates levels of dilution (water), salts and other small molecules in the filtrate. Any excess or undesired products travel through each ureter and are deposited into the reservoir of the bladder, while purified blood re-enters the circulatory system by way of the renal vein. Urine is stored in the bladder until the urinary nervous system releases the contents through the urethra and out of the body. The passing of urine is known as micturition or urination. The urinary system is split into the upper and lower urinary tract. The former consists of the kidneys and ureters, the latter of the bladder and urethra. The main urinary system function is to filter the blood of excess water, salts, and waste products, temporarily store these within a reservoir, and intermittently expel these products from the body. Filtration, Reabsorption, and Secretion Each kidney contains approximately one million nephrons. Nephrons play a crucial role in removing waste products and adjusting concentrations of water, ions and smaller molecules in the blood. A single kidney contains enough nephrons to filter the blood and produce urine – for this reason, kidney transplants can use organs from living donors. When both kidneys are damaged, ions, salts, water, and small molecules accumulate in the body, causing complete organ failure and death if left untreated. A kidney cannot generate new nephrons – once they are damaged, they can not be replaced. The image below shows the blood flow into the kidney and nephron (in red), the production and excretion of urine (yellow), and the return of reabsorbed products and filtered blood into the circulatory system (blue) (Figure 5). Figure 5. Blood filtration in the kidney. Filtration removes excess water, salt and waste products, and returns filtered blood back to the body. The filtrate (urine) is collected in the bladder. A nephron has three functions: glomerular filtration (of water and solutes within the blood), tubular reabsorption (the return of water and required molecules to the circulation), and tubular secretion (of waste or excess molecules – including water) into a distal tube. This secreted fluid is known as urine. Every minute, approximately 125 ml of blood is filtered by the nephrons of both kidneys. The majority of filtrate is reabsorbed – meaning in a period of 24 hours, around 180 liters of filtrate are produced but only 1.5 liters of this is excreted as urine. Filtration occurs within the glomerulus, tubular reabsorption in the proximal convoluted tubule, and tubular secretion in the distal convoluted tubule. The loop of Henlé maintains a concentration gradient so that water and ions are more easily reabsorbed. In the image below, the achievement of equal concentrations of a solute on either side of a membrane by the process of diffusion is depicted. Both osmosis and diffusion occur within nephrons. Osmosis is the movement of water, not solute, through a semi-permeable membrane (Figure 6). Figure 6. Transport of solutes (ions and small molecules) and water across the cell membrane during filtration, reabsorption and secretion. Both osmosis and diffusion are passive processes which depend on the relative concentration of water and solutes across