Excretory System - High School Biology

Kidneys are bilateral organs that function to filter metabolic waste products from the blood. These waste products are processed and excreted via urine. Once it is formed in the kidneys, urine exits via the ureter. The ureter carries the urine to the bladder, where it is stored until it can be excreted. Once the bladder is full, smooth muscles surrounding the bladder contract and push urine out of the bladder and into the urethra. The urethra is the last stop for urine, after which it is excreted.

The question states that the organ stores urine. Recall that bladder stores urine until excretion; therefore, the researcher must be analyzing a bladder. Smooth muscle and skeletal muscle are both muscle types found in the human body. Smooth muscle is involuntary, whereas skeletal muscle is voluntary. Recall that the muscle surrounding the bladder functions to empty the bladder. We cannot control bladder emptying; therefore, the muscle surrounding the bladder is involuntary and is classified as smooth muscle.

The excretory system for urine consists of the following components: kidneys, ureter, bladder, and urethra. These components are the same in both males and females. The urine is made in the kidneys. It exits the kidneys and travels through the ureter, bladder, and urethra, after which it is excreted.

The uterus is only found in females and the prostate is only found in males; however, these are reproductive structures and are not part of the excretory system. Age does not determine the structures of the excretory system. A teenager and an eighty-year old will have the same excretory system.

The excretory system functions to remove waste from the body. This can happen via two pathways. First, kidneys filter metabolic wastes from the blood and excrete them via urine. Second, organs of the GI tract (such as the liver, small intestine, and colon) filter toxic substances and waste products from food and excrete it via feces; therefore, the excretory system functions to remove waste via urine and feces.

Another main function of the excretory system is to maintain homeostasis. To maintain homeostasis, the kidneys have the ability to sense changes in certain substances in the blood and can excrete them accordingly. For example, if there is an increase in the concentration of bicarbonate ions in the blood, the kidneys will sense this change and increase the excretion of bicarbonate ions via urine. This will bring the concentration of bicarbonate ions back to normal and will maintain homeostasis.

The word "renal" refers to the kidneys. Since it plays a huge role in excretion, kidney failure can alter excretory system functions.

Transitional epithelium consists of multiple layers of epithelial cells which can contract and expand. When the cells are not stretched they appear cuboidal with a domed apex, however once stretched the the tissue compresses and the cells appear flattened. This type of tissue is found in the urinary bladder, the ureters, the superior urethra, and gland ducts of the prostrate.

All of these are functions of the kidney except for excretion of proteins. In fact, the kidney should never secrete protein or glucose (sugars) unless there is a disease state present. For example, in diabetics who have damaged kidneys (diabetic nephropathy), there is protein in the urine. To a degree, the amount of protein in the urine determines how bad their kidney disease is.

Glucose is initially filtered at the glomerulus but then all reabsorbed in the proximal convoluted tubule. If it appears in the urine, it is termed glucosuria and indicates that the patient has diabetes (impaired insulin secretion or sensitivity that causes elevated levels of glucose in the blood).

Antidiuretic hormone (ADH) is useful if an individual becomes dehydrated. When dehydrated, ADH will make the collecting ducts permeable to water. This allows water to be reabsorbed into the body from the filtrate in the kidneys. By moving water from the filtrate to the body, the urine becomes more concentrated and the body retains water. This process helps to prevent dehydration.

Oxytocin and antidiuretic hormone (ADH) are both produced by the hypothalamus and released from the posterior pituitary. In women, oxytocin is important during childbirth because it regulates uterine contractions. It also acts as a hormone in the brain and is important to sexual arousal, recognition, and trust. Antidiuretic hormone, also known as vasopressin, is an important hormone that induces increased water reabsorption in the kidney to increase blood volume.

Thyroid hormone (thyroxine or T4) and calcitonin are both produced in the thyroid. Thyroid hormone is extremely important in the regulation of metabolism. Calcitonin in the bones inhibits the activity of osteoclasts, which break down bone matrix. This decreases the amount of calcium in the blood. In the kidney, calcitonin inhibits the reabsorption of calcium. Thyroid-stimulating hormone is used to stimulate the thyroid to release thyroid hormone, and is produced by the anterior pituitary.

Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland and plays an important role in reproduction, growth, and development, particularly during puberty.

The nephrons of the kidney serve to balance and concentrate the filtrate in order to generate urine and eliminate nitrogenous wastes, while conserving valuable nutrients and ions. Filtration separates fluids from the cells of the blood in Bowman's capsule. Reabsorption refers to the process by which particulates and ions are removed from the filtrate and returned to the blood, while secretion refers to the addition of compounds from the blood into the filtrate. These processes are critical to establishing the proper urine composition.

While much of reabsorption and secretion occurs passively due to the manipulation of ion gradients, it is important to note that the generation of ion gradients usually requires active transport and the expenditure of energy. As such, the excretory system does utilize energy and involves active transport.

Filtrate that enters the renal corpuscle in Bowman’s capsule is forced in due to hydrostatic blood pressure; this is a passive mechanism. However, as the filtrate is reabsorbed in the proximal convoluted tubule (PCT), some molecules exit the tubule via active transport mechanisms. Later, as filtrate travels through the descending limb of the loop of Henle, water moves out passively with its concentration gradient, as do sodium ions in the thin ascending limb. In the thick ascending limb, the concentration of salt has increased in the medulla, causing salt to exit the tubule via active transport mechanisms. Finally, in the distal convoluted tubule and collecting duct, water reabsorption occurs passively through aquaporins, but some reabsorption and secretion may occur through active transport.

Active transport occurs in the proximal convoluted tubule, ascending loop of Henle, distal convoluted tubule, and collecting duct.

Renin is released from these cells in response to decreased arteriole blood pressure. The renin-angiotensin-aldosterone system is then activated to maintain blood pressure by various ways including stimulating thirst, reabsorbing sodium in the kidney, and increasing vasopressin release.

Aldosterone is released by the adrenal glands and works to increase sodium retention which increases water retention and thereby increases blood volume to increase blood pressure. If sodium flows back into the cells from the kidney tubules (so that it is not secreted as urine), water also flows back as an osmotic effect. Additionally, aldosterone also stimulates thirst and decreases vasopressin secretion, all to increase blood volume to increase blood pressure.

The nephron is composed of multiple sections, which are used to control filtrate solute concentration and form urine. The proximal convoluted tubule does not alter solute concentration, and the distal convoluted tubule decreases solute concentration in the filtrate. It is the loop of Henle that increases the solute concentration of the filtrate.

Filtration occurs in the renal corpuscle, which is composed of the glomerulus and Bowman's capsule. The glomerulus filters the blood, resulting in the filtrate, which will then go through the tubules. The proximal and distal convoluted tubules are primarily involved in solute reabsorption, and the loop of Henle helps concentrate the urine.

Water, small molecules, and ions are forced through the glomerular capillaries into Bowman’s capsule in the process of filtration, largely due to the high blood pressure in the capillaries of the glomerulus. Nitrogenous compounds, namely urea, are filtered into Bowman's capsule for excretion. Glucose is also filtered into the capsule, but is reabsorbed in the proximal convoluted tubule; the presence of glucose in the final filtrate or urine can be indicative of diabetes or other maladies.

The correct answer is that two answer options are correct: small ions, glucose, amino acids, water, and nitrogenous compounds are all found in the initial filtrate.

Large molecules and cells are never filtered through the podocytes (cells lining the capsule), so we never expect to find blood cells or large blood proteins like albumin in the filtrate.

The nephrons are the basic filtration units of the kidneys. In the cortex (plural "cortices") or outer portion of the kidney, blood enters a nephron consisting of a glomerulus (mass of capillaries), renal tubules and loop of Henle. Urine is formed by filtration in the glomerulus and reabsorption and secretion in the tubules. The inner medulla of the kidney contains the collecting ducts, which merge together to form papillary ducts that enter into a calyx (plural "calyces") or funnel-shaped structure. The hilum is the part where the renal artery enters and renal vein and ureter leave the kidney.

The correct answer is antidiuretic hormone (ADH), also known as vasopressin. ADH controls the permeability of the collecting duct of the nephron to water. If ADH levels are high in the blood, more water will be reabsorbed into the bloodstream, concentrating the urine. Alternatively, if ADH levels are low, less water is reabsorbed into the bloodstream, and the urine is dilute.

Many desert animals have adapted physiologically with certain mechanisms to retain more water and survive the in a dry, desert environment. Such desert animals have long loops of Henle, allowing greater opportunity to reabsorb water in the medulla in the descending loop of Henle.