Excretory System - 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).

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.

The correct answer is the glomerulus, a convoluted capillary bed that is directly adjacent to the nephron. Solutes from the blood are filtered in the glomerulus and enter the nephron, specifically, Bowman's capsule. Both blood pressure and oncotic force (pressure in the glomerulus due to proteins) affect filtration rate. Together, the glomerulus and Bowman's capsule are known as the renal corpuscle.

The glomerular basement membrane—which sits between capillary endothelial cells and the selectively permeable podocytes that line Bowman's capsule—is comprised of negatively charged proteins that repel other negatively charged proteins. This helps prevent these compounds from entering the nephron with the filtrate.

Sodium is coupled with glucose via a cotransporter, allowing it to enter the proximal tubule cells from the tubule lumen. Once in the proximal tubule cells, glucose then passes through a Glut-2 transmembrane carrier protein, but only on the basolateral membrane of the proximal tubule cells (from proximal tubule cells to the interstitual fluid).

Kidneys contain small structures called nephrons that function to create urine. Urine is created by filtering the incoming blood. The blood enters the nephron via the glomerulus. It enters the Bowman’s space and then is transported through a series of nephron segments. Each segment has varying permeabilities for ions and water. The nephron segment that is only permeable to water is the descending loop of Henle. As urine travels down this segment, water gets reabsorbed back into the blood whereas the solutes remain in the urine (because the descending loop of Henle is impermeable to solutes). This makes the urine very concentrated.

The ascending loop of Henle has opposite permeabilities. Solutes, such as ions, are permeable and can cross the wall and be reabsorbed into the blood; however, water is impermeable. The proximal convoluted tubule is the site of most of the reabsorption and secretion of molecules; therefore, both water and solutes are permeable here. The collecting duct is not part of the nephron. Multiple nephrons drain into a collecting duct. Collecting ducts are sites of further reabsorption and secretion; therefore, both water and ions are permeable.

Note that reabsorption is the process of moving substances (solutes or water) from urine to the blood whereas secretion is the process of moving substances from the blood to urine.

Blood from afferent arterioles enter the Bowman’s space via glomerulus. The filtered blood in the Bowman’s space traverses through different nephron segments before being excreted as urine. The glomerulus and Bowman’s space together are called the renal corpuscle; therefore, blood is first filtered here.

The urine gets processed (reabsorption or secretion) in different nephron segments before being ultimately excreted. During this process, the urine (also called tubular fluid) is transported via tubes called renal tubules; therefore, this is the site of reabsorption and secretion.

A nephron begins at the renal corpuscle and ends at the distal convoluted tubule. The renal corpuscle consists of the glomerulus and the Bowman’s capsule; therefore, the glomerulus is a part of the nephron. The collecting duct, on the other hand, is not part of a nephron. Multiple nephrons drain their tubular fluid into a single collecting duct. The tubular fluid is further filtered in the collecting duct and is ultimately excreted as urine. The distal convoluted tubule (last part of a nephron) is connected to the collecting duct.

Most of the reabsorption and secretion of ions and water happens in the proximal tubule, which is the nephron segment located right next to the Bowman’s space. This means that most of the fluid that enters the Bowman’s space is immediately processed in the proximal tubule. The latter third of a nephron also processes the fluid; however, it is negligible when compared to the proximal tubule.

Most of the urine is created in the nephron. The tubular fluid traverses each nephron segment, where it is processed. The final fluid that enters the collecting duct is processed further, but most of the urine that is excreted comes from the nephron.

Kidneys can be divided into two parts: renal cortex and renal medulla. The cortex is the outer part of the kidneys that contains the renal corpuscles (glomerulus and Bowman’s space) and most of the renal tubules (except the loop of Henle). The medulla is the inner part; it contains the ascending and descending limbs of the loop of Henle, and the portion of renal tubule that descends into the medulla. The proximal convoluted tubule and the distal convoluted tubule are not part of the loop of Henle and, therefore, are found in the renal cortex.

Note that the nephron begins at the renal corpuscle in the cortex, becomes the proximal tubule (in the cortex), descends into the medulla as loop of Henle, and rises back into the cortex as the distal tubules. The tubular fluid in distal tubules drains into the cortical collecting duct (the region of the collecting duct found in the cortex), which eventually enters the medullary collecting duct and leaves the kidney as urine.

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.