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The renal system, which is also called the urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. The renal system organs include the kidneys, ureters, bladder, and urethra. Metabolic wastes and excess ion are filtered out of the blood, along with water, and leave the body in the form of urine.
Osmoregulation: control of blood volume and body water content
Blood pressure homeostasis: the renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.
Regulation of acid-base homeostasis and blood pH, a function shared with the respiratory system.
Many of these functions are related to one another as well. For example, water follows ions via an osmotic gradient, so mechanisms that alter sodium levels or sodium retention in the renal system will alter water retention levels as well.
Organs of the Renal System
Kidneys are the most complex and critical part of the urinary system. The primary function of the kidneys is to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. The kidneys have extensive blood supply via the renal arteries which leave the kidneys via the renal vein.
Nephrons are the main functional component inside the parenchyma of the kidneys, which filter blood to remove urea, a waste product formed by the oxidation of proteins, as well as ions like potassium and sodium. The nephrons are made up of a capsule capillaries (the glomerulus) and a small renal tube. The resulting urine passes from the renal tube through tubes called ureters and into the bladder. The renal tube of the nephron consists of a network of tubules and loops that are selectively permeable to water and ions. Many hormones involved in homeostasis will alter the permeability of these tubules, changing the amount of water that is retained by the body.
The bladder is flexible and is used as storage until the urine is allowed to pass through the urethra and out of the body. The female and male renal system are very similar, differing only in the length of the urethra.
Kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), renin, aldosterone, and angiotensin I and II. A basic example is that a decrease in water concentration of blood is detected by osmoreceptors in hypothalamus, which stimulates ADH release from pituitary gland to increase the permeability of the wall of the collecting ducts and tubules in the nephrons. Therefore a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.
The extent of blood volume and blood pressure regulation facilitated by the kidneys is a complex process. Besides ADH secretion, the renin-angiotensin feedback system is critically important to maintain blood volume and blood pressure homeostasis.