Welcome to this lesson on the urinary system. Today you are going to be learning about how the body maintains homeostasis of its body fluids. Specifically, you will learn about:
The urinary system helps to maintain homeostasis of extracellular fluids. Remember, extracellular fluids are fluids found on the outside of cells, such as blood. Extracellular fluids, have a tendency to change as your body takes in water and solutes or loses water and solutes. In order to maintain homeostasis, your body has to try and maintain these extracellular fluids at a certain level, concentration, and pH.
Extracellular Fluid
Fluid of the body that is contained outside of cells. Plasma is an example of extracellular fluid outside of blood cells.
The intake of water and solutes comes primarily from food and liquid that is ingested. Intake can also be through respiration or through metabolism. In summation, these are all ways that water and solutes can be added to your system. Water and solutes are primarily lost via urine, feces, sweat, and evaporation.
The kidneys play a really important role in homeostasis of these extracellular fluids by forming urine. Urine is produced and excreted by the body and hormones also play a role in this process of urine formation and extracellular fluid homeostasis.
The kidneys can conserve or excrete fluids as necessary which means they can produce concentrated or diluted urine. If there needs to be more water in extracellular fluid, for example in dehydration, then the kidneys will produce more concentrated urine by holding in more water.
Nephrons are structures in the kidneys that filter water and solutes from blood and produce urine. Nephrons span the cortex and medulla of the kidneys and produce urine through the three following steps: filtration, reabsorption, and secretion.
Filtration is the first step in urine formation, and it takes place within Bowman's capsule. Here, water and solutes are filtered out of the blood through glomerular capillaries within the Bowman's capsule.
Reabsorption, which is the second step in urine formation, takes place mostly in the proximal tubule, which comes off of Bowman's capsule. This is where needed minerals and the correct amount of water are returned to the blood.
Secretion, which is the third step in urine formation, starts at the proximal tubule and continues along the loop of Henle and continues through the distal tubule. Once urine has formed, it will move into the collecting duct and the collecting duct will deliver that urine into the renal pelvis of the kidney. From here it will filter down into the ureters and then down into the urinary bladder where, it's stored until it's excreted.
Filtration
The first step in urine formation in which urine is filtered from afferent arterioles in the Bowman's capsule of a nephron.
Reabsorption
The second step in urine formation in which valuable water and solutes are reabsorbed back into the blood.
Secretion
The third step in urine formation in which filtrate (waste) moves toward collecting ducts; secretion helps maintain the body's acid-base balance.
Filtration is the first step in urine production; it's purpose is to separate blood plasma from blood cells and platelets so that dissolved waste can be filtered from the plasma before it is returned to the blood. Filtration only occurs within a specific part of the nephron: the Bowman's capsule. During filtration blood enters the capillaries within Bowman's capsule, called the glomerulus (the plural is glomeruli), through the afferent arteriole. The afferent arteriole is narrow compared to other blood vessels within the body. As a result, blood is delivered towards the nephron through the afferent arteriole at a high pressure due to the more narrow diameter these arterioles have. This high pressure will force water and solutes from the blood into the Bowman's capsule of the nephron.
The solutes that are forced out of the blood include: glucose, sodium, urea, vitamins, amino acids, et cetera. Water and solutes that are pushed out of the blood would then flow into the proximal tubule where the next stage of urine formation (reabsorption) occurs.
Larger solutes in the blood, such as blood cells or platelets, will stay in the blood, and follow the arteriole as it weaves around the nephron.
Bowman's Capsule
A section of a nephron where afferent arterioles deliver blood to the nephron for water and solutes to be filtered out.
Glomerulus
A tangle of blood vessels in the bowman's capsule where water and solutes are forced out of blood and into the bowman's capsule to begin the next stage of urine formation.
Reabsorption is the second step in urine formation and reabsorption is also sometimes referred to as tubular reabsorption. In this step of urine formation water and solutes that have been filtered from blood in the first step (filtration) move throughout the rest of the nephron. Water and other solutes sometimes are very valuable and our body needs to retain them. These water and solutes need to be reabsorbed back into blood so the body can retain them.
Reabsorption takes place primarily across the walls of the proximal tubules. The walls of the proximal tubule are very thin, about one cell thick. Solutes and water will leak out or be pumped out of nephron tubules and will then be reabsorbed into peritubular capillaries.
Peritubular capillaries interweave within the nephron and from there, they will return solutes to the bloodstream via the renal vein. Any remaining solutes that weren't reabsorbed from the nephron tubules into the bloodstream will become a part of urine.
Peritubular Capillaries
Capillaries associated with nephrons which allow water and solutes to be exchanged between the nephron and the blood during urine formation.
This step of reabsorption returns the vast majority of water and solutes, such as glucose, amino acids, and sodium back to the blood. Between 95 to 100% of those solutes get returned back into the blood because our body needs them. Any remaining solutes that do not get returned back to the blood will then become a part of urine.
Tubular secretion starts at the proximal tubule; it occurs along all the tubular parts of the nephron. Substances such as hydrogen, potassium and urea will move from the peritubular capillaries into nephron tubules, joining with the urine that is forming.
In reabsorption, water and extra solutes that haven't been reabsorbed back into the blood will become a part of urine. Hydrogen, potassium, and urea from the peritubular capillaries will join with them, forming urine. Urine will then move toward the collecting duct, which will then move the urine into the ureters, and then the urinary bladder.
Secretion helps to maintain the body's electrochemical balance, and makes sure that substances don't build up in the blood. It starts at the proximal tubule and will work its way down through the Loop of Henley and distal tubule.
To better understand, take a look at the diagram below:
Here have the kidney and the kidney is where the nephrons are located. The nephrons produce urine and carry it to the collecting ducts. Collecting ducts move urine to the renal pelvis of the kidney, which funnels it into the ureter. Ureters will funnel that urine down to the urinary bladder where it will be stored until it's excreted through the urethra. The urethra connects the urinary bladder to the outside, as urine is excreted. When urine is expelled from the urinary bladder through the urethra to the outside, we call that urination, which is a reflex response.
Urination
A reflex that occurs when the urinary bladder fills to capacity and is stimulated to empty itself.
As the bladder fills with urine that it's storing, the tension across the muscles of the urinary bladder will increase, causing the internal sphincter to relax. The bladder will then contract, or shorten, forcing urine out through the urethra.
This lesson has been an overview on the urinary system and how the urinary system contributes to maintaining homeostasis of extracellular fluid. Specifically, you looked at the process of intaking and losing water and solutes, as well as the role nephons in filtration, reabsorption and secretion.
Keep up the learning and have a great day!
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND
