4.3 Excretion in humans

• Bladder: The bladder is a hollow, muscular organ located in the pelvis that serves as a reservoir for urine. Its main function is to store urine until it is expelled from the body.
• Excretion: Excretion is the process by which metabolic waste products, such as urea and excess salts, are removed from the body. In the context of the urinary system, excretion primarily refers to the elimination of urine containing these waste products.
• Kidneys: The kidneys are bean-shaped organs located on either side of the spine, near the middle of the back. They play a central role in filtering waste products from the blood to form urine. The kidneys also regulate electrolyte balance, blood pressure, and red blood cell production.
• Renal: "Renal" refers to anything related to the kidneys. For example, renal function refers to the kidney's ability to filter blood and produce urine.
• Urea: Urea is a nitrogenous waste product produced in the liver as a result of protein metabolism. It is excreted from the body in urine and plays a vital role in maintaining nitrogen balance.
• Ureter: The ureters are narrow tubes that connect each kidney to the bladder. Their primary function is to transport urine from the kidneys to the bladder using peristaltic contractions.
• Urethra: The urethra is a tube that carries urine from the bladder to the outside of the body during urination. In males, it also serves as the passageway for semen during ejaculation.
• Urine: Urine is a liquid waste product produced by the kidneys through the filtration of blood. It consists of water, urea, electrolytes, and other waste products. Urine is stored in the bladder until it is expelled from the body during urination.
• Nephron: A nephron is the functional unit of the kidney responsible for filtering blood and producing urine. Each kidney contains approximately one million nephrons.
• Filtrate: Filtrate is the fluid formed at the beginning of the nephron's journey through the kidney. It contains most of the substances dissolved in the blood plasma but lacks proteins and large molecules like glucose.
• The main job of the kidneys is to filter the blood and remove waste products like urea, which is made in the liver during deamination
• The kidneys also regulate the levels of ions like sodium and potassium, as well as water in the body
• Water regulation is crucial because cells can lose or gain water through osmosis, affecting their size and function
• Inside each kidney are around a million structures called nephrons, which help in the filtration and reabsorption of substances
• Kidney tubules absorb small substances like water, glucose, amino acids, and urea during filtration
• Selective reabsorption occurs where only the substances that the body needs are reabsorbed, while the rest forms urine
• Water levels in the body are regulated by the hypothalamus in the brain, which detects water concentration in the bloodstream
• If water levels are too low, the hypothalamus signals the pituitary gland to release antidiuretic hormone (ADH)
• ADH travels to the kidneys and instructs the tubules to reabsorb more water into the blood, reducing urine production
• If water levels are too high, the hypothalamus stops sending signals to the pituitary gland, leading to less ADH release
• Less ADH results in less water reabsorption, causing more water to stay in the tubules and increasing urine production
• This process of water regulation is an example of a negative feedback loop, where the body constantly adjusts water levels to maintain balance
• The three main roles of the kidneys are to:

  • Remove waste products like urea from the blood
  • Regulate the levels of ions in the blood
  • Regulate the amount of water in the blood
• Blood cycles through the kidneys, passing in through the renal arteries and out through the renal veins
• The kidneys produce urine from the blood, which passes down the ureters and is stored in the bladder until it is released through the urethra
• Filtration process in the kidneys:

  • Filtration occurs in the glomerulus, where some liquid part of the blood is forced into the Bowman's capsule
  • Only small substances like water, amino acids, urea, glucose, and ions can be filtered through to the Bowman's capsule
  • Only about 20% of the plasma in the blood is filtered through to the Bowman's capsule
• Selective reabsorption in the kidneys:

  • The kidneys reabsorb useful substances like glucose and amino acids back into the blood vessels
  • Water and ions are reabsorbed to some extent, while urea is not reabsorbed
  • This process is called selective reabsorption and occurs throughout the tubules
• The three main roles of the kidneys are to:

  • Remove waste products like urea from the blood
  • Regulate the levels of ions in the blood
  • Regulate the amount of water in the blood
• Blood cycles through the kidneys, passing in through the renal arteries and out through the renal veins
• The kidneys produce urine from the blood, which passes down the ureters and is stored in the bladder until it is urinated out through the urethra
• Only small substances like water, amino acids, urea, glucose, and ions can be filtered through to the Bowman's capsule
• Selective reabsorption is the process where the kidneys reabsorb useful substances like glucose and amino acids back into the blood vessels
• Fluid that passes out of the collecting duct is classified as urine, which then passes down the ureter to the bladder and can be released as waste
• The three main roles of the kidneys are to:

  • Remove waste products like urea from the blood
  • Regulate the levels of ions in the blood
  • Regulate the amount of water in the blood
• Blood cycles through the kidneys, passing in through the renal arteries and out through the renal veins
• The kidneys produce urine from the blood, which passes down the ureters and is stored in the bladder until it is urinated out through the urethra
• Filtration is where some of the liquid part of the blood is forced from the glomerulus into the Bowman's capsule
• Only small substances like water, amino acids, urea, glucose, and ions can be filtered through to the Bowman's capsule
• The kidneys reabsorb substances like glucose and amino acids back into the blood vessels
• Selective reabsorption is the process of reabsorbing only the substances that are needed back into the blood vessels