Kidneys and associated organs:
- Kidneys are connected to ureters and subsequently to the bladder and urethra.
- Ureter connects the kidneys to the bladder.
- Urethra connects the bladder to the external environment.
- Blood enters the kidney through the renal artery and leaves through the renal vein.
- The renal artery branches into arterioles and then into capillaries.
- These capillaries form many minute knots called glomeruli, which are contained inside a kidney tubule called a nephron.
- There are two distinct regions in the kidneys: medulla and cortex.
  - Medulla in Latin means “middle” whereas cortex in Latin means “outer”.
  - This is where urine is produced.
- The urine will then go to the renal pelvis, which acts like a funnel to direct it to the ureter and then into the bladder.

Functions of kidneys:
- Kidney produces urine.
- Urine contains excess water, ions, and urea.
  - Urea is toxic to our body in high amounts.
  - Thus, the kidneys excrete urea from the body as part of urine.

Functions of different parts of nephron:
- Firstly, blood enters the glomerulus, where ultrafiltration takes place.
  - Ultrafiltration is a process to filter out molecules based on size.
  - This process filters out smaller molecules while retaining the larger ones. Hence, large molecules such as proteins are not filtered out.
  - On the other hand, smaller molecules such as water, ions, urea, glucose, and amino acids are filtered out.
- After ultrafiltration, selective reabsorption takes place in the tubules.
  - In this process, all glucose molecules and most amino acids are reabsorbed back into the blood.
  - Likewise, some of the ions and most of the water are also reabsorbed to ensure an optimal concentration in the blood.
- Finally, the loop of Henle and collecting duct play a vital role in salt and water regulation (osmoregulation).
- All these parts of the nephron lead to the formation of urine containing urea, excess water, and excess ions.

Factors affecting urine production:
- Urine production is primarily affected by water intake, temperature, and physical activity.
  - When you drink more water, there is an excess amount of water in your body.
    - This makes your blood more dilute.
    - Thus, the kidneys reabsorb less water back into the body.
    - This means more urine is produced.
- Likewise, if you exercise and sweat more, or during a hot day, you tend to lose more water from the body.
  - The kidneys will then reabsorb more water back into the body.
  - This means less urine is produced.
- On the other hand, during a cold day, you do not sweat or sweat less, which means more water will be retained in the body.
  - Hence, more urine is produced.

13.02 - Kidneys

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