Renal tube of nephron -->collecting duct-->papillary duct-->minor calyces-->major calyces-->pelvis-->ureter-->urinary bladder -->urethra.

Passage of Urine from Kidneys to the Outside in Humans

The passage of urine from the kidneys to the outside in humans involves a series of structures and organs collectively known as the urinary system. It begins in the kidneys, where urine is produced, and ends at the external urethral opening.

1. Kidneys: The kidneys are vital organs responsible for filtering waste products, excess water, and other substances from the bloodstream to produce urine.

2. Renal Pelvis: Once urine is formed in the kidneys, it collects in the renal pelvis, a funnel-shaped structure that acts as a reservoir.

3. Ureters: From the renal pelvis, urine travels down the ureters, which are long muscular tubes that connect the kidneys to the urinary bladder. The ureters propel urine using peristaltic contractions, squeezing it towards the bladder.

4. Urinary Bladder: The urinary bladder is a hollow, muscular organ that stores urine until it is ready to be expelled. It expands as it fills with urine and contracts during urination to expel the urine out of the body.

5. Urethra: The urethra is a tube that connects the urinary bladder to the external urethral opening. In males, the urethra also serves as a passage for reproductive fluids, whereas in females, it is solely dedicated to the removal of urine.

Prevention of Urine Flowing Back into the Ureters:

To prevent the backward flow of urine, a mechanism called the ureterovesical valve, or the vesicoureteral valve, comes into play. This valve is located at the junction where the ureters enter the urinary bladder. Here's how it works:

1. Valve Structure: The ureterovesical valve consists of a muscular and elastic flap of tissue that forms a one-way valve. It allows urine to flow from the ureters into the bladder but prevents its backflow.

2. Passive Closure: When urine enters the bladder, the pressure within the bladder increases, causing the ureterovesical valve to close passively. This closure is facilitated by the elasticity of the valve tissue.

3. Preventing Reflux: The closed valve prevents urine from flowing back into the ureters by creating a barrier. This prevents the backward flow of urine, known as vesicoureteral reflux, which could potentially cause urinary tract infections and other complications.

4. Active Closure: Additionally, the ureterovesical valve is further supported by the muscular wall of the ureter, which contracts to close the valve actively during bladder contraction and urination. This ensures a more secure closure and prevents any residual urine from flowing back towards the kidneys.

In conclusion, urine passes from the kidneys to the outside in humans through a series of organs and structures, including the kidneys, renal pelvis, ureters, urinary bladder, and urethra. The backward flow of urine into the ureters is prevented by the ureterovesical valve, which closes passively due to increased bladder pressure and actively during bladder contraction. This mechanism ensures the unidirectional flow of urine, maintaining the integrity and functionality of the urinary system.