How many times a molecule of urea have to pass through the heart in its journey from liver to kidney ?

Options

A. 0
B. 1
C. 2
D. 4

Answer

To determine how many times a molecule of urea passes through the heart on its journey from the liver to the kidney, we can break down the process step by step:

### Step 1: Formation of Urea in the Liver
- Urea is formed in the liver from the breakdown of amino acids through the urea cycle. This is the starting point of the journey for the urea molecule.

**Hint:** Remember that urea is produced in the liver as a result of protein metabolism.

### Step 2: Urea Enters the Hepatic Vein
- Once urea is formed in the liver, it enters the hepatic vein. This is the first step where the urea molecule leaves the liver.

**Hint:** The hepatic vein is responsible for transporting blood away from the liver.

### Step 3: Urea Reaches the Right Side of the Heart
- The hepatic vein drains into the inferior vena cava, which then delivers the blood (containing urea) to the right atrium of the heart. This is the first time the urea passes through the heart.

**Hint:** The right atrium collects deoxygenated blood from the body, including the urea from the liver.

### Step 4: Urea Moves to the Lungs for Oxygenation
- From the right atrium, the blood moves to the right ventricle and then is pumped into the pulmonary artery, leading to the lungs where it gets oxygenated. This is not a direct passage of urea but part of the blood circulation process.

**Hint:** The right ventricle pumps deoxygenated blood to the lungs.

### Step 5: Oxygenated Blood Returns to the Left Side of the Heart
- After oxygenation in the lungs, the blood returns to the heart via the pulmonary veins, entering the left atrium. This is the second time the urea molecule is indirectly associated with the heart.

**Hint:** The left atrium receives oxygenated blood from the lungs.

### Step 6: Blood is Pumped to the Aorta
- From the left atrium, the blood moves into the left ventricle, which then pumps the oxygenated blood into the aorta. This blood, now containing urea, enters systemic circulation.

**Hint:** The left ventricle is responsible for pumping oxygenated blood to the rest of the body.

### Step 7: Urea Reaches the Kidneys
- The aorta branches into the renal arteries, which carry the blood (containing urea) to the kidneys for filtration and excretion.

**Hint:** The renal arteries are the vessels that supply blood to the kidneys.

### Conclusion
In summary, the molecule of urea passes through the heart **twice** during its journey from the liver to the kidney: once when it enters the right side of the heart and once when it exits from the left side of the heart.

**Final Answer:** The molecule of urea passes through the heart **2 times**.