Why someone who is haemorrhaging with falling systemic BP and volume has a decreased urine output?
A decreased urine output in a person who is hemorrhaging with falling systemic blood pressure (BP) and volume is primarily due to the body's compensatory mechanisms to maintain blood pressure and preserve vital organ perfusion. Here are the physiological processes that contribute to decreased urine output in this scenario:
1. Activation of the Renin-Angiotensin-Aldosterone System (RAAS):
- Hemorrhage and the subsequent decrease in circulating blood volume activate the RAAS.
- Angiotensin II, a key hormone in the RAAS, causes vasoconstriction, which increases systemic vascular resistance and helps maintain BP.
- Angiotensin II also stimulates the release of aldosterone from the adrenal glands.
- Aldosterone promotes sodium and water reabsorption in the kidneys, leading to fluid retention and a decrease in urine output.
2. Increased Sympathetic Nervous System Activity:
- Hemorrhage triggers an increase in sympathetic nervous system activity.
- This activation leads to vasoconstriction in the renal vasculature, reducing blood flow to the kidneys.
- Decreased renal blood flow impairs the kidneys' ability to filter and produce urine, resulting in reduced urine output.
3. Antidiuretic Hormone (ADH) Release:
- The decrease in blood pressure and volume stimulates the release of ADH (also known as vasopressin) from the posterior pituitary gland.
- ADH acts on the kidneys to increase water reabsorption in the collecting ducts, reducing urine output and conserving body water.
4. Impaired Glomerular Filtration Rate (GFR):
- Severe hemorrhage and the associated hypotension can lead to a decrease in GFR, which is the rate at which blood is filtered by the kidneys.
- When systemic BP drops significantly, the pressure gradient necessary for efficient glomerular filtration is compromised.
- Reduced GFR results in the formation of less filtrate, leading to decreased urine production.
5. Myoglobinuria and Tubular Damage:
- In cases of severe hemorrhage and hypotension, muscle cells may break down, releasing myoglobin into the bloodstream.
- Myoglobin can damage the renal tubules, further impairing urine production.
It's important to note that decreased urine output in the context of hemorrhage and hypotension is a protective mechanism to maintain circulating blood volume and BP. However, if the underlying cause is not promptly addressed, it can progress to acute kidney injury (AKI) and further complications. Therefore, individuals with these symptoms require immediate medical attention and treatment to restore fluid balance and prevent kidney damage.