Final answer:
The afferent arteriolar constriction in the glomerulus is caused by the myogenic mechanism and the tubuloglomerular feedback mechanism. The myogenic mechanism works by contracting smooth muscle cells in response to increased blood pressure, while the tubuloglomerular feedback mechanism involves specialized cells that release paracrine factors to constrict or dilate the afferent arteriole based on changes in glomerular filtration rate.
Step-by-step explanation:
The afferent arterioles of the glomerulus can constrict due to two main mechanisms: the myogenic mechanism and the tubuloglomerular feedback mechanism.
The myogenic mechanism works by contracting the smooth muscle cells in the wall of the arteriole when it is stretched due to increased blood pressure. This helps to maintain a constant flow rate of blood. When blood pressure drops, the smooth muscle cells relax to lower resistance, allowing for continued blood flow.
The tubuloglomerular feedback mechanism involves specialized cells called macula densa cells in the distal convoluted tubule (DCT). These cells respond to changes in fluid flow rate and sodium concentration. When the glomerular filtration rate (GFR) increases, the macula densa cells release paracrine factors like ATP and adenosine, which stimulate the afferent arteriole to constrict. This reduces GFR. Conversely, when GFR decreases, the release of ATP and adenosine decreases, allowing the afferent arteriole to dilate and increase GFR.