Renal blood flow

The kidneys receive a disproportionate fraction of cardiac output. Although the combined weight of the kidneys accounts for less than 1% of total body weight, these organs receive 20 to 25% of the cardiac output. This magnitude of blood flow, which is in profound excess to their metabolic needs, enables them to carry out their multiple homeostatic functions more efficiently. Assuming a resting cardiac output of 5 l/min, the renal blood flow (RBF) is approximately 1.1 l/min.

Renal blood flow has a direct effect on GFR, which in turn has a direct effect on urine output. As RBF increases, GFR and urine output increase. Conversely, as RBF decreases, GFR and urine output decrease. Furthermore, any change in urine output affects plasma volume and blood pressure. Therefore, the regulation of RBF and GFR are important considerations. According to Ohm's law (Q = DP/R), RBF is determined by mean arterial pressure (MAP) and the resistance of the afferent arteriole (Rf art):

R ff t art art

Autoregulation. The equation for RBF predicts that an increase in MAP will increase blood flow through the kidneys and a decrease will decrease blood flow through them. Physiologically, this response is not always desired. For example, during exercise, MAP increases in order to increase blood flow to the working skeletal muscles. A corresponding increase in RBF would lead to an increase in GFR and an undesired loss of water and solutes in the urine. On the other hand, a profound decrease in MAP could decrease RBF and GFR. In this case, elimination of wastes would be impaired. Maintaining a constant RBF and GFR, even when MAP changes, is advantageous in some physiological conditions.

Interestingly, RBF remains relatively constant when MAP changes in the range of 85 to 180 mmHg. This ability to maintain a constant blood flow in spite of changes in MAP is referred to as autoregulation. The mechanism of autoregulation involves corresponding changes in the resistance of the afferent arteriole. For example, when MAP increases, the resistance of the afferent arteriole increases proportionately so that RBF remains unchanged. It is important to note that the major site of autoregulatory changes is the afferent arteriole. As this arteriole constricts, glomerular capillary pressure and therefore the GFR are reduced toward their normal values.

Autoregulation of RBF is an intrarenal response. In other words, the mechanisms responsible for autoregulation function entirely within the kidney and rely on no external inputs. Two mechanisms elicit this response:

• Myogenic mechanism

• Tubuloglomerular feedback

Myogenic mechanism. As discussed in Chapter 16 on the circulatory system, the myogenic mechanism involves contraction of vascular smooth muscle in response to stretch. For example, an increase in MAP would tend to increase RBF, leading to an increase in pressure within the afferent arteriole and distension, or stretch, of the vessel wall. Consequently, the vascular smooth muscle of the afferent arteriole contracts, increases the resistance of the vessel, and decreases RBF toward normal.

Tubuloglomerular feedback. Tubuloglomerular feedback involves the activity of the juxtaglomerular apparatus (see Figure 19.1). This structure is located where the distal tubule comes into contact with the afferent and efferent arterioles adjacent to the glomerulus. The juxtaglomerular apparatus is composed of the following:

• Granular cells

The macula densa consists of specialized cells of the distal tubule adapted to monitor GFR. In other words, they are sensitive to changes in the rate of filtrate flow through the distal tubule. Granular cells are specialized smooth muscle cells of the arterioles, in particular the afferent arteriole. These cells are adapted to monitor RBF — they are sensitive to changes in blood flow and blood pressure in the afferent arteriole. As such, they are also referred to as intrarenal baroreceptors. It is the granular cells of the juxtaglomerular apparatus that secrete renin. Further discussion of granular cell function is found in a subsequent section. Tubuloglomerular feedback involves the function of the macula densa. This mechanism may be summarized with the following example in which MAP increases:

T Mean arterial pressure

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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