- Describe how the excretion of salt and water serves the needs of the cardiovascular system.
- Name the major regulators of sodium excretion.
- Describe the systemic renin-angiotensin-aldosterone system and where its components are formed.
- State the major actions of angiotensin II.
- State the major actions of aldosterone.
- Describe the 3 major regulators of renin secretion.
- Describe the functions of the macula densa.
- Define tubuloglomerular feedback and describe the mechanism for tubuloglomerular feedback and autoregulation of glomerular filtration rate.
- State the origin of atrial natriuretic peptides, the stimulus for their secretion, and their effect on sodium reabsorption and glomerular filtration rate.
- State the 2 urinary factors that determine water excretion.
- Describe the origin of antidiuretic hormone and the major controls of its secretion.
Sodium and water excretion are regulated by an array of control systems. Seemingly every hormone, cytokine, sympathetic transmitter, and paracrine agent exerts an influence somewhere in the kidney. Given this complexity it might seem impossible to present a meaningful description of how sodium and water excretion are regulated. Certainly we cannot begin with each of the known regulated processes and predict their behavior in every situation. However, if we look at the goals of regulation—what regulation accomplishes—we can fit the various regulated processes into a framework of cooperative actions that serve the needs of the body.1
The overriding goals of regulating sodium and water excretion are to support the requirements of the cardiovascular (CV) system. This is manifested in 2 ways: (1) the kidneys maintain a sufficient ECF volume to fill the vascular space and (2) keep the osmolality of the ECF at a level consistent with cellular health. As explained in more detail in the next section, the kidneys and the CV system work cooperatively to ensure that peripheral tissue is sufficiently perfused. An adequate circulating volume is one of the essential requirements for tissue perfusion and it is the kidneys that control this volume. Osmolality is the ratio of solute content to water content. Sodium and chloride together account for 80% of the normal extracellular solute; thus the excretion of sodium and water by the kidneys regulates osmolality in the tight range that is needed for the health of tissue cells. These goals are another way of looking at the concept of balance that we described earlier. When the kidneys alter excretion of sodium and water to preserve proper amounts in the body, they are maintaining balance.
There is a separate goal of regulation that differs from those stated above. Variations in renal blood flow (RBF) and glomerular filtration rate (GFR) are major means of regulating sodium excretion. However, the kidneys cannot change blood flow and filtration to such extreme values that they compromise the metabolic health of the kidneys or interfere with the excretion of substances other than sodium, particularly organic waste. Thus, another goal is to limit the sodium-related changes in RBF and GFR that might otherwise reach deleterious levels.