Learning Objectives
By the end of this section, you will be able to:
- Identify the function of the kidneys.
- Describe the path of blood flow through the kidney.
- Describe the types and functions of nephrons and describe each of the five components of a nephron.
- Describe the structure and function of glomerular capillaries.
- Describe the structure and function of the peritubular capillaries and vasa recta.
- Define glomerular filtration rate (GFR) and identify the major factors affecting GFR, including myogenic and tubuloglomerular regulation.
- Explain how the reninangiotensin mechanism affects GFR and blood pressure.
- Define tubular reabsorption and explain where it occurs and why it is necessary. Include the mechanism of sodium reabsorption and the process of obligatory water reabsorption.
- Explain how hormones can affect reabsorption in the distal convoluted tubule.
- Explain how ADH affects water reabsorption in the collecting duct.
- Define tubular secretion and identify the major portion of the nephron where it occurs and explain why tubular secretion is necessary.
- Describe the normal chemical composition of urine in terms of solvent, nitrogenous wastes, and other solutes.
- Describe the structure and function of the ureters.
- Describe the structure, and function of the urinary bladder.
- Describe the structure and functions of the urethra.
- Define micturition and describe the micturition reflex.
Figure 1. Sewage Treatment Plant (credit: “eutrophication&hypoxia”/flickr.com)
The urinary system has roles you may be well aware of: cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions played by the system. Take for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production, converting calcidiol to calcitriol, the active form of vitamin D.
If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis. The affected individual might experience weakness, lethargy, shortness of breath, anemia, widespread edema (swelling), metabolic acidosis, rising potassium levels, heart arrhythmias, and more. Each of these functions is vital to your well-being and survival. The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body. Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence.
This chapter will help you to understand the anatomy of the urinary system and how it enables the physiologic functions critical to homeostasis. It is best to think of the kidney as a regulator of plasma makeup rather than simply a urine producer. As you read each section, ask yourself this question: “What happens if this does not work?” This question will help you to understand how the urinary system maintains homeostasis and affects all the other systems of the body and the quality of one’s life.