13.1: Overview of the Renal (Urinary) System
- Page ID
- 100216
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The renal (urinary) system filters the blood, removes wastes, and helps maintain fluid and chemical balance in the body.
- List the organs of the renal system and state their primary roles.
- Describe how the renal system helps maintain a stable internal environment.
Structural Overview of the Renal System
The renal system — also commonly called the urinary system — is a group of only four organs:
- two kidneys,
- two ureters,
- one urinary bladder, and
- one urethra.
(See image on the right.)
Functional Overview of the Renal System
Beyond forming urine, the kidneys carry out several essential homeostatic roles that influence nearly every other body system:
-
Removal of Metabolic Wastes and Toxins
The kidneys filter the blood to eliminate nitrogenous wastes, drugs, and other toxins that the body cannot use or must keep at very low levels. -
Regulation of Blood Volume and Blood Pressure
By adjusting how much water and sodium are returned to the bloodstream, the kidneys help determine blood volume. Through the renin-angiotensin-aldosterone system (RAAS), they also influence vascular resistance. Together, these processes are central to maintaining stable blood pressure. -
Maintenance of Electrolyte and Metabolite Balance
Electrolytes such as sodium, potassium, calcium, and bicarbonate are constantly filtered and then selectively reabsorbed or excreted. This careful regulation supports nerve signaling, muscle contraction, and normal cellular metabolism. -
Control of Blood pH
The kidneys help stabilize the body’s acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate. This complements the work of the respiratory system and chemical buffers in the blood. -
Stimulation of Red Blood Cell Production
When oxygen levels drop, special peritubular cells in the renal cortex and outer medulla release erythropoietin (EPO), a hormone that signals the bone marrow to increase red blood cell production, improving the blood’s ability to carry oxygen. -
Activation of Vitamin D
The kidneys convert calcidiol into calcitriol, the active form of vitamin D. Calcitriol is required for calcium absorption in the intestines and plays a major role in bone health.
Although we often think of the kidneys as simple urine producers, it is more accurate to view them as regulators of plasma composition. By adjusting how much water and which solutes are returned to or removed from the bloodstream, the kidneys help maintain homeostasis and indirectly influence the function of every other body system.
Once urine is formed in the kidneys, the two ureters carry it from the kidneys to the urinary bladder through peristaltic movements. The urinary bladder stores urine until stretch receptors signal that it is time to empty. When the appropriate sphincters relax, urine flows out of the bladder and exits the body through the urethra.