Urinary System        (updated 11/17/05)    A&P2

The following 31 illustrations should help you to learn and understand the structure and functions of the urinary system and its interactions with the body.

Urinary System - Be able to identify the following structures on illustrations, models, and/or microscope slides.
1. afferent arteriole
2. ascending limb of Henle
3. Bowman's (glomerular) capsule
4. collecting duct
5. cortex
6. descending limb of Henle
7. distal convoluted tubule
8. efferent arteriole
9. glomerulus
10. juxtaglomerular apparatus
11. major calyx
12. minor calyx
13. medulla
14. proximal convoluted tubule
15. renal capsule
16. renal column
17. renal papilla
18. renal pelvis
19. renal pyramids
20. ureter
21. macula densa
22. juxtaglomerular cells

The kidneys are among the most complicated of organs in terms of structure and physiology. They function by first filtering approximately 1/5 of the blood each minute, in which about 99% of the filtrate is then reabsorbed, and finally certain wastes are secreted. They regulate blood volume and pressure by controlling the amount of water and electrolytes conserved or eliminated. Osmolarity of body fluids is controlled by the amounts of solutes and water that are eliminated. They secrete the hormone renin, which helps control blood pressure and electrolyte balance, and the hormone erythropoietin, which helps produce red blood cells. They aid the lungs and buffers in the blood in regulating the acid-base balance of body fluids. They play a role in synthesizing calcitriol (vitamin D), thus contributing to calcium homeostasis. Each region of the kidneys contains specialized structures that contribute to each of these functions.  

Be able to trace the flow of blood to and through the kidneys, explaining the significance of the difference in diameter of the afferent and efferent arterioles..

Notice the difference in diameter of the afferent and efferent arterioles in the scanning EM below.

Scanning EM of glomerulus and capsule.

In this SEM, the pedicels and slit pores (fenestrations) of the podocytes are visible.

Both cortical (85%) nephrons (having short or no nephron loops) and juxtamedullary nephrons (15%), which possess long loops extending to the apex of the renal pyramids, are present. Both illustrations pictured below are juxtamedullary nephrons. Labeled structures in cortex.

Labeled structures in medulla.

Between the glomerular capsules is an interlobular artery. The proximal convoluted tubules (PCT) contain many microvilli extending into the lumen (and appear pink), while the distal convoluted tubules (DCT) have fewer and shorter microvilli (and appear to have a clear lumen).

Describe the functions of the juxtaglomerular apparatus, including the functions of the macula densa and juxtaglomerular cells. What is the role of this region in causing chronic hypertension and eventual renal failure? How do ACE inhibitors help to lower blood pressure?

    The kidneys are capable of renal autoregulation (as well as control of blood volume and fluid composition and volume throughout the body) through the specialized activities of the JGA. The juxtaglomerular cells control pressure within the afferent and efferent arterioles by dilating or constricting them. In response to decreasing blood pressure (or blood loss), these cells produce the enzyme renin, which removes 10 amino acids from angiotensinogen. These amino acids become angiotensin I. When AT I passes through the lungs and kidneys, Angiotensin Converting Enzyme (ACE, {kininase II}) removes two more amino acids, producing angiotensin II. Angiotensin II causes the zona glomerulosa of the adrenal cortex to produce aldosterone, and also stimulates the secretion of ADH, which increases water absorption in the DCT and collecting ducts.. Aldosterone increases Na+ absorption and K+ secretion, which increases water absorption, and thus increases blood pressure. AT II also stimulates widespread vasoconstriction throughout the body, which raises the mean arterial pressure, and also constricts both the afferent and efferent arterioles, which reduces glomerular filtration rate and water loss. Some of the newer treatments for hypertension involve ACE inhibitors, which block the conversion of AT I to AT II. Similar drugs function as AT II receptor antagonists to prevent aldosterone secretion.   

    Mesangial cells (not shown), found between the afferent and efferent arterioles, are connected to both the Macula Densa cells and JG cells by gap junctions, and probably communicate information between these cells. The kidneys produce the hormone erythropoietin, which increases production of red blood cells, aiding the increase of blood pressure. In kidney disease, a synthetic hormone E-poietin must be given to stimulate erythrocyte production. 

What is the difference in function of these two types of nephrons?

Be able to name the structures that comprise a nephron, and give the functions of each. Each nephron ends at a collecting duct. Several collecting ducts empty into larger papillary ducts, which drain into a minor calyx. Note that only the juxtamedullary nephrons contain vasa recta.

Concentrations of ions in plasma, glomerular filtrate, and urine. Nitrogenous wastes include ammonia (which is highly toxic), urea (a byproduct of protein catabolism, which is made in the liver from ammonia), uric acid (from catabolism of nucleic acids) and creatinine (from catabolism of creatine phosphate). These wastes are often referred to as blood urea nitrogen (BUN), which is normally between 7 and 18 mg/dL. An elevated BUN is called azotemia, and it may lead to uremia, which includes diarrhea, vomiting, and cardiac arrhythmia because of the toxicity of the nitrogenous wastes. The maximum rate of reabsorption of any substance into the renal tubules depends upon the number of transport proteins in the plasma membranes of tubular cells. The transport maximum is the maximum amount of a substance that can be reabsorbed in a given time. For glucose, this is about 320mg/min.

Structures within Bowman's (glomerular) capsule.

Glomerular filtrate fills the capsular space, and is formed by a net filtration pressure (NFP) of 10 mm Hg.   (Blood Hydrostatic Pressure = 60 mm Hg) - (Blood Colloid Osmotic P = 32 mm Hg + Capsular Hydrostatic Pressure = 18 mm Hg).  (60 - 50 = 10)

Many drugs that control hypertension function by blocking one of the steps in this process, thus decreasing the reabsorption of salt and water into the blood stream. ACE inhibitors block the Angiotensin Converting Enzyme (ACE).

Relationship of sodium, chloride, potassium, bicarbonate, and phosphate ions, and water absorption from kidney tubules into peritubular capillaries.

When all the pressures are accounted for, what is the approximate difference in pressure that allows filtration to occur?

What chemical mechanisms occur within the distal tubules to eliminate H+ in the urine? 

Which tissue in the distal tubules and collecting ducts is affected by ADH? 

Blood has an approximate 300 mOsm (milliosmolar) concentration; in the presence of aldosterone and ADH, the concentration of fluid in the loop of Henle and in the adjacent collecting tubules = 1200 mOsm. In hot weather, how will this influence urine production? In cooler weather, less aldosterone and ADH are produced, which will lower the filtrate and urine to about 70 mOsm. How will this influence urine production?

The role of the countercurrent mechanism in the vasa recta and loop of Henle in producing hypertonic urine..

Role of ADH in regulating urine concentration and volume. The renin-angiotensin reaction also plays a major role in this function.

Functions of the nephron components.

The mucous membrane from the minor calyces through the urinary bladder is stratified transitional epithelium.

Layers of the urinary bladder wall. The mucosa is what kind of tissue? The submucosa contains not only connective tissue but also blood vessels and nerves.

Posterior view of male urinary bladder, showing vas deferens, seminal vesicles, and prostate gland.

Male urinary bladder (anterior view) and urethra.

Photomicrograph of urethra in cross section.

Female urinary bladder and urethra.

Interactions of urinary system and other body systems.

Urinary System