I. Function: maintains the constancy of the internal environment.

II. Organs

A. Kidneys

1. External structure

a. lima bean shaped; 11 x 7 x 3 cm.

b. cushioned by fat.

c. renal fasciae anchors kidneys to surrounding structures.

d. hilum: concave notch on medial surface through which blood vessels, lymphatic vessels, and ureter pass through.

e. renal sinus: hollow chamber at hilum.

f. capsule: tough white fibrous covering encasing kidney.

2. Location: posterior to the parietal peritoneum on either side of the vertebral column.

3. Internal structures

a. cortex: outer region with granular appearance. forms renal column, which dip between pyramids.

b. medulla: inner region.

1. renal pyramids: triangular wedges.

2. papilla: narrow projection facing hilum.

3. calyx: cup shaped structure at end of papilla.

c. renal pelvis: superior expanded end of ureter; funnel-shaped.

4. Function

a. remove metabolic wastes from blood; excrete them to the outside.

b. secrete erythropoeitin to help control rate of RBC formation.

c. secrete renin to help control blood pressure.

d. regulate volume, composition, and pH of body fluids.

B. Ureter

1. Structure: 28 cm long tube.

a. mucous coat: inner layer. forms a fold to act as a valve to prevent urine from backing into the ureter.

b. muscle layer: middle layer of smooth muscle.

c. fibrous coat: outer layer of connective tissue.

2. Location: behind parietal peritoneum, parallel to vertebral column.

3. Function: propels urine by peristalsis

C. Urinary Bladder

1. Structure

a. collapsible bag with three openings.

b. trigone: triangle shaped floor with openings at corners.

1. ureters: 2 openings in back.

2. urethra: 1

c. four layers of wall

1. mucous coat: transitional epithelium.

2. submucosa coat: connective tissue and elastic fibers.

3. detrusor muscle: smooth muscle of wall formed by criss-crossing bundles of fibers.

4. serous coat: upper surface of bladder is parietal peritoneum. other surfaces are covered by fibrous connective tissue.

2. Location: behind symphysis pubis beneath parietal peritoneum

3. Function

a. reservoir for urine

b. expels urine from body

D. Urethra

1. Structure: small mucous-membrane lined tube with a thick layer of smooth muscle.

2. Location

a. in females, behind symphysis pubis, anterior to vagina. 3 cm.

b. in males, winds 20 cm passing through prostate gland, joined by 2 ejaculatory ducts, extends down, forward, then up to the base of the penis; travels through the center of the penis to the tip.

3. Function: conveys urine from bladder to exterior

III. Nephrons

A. Bowman's capsule: cup-shaped expansion of the renal tubule in the cortex.

1. Structure: 2 layers of epithelial cells with space between them.

2. Function: allows for filtration

B. Glomerulus: fine capillary network found in the cortex.

C. Proximal convoluted tubule: highly coiled tube leading from the capsule. made of one layer of epithelial cells.

D. Loop of Henle: pct dips to form the descending limb (thin), then turns, and returns toward the renal corpuscle, forming the ascending limb (thick). located in the medulla.

E. Distal convoluted tubule: highly coiled, thick walled tube in the cortex.

F. Collecting tube: straight tubule joined by distal tubules of several nephrons. merge with other collecting ducts passing through the medulla to converge in calyx and empty into renal pelvis.

IV. Blood Vessels

A. Renal artery branches from aorta and enters kidney through the hilum.

1. interlobar arteries: between pyramids of medulla.

2. arcuate arteries: arch over pyramid bases.

3. interlobular arteries: penetrate cortex.

4. afferent arterioles: branch into glomeruli.

5. glomerular capillaries: inside Bowman's capsule.

6. efferent arterioles: lead away from glomeruli.

7. peritubular capillaries: capillaries around tubules.

8. vasa recta: loops down into medulla and back to supply loop of Henle and collecting ducts

B. Renal vein: formed by converging venules; empties into inferior vena cava.

1. interlobular veins

2. arcuate veins

3. interlobar veins

V. Function of Kidneys

A. Urine formation

1. Filtration

a. movement of water and solutes from plasma in the glomerulus into Bowman's capsule.

b. filtration rate is affected by diameters of afferent and efferent arterioles and changes in systemic blood pressure.

2. Reabsorption: movement of molecules out of renal tubule and into peritubular blood, changing fluid composition.

a. proximal tubule:

1. Na⁺: active transport

2. Glucose, amino acids: facilitated diffusion

3. Cl^-: due to the imbalance in electrical charge.

4. H₂O: osmosis

5. urea: passively leaves.

b. loop of Henle: counter current exchange keeps solute concentration of medulla extremely high.

1. descending loop: thinner wall; permeable to water and urea.

a. H₂O reabsorbed from tubule fluid

b. picks up urea from ECF

2. ascending loop: thicker wall; impermeable to water, Na⁺, Cl^-, and urea. reabsorbs Na⁺, Cl^- from tubule fluid.

a. distal tubule: reabsorbs some Na⁺ by active transport. impermeable to water.

b. collecting duct: generally impermeable to water; urea diffuses out due to water leaving.

3. Tubular Secretion: movement of molecules out of peritubular capillary into tubule for excretion. distal and collecting tubules receive K⁺, H⁺, NH₄^-, descending limb receives urea

B. Regulation of Urine Volume

1. Antidiuretic Hormone: controls solute concentration of urine. reduces water loss.

2. Aldosterone: decreases urine volume, conserves water by increasing distal and collecting tubule absorption of Na+.

3. Atrial natriuretic hormone: promotes Na+ loss.

C. Urine Composition

1. 95% water

2. nitrogenous waste: urea, uric acid, ammonia

3. electrolytes

4. toxins

5. pigments

6. hormones

7. abnormal substances: blood, glucose, albumin, casts, calculi

D. Micturition

1. contraction of detrusor muscle, aided by contraction of abdominal wall muscles, and relaxation of external urethral sphincter

2. Path of urine

a. nephrons

b. collecting ducts

c. calyces

d. renal pelvis

e. ureter

f. urinary bladder

g. urethra

anatomy page