Vertebrate kidney structure and functionap biology chapter 44

Human Excretory System Kidneys (2) Ureters (2) Urinary bladder Urethra

Nephron Functional unit of the kidney 3 main jobs:

Filtration of water and solutes from blood

Reabsorption of water and molecules back into blood or interstitial fluid

Secretion of ions and other waste products from capillaries

Remember: the idea is to concentrate the fluids (filtrate) in the tubes of the kidney to produce urine that has higher osmolarity than that of blood

Filtration: Glomerulus Bowman’s capsule

Reabsorption Proximal convoluted tubule

Reabsorbs 75% of the water, salts, glucose, and amino acids

Reabsorption/Secretion Loop of Henle

Countercurrent exchange, which maintains the gradient

Distal convoluted tubule Tubular secretion of H ions, potassium, and certain

drugs

Overview: Flow of Filtrate Bowman’s capsule proximal

convoluted tubule descending loop of Henle ascending loop of Henle distal convoluted tubule collecting duct renal pelvis ureter bladder urethra

Urine production through filtration, secretion, and reabsorption 1. Blood pressure in

glomerulus forces fluid into Bowman’s capsule (BC) [example of bulk flow!] Blood is now called filtrate

and is composed of glucose, salts, vitamins, nitrogenous wastes (urea)

The osmolarity of blood and filtrate are about equal at this point since both water and salt are removed

2. As the blood flows from BC to proximal convoluted tubule (PCT) solutes are reabsorbed from the tubule Water and salts are

reabsorbed back into interstitial fluid

Ammonia, drugs, ions (H+) are secreted into tubule

Some ions are actively transported

Water flows passively out of PCT

3. From the PCT filtrate moves down the descending loop of Henle Water is reabsorbed by the

interstitial fluid out of the tubule This section is permeable to

water but not to solutes As water is reabsorbed, the

concentration within the tubule increases and is at its maximum at the elbow of the loop

4. As the filtrate makes the turn at the bottom of the loop, the epithelium is change and becomes permeable to salt but not to water Salt is transported out of the

ascending loop of Henle, which makes the interstitial fluid on the inner medulla of the kidney very saline

The filtrate within the loop is less concentrated

Through the thick portion of the loop, salt is actively pumped out and the filtrate continues to decrease in osmolarity

5. Filtrate enters the distal convoluted tubule (DCT) where ions are secreted into the tubule; water and bicarbonate are reabsorbed by the interstitial fluid Concentration of the filtrate is at its

lowest point along the DCT

6. From the DCT the filtrate again turns toward the inner medulla down through the collecting duct Water is reabsorbed and the filtrate

becomes increasingly more concentrated

Interstitial fluid increases in osmolarity from the cortex to the inner medulla so what is constantly diffusing out of the tube back into the interstitial fluid

This creates a urine that is hypertonic to most of the body fluids (since urea is present), becoming more concentrated as the water diffuses out of the duct

Hormone Control Antidiuretic

Any chemical that prevents excessive urine production Example: antidiuretic

hormone (ADH) Diuretic

Any chemical that increases urine production Example: coffee, tea,

alcohol Alcohol inhibits secretion

of ADH, increasing urine production