Unit 10: Excretion Flashcards
Excretion definition
Removal of metabolic wastes from the body; the useless and potentially dangerous byproducts of cellular reactions.
Kidneys structure and function
Bean shaped, paired organs on the back region on either side of the vertebrae.
- Produce and excrete urine, which contains nitrogenous wastes such as urea, uric acid, ammonium and creatine.
- Maintain water-salt balance in blood
- Maintain blood pH levels
- Secrete hormones
Ureters structure and function
Two small, muscular tubes connected to each kidney.
Transportation of urine from the kidney to the bladder.
Urinary bladder structure and function
Located around the pelvic area with three openings: two for the ureters and one for the urethra.
Storage of urine until excretion.
Urethra structure and function
Small tube that extends from the urinary bladder to an external opening. Varies in length from male and female.
Function is to remove urine from the body, in males, it carries urine during urination and sperm during ejaculation.
Kidney anatomy
Kidney is composed of three sections:
1. An outer, grainy region called the renal cortex
2. Lined layer called the renal medulla
3. Inner cavity called the renal pelvis
The cortex and medulla houses the nephrons, which cleanses the blood of wastes. The pelvis is continuous with the ureter, and collects urine.
Blood supply to nephron
Each nephron has its own supply, with two capillary regions.
- Afferent arteriole, brings blood to a knot of capillaries called
- Glomerulus, which is surround by the
- Bowman’s capsule.
- Efferent arteriole leads out of the glomerulus and splits into the
- Peritubular capillary networks that surrounds the rest of the nephron
- After filtration, blood goes into the renal vein to return back to the heart
Nephron anatomy
- Bowman’s capsule; closed-end of the nephron that surrounds the glomerulus. Inner layer of the capsule is composed of extensions called podocytes that cling onto the capillary and allow passage of small molecules to the inside of the capsule.
- Proximal convoluted tubule (PCT) : lined with microvilli for reabsorption of water and other materials
- Loop of nephron: U-turn of the PCT tube
- Distal convoluted tubule (DCT): Have numerous mitochondria, but no microvilli. Move molecules from blood into the tubule, instead of absorption.
- Collecting ducts: Many nephrons share a collecting duct, which carries urine to the renal pelvis.
Urine formation: Pressure filtration
First step in urine formation that occurs at the glomerulus
- Blood enters glomerulus through the afferent arteriole
- Due to glomerular blood pressure, water, nutrients, and wastes are filtered into Bowman’s capsule
- Large molecules like blood cells, platelets and plasma proteins remain in the blood as they are too big
- Efferent arteriole carries out the blood, now with less water, nutrients and wastes
The fluid in the nephron is now called filtrate
Urine formation: Selective reabsorption
Second step in urine formation that occurs at the PCT
- PCT walls are lined with villi and contain many mitochondria for the active transport of essential materials back into the blood
- Water cannot be actively transported, thus Na ions are actively transported, and Cl ions passively follow
- The presence of NaCl in the blood creates osmotic pressure as most water diffuse back into blood
- With the help of carrier proteins, nutrients like glucose/amino acids are actively transported back
- Carrier protein has a capacity; not all nutrients are absorbed back, creating osmotic pressure in the filtrate that leaves some water in the nephron
The fluid in the nephron is now called tubular fluid
Urine formation: Tubular secretion
Third step in urine formation that occurs at the DCT
Through active transport, unwanted substances in the blood, such as excess H ions, histamine, penicillin, and ammonia are secreted into the tube.
The fluid in the nephron is now substances that undergone pressure filtration, have not been reabsorbed, and undergone tubular secretion: urine.
Reabsorption of salt and hormones that promotes it
Reabsorption of water occurs all along the nephron
First, the reabsorption of salt is required, which is regulated by several hormones.
Aldosterone: secreted by adrenal cortex; promotes excretion of K ions and the reabsorption of Na ions.
Renin: secreted by juxtaglomerular apparatus (region of contact between afferent arteriole & DCT); converts angiotensinogen into angiotensin, which stimulates the adrenal cortex to release aldosterone
The following hormones promote the reabsorption of sodium ions, which is followed by reabsorption of water. Therefore, the hormones increases blood volume and blood pressure so that pressure filtration is possible.
Reabsorption of salt and hormone that inhibits it
Atrial natriuretic hormone (ANH): secreted by the atria of the heart when cardiac cells are stretched due to increased blood volume; inhibits the secretion of aldosterone and renin, resulting in less salt absorption = less water absorption.
Ultimately, ANH decreases blood volume and pressure, while promoting excretion of more sodium called natriuresis along with more water.
Establishment of a solute gradient and loop of nephron
Once salt is reabsorbed, a solute gradient is established that creates an osmotic pressure to diffuse water from the nephron into the capillaries.
Loop of nephron is specialized to create a large osmotic gradient; the ascending limb is impermeable to water, but actively secretes salt out. This along with urea leakage at the collecting duct contributes to a high solute concentration of blood and water is reabsorbed at the descending limb of loop of nephron and the collecting duct.
Reabsorption of water and hormone that promotes it
Antidiuretic hormone (ADH): secreted by the posterior pituitary (at the brain) when blood concentration is increasing - not enough water. High blood concentration is detected in the hypothalamus that sends a message to the posterior pituitary to secrete ADH. Increases the permeability of water at the DCT and the collecting duct. Does this by increasing the number of channels through water can pass through. Ultimately, ADH increases blood volume and pressure.