Unit 10: Excretion

Question 1

Excretion definition

Answer 1

Removal of metabolic wastes from the body; the useless and potentially dangerous byproducts of cellular reactions.

Question 2

Kidneys structure and function

Answer 2

Bean shaped, paired organs on the back region on either side of the vertebrae.

1. Produce and excrete urine, which contains nitrogenous wastes such as urea, uric acid, ammonium and creatine.
2. Maintain water-salt balance in blood
3. Maintain blood pH levels
4. Secrete hormones

Question 3

Ureters structure and function

Answer 3

Two small, muscular tubes connected to each kidney.

Transportation of urine from the kidney to the bladder.

Question 4

Urinary bladder structure and function

Answer 4

Located around the pelvic area with three openings: two for the ureters and one for the urethra.
Storage of urine until excretion.

Question 5

Urethra structure and function

Answer 5

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.

Question 6

Kidney anatomy

Answer 6

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.

Question 7

Blood supply to nephron

Answer 7

Each nephron has its own supply, with two capillary regions.

1. Afferent arteriole, brings blood to a knot of capillaries called
2. Glomerulus, which is surround by the
3. Bowman’s capsule.
4. Efferent arteriole leads out of the glomerulus and splits into the
5. Peritubular capillary networks that surrounds the rest of the nephron
6. After filtration, blood goes into the renal vein to return back to the heart

Question 8

Nephron anatomy

Answer 8

1. 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.
2. Proximal convoluted tubule (PCT) : lined with microvilli for reabsorption of water and other materials
3. Loop of nephron: U-turn of the PCT tube
4. Distal convoluted tubule (DCT): Have numerous mitochondria, but no microvilli. Move molecules from blood into the tubule, instead of absorption.
5. Collecting ducts: Many nephrons share a collecting duct, which carries urine to the renal pelvis.

Question 9

Urine formation: Pressure filtration

Answer 9

First step in urine formation that occurs at the glomerulus

1. Blood enters glomerulus through the afferent arteriole
2. Due to glomerular blood pressure, water, nutrients, and wastes are filtered into Bowman’s capsule
3. Large molecules like blood cells, platelets and plasma proteins remain in the blood as they are too big
4. Efferent arteriole carries out the blood, now with less water, nutrients and wastes

The fluid in the nephron is now called filtrate

Question 10

Urine formation: Selective reabsorption

Answer 10

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

Question 11

Urine formation: Tubular secretion

Answer 11

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.

Question 12

Reabsorption of salt and hormones that promotes it

Answer 12

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.

Question 13

Reabsorption of salt and hormone that inhibits it

Answer 13

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.

Question 14

Establishment of a solute gradient and loop of nephron

Answer 14

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.

Question 15

Reabsorption of water and hormone that promotes it

Answer 15

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.

Question 16

Maintenance of blood pH: buffers

Answer 16

Blood pH must be maintained near 7.4 for proteins and enzymes to function.
Acid-base buffer systems: Many buffers exist in the blood that can temporarily take up H or OH ions. Most important buffer is the combination of bicarbonate ions (HCO3) and carbonic acid (H2CO3). HCO3 takes up excess H ions to from H2CO3, while H2CO3 takes up excess OH ions to form HCO3 + H20.

Question 17

Maintenance of blood pH: breathing

Answer 17

Respiratory center: Increase in H ion concentration alerts the respiratory center to increase breathing rate. This exhales more CO2 (remember H + HCO3 -> H2CO3 -> H20 + CO2) and reduces the amount of H ions.

Question 18

Maintenance of blood pH: kidneys

Answer 18

Kidneys: Reabsorbs HCO3 and excretes H ions as needed to maintain pH. If blood is acidic, H ions will remain in the nephron system and excrete out in urine, and HCO3 are reabsorbed to act as buffer. If blood is basic, H ions will not excrete out and HCO3 will not be reabsorbed. Ammonia and phosphate are buffers that exist in the urine that maintains urine pH by removing H ions.