Chapter 8: Excretion in Humans

Define the term excretion and explain its importance

Excretion is the removal of metabolic waste products, toxic substances, and substances in excess to the body’s requirements. Excretion is important to prevent the accumulation of metabolic waste products which in excess can be toxic

What are examples of metabolic waste products and what are their excretory organs?

1. carbon dioxide → lungs

2. urea → kidney

3. excess mineral salts → kidneys

4. excess water → kidneys

Explain the function of the glomerulus

Ultrafiltration occurs in the glomerulus. Small substances such as urea is filtered from the blood capillaries into the Bowman’s capsule.

Describe how the nephron is adapted for ultrafiltration

1. The diameter of the affarent arteriole is wider than the efferent arteriole, creating a high hydrostatic blood pressure in the glomerulus which forces out small substances such as urea into the Bowman’s capsule.

2. The glomerulus is covered by a filter, basement membrane, a thin partially permeable membrane that only allow soluble ions and molecules to filter through and blood cells and large proteins don’t filter into the capsule.

3. Walls of blood capillaries are one cell thick to provide short distance for small substances such as urea to be forced into the Bowman’s capsule

Describe how selective reabsorption occurs at proximal convoluted tubule

• Water is reabsorbed by osmosis

• Most mineral salts are reabsorbed by diffusion and active transport

• ALL glucose and amino acids are reabsorbed by active transport and are reabsorbed through walls of tubules into surrounding blood capillaries

Describe how selective reabsorption occurs at distal convoluted tubule

• Some water reabsorbed by osmosis and some mineral salts absorbed by active transport

Describe how selective reabsorption occurs at collecting duct

• Reabsorbs some water and osmosis (controlled by ADH)

Describe how selective reabsorption occurs at loop of Henlé

• Reabsorbs some water and mineral salts

What happens when water potential of blood plasma decreases below normal

When the water potential of blood plasma decreases, the pituitary gland secretes more ADH into blood plasma. ADH makes the cells of the collecting ducts more permeable to water and more water is reabsorbed from the collecting duct into blood capillaries. Thus, the volume of water decreases and urine produced is more concentrated (less water, more urea/mineral ions)

What happens when water potential of blood plasma increases above normal

When the water potential of blood plasma increases, the pituitary gland secretes less ADH into blood plasma. ADH makes the cells of the collecting ducts less permeable to water and less water is reabsorbed from the collecting duct into blood capillaries. Thus, the volume of water increases and urine produced is more diluted (more water, less urea/mineral ions)

Why is the kidney important?

• They secrete metabolic waste products

• They are osmoregulators that regulate water potential in the blood

Explain how a dialysis machine prevents the loss of plasma and red blood cells

Plasma proteins and red blood cells are too large to pass through the pores of the partially permeable dialysis membrane

Explain how a dialysis machine maintains normal glucose concentration

The dialysis fluid contains the same glucose concentration as the blood plasma. Thus, there is no net diffusion or concentration gradient for glucose to diffuse across the membrane.

Explain how a dialysis machine ensures efficient removal of metabolic waste products

1. The dialysis fluid does not contain metabolic waste products which sets up a concentration gradient that allows waste products to diffuse into dialysis fluid

2. The tubing in the machine is narrow and coiled to increase surface area to volume ratio for faster exchange

3. The direction of blood flow is opposite the flow of dialysis fluid to maintain a concentration gradient.