Excretion in Humans (chapter 8)

Metabolic Waste Products

• Carbon dioxide

• Urea

• Mineral salts or Ions

• Water

Excretion definition

Excretion is the removal of metabolic waste products, toxic substances and substances ion excess of the body’s requirement

Blood capillaries around the nephron

1. The small branch of the renal artery called the afferent arteriole carries blood into the glomerulus

2. The glomerulus is a knot of blood capillaries in the Bowman’s capillaries

3. An efferent arteriole transports blood away from the glomerulus.

4. The blood continues into the blood capillaries surrounding the nephron . These blood capillaries eventually lead into a branch of renal vein.

Ultrafiltration

• The afferent arteriole being wider than the efferent arteriole creates a high blood pressure in the glomerulus.

• Blood plasma is forced out of the glomerular blood capillaries into the Bowman’s capsule

• The filtered blood plasma is known as the glomerular filtrate. It contains small, soluble molecules that are forced out into the Bowman’s capsule.

How is the Glomerulus suited to Urine formation

• the glomerulus is a knot or network of blood capillaries. These provide a large surface area for filtration process.

• The blood capillaries have walls one-cell thick. There are tiny pores in the capillary wall.

• The blood capillaries are covered by a thin partially permeable membrane. It only allows very small soluble molecules or ions to pass through. It is impermeable to blood cells, platelets and large molecules such as proteins

Selective Reabsorption

The glomerular filtrate passes along the nephron, useful substances are reabsorbed into the surrounding capillaries.

• Most of the water is reabsorbed by osmosis.

• Some salts are reabsorbed by active transport

• All the glucose man’s amino acids are reabsorbed by active transport.

1. Proximal convoluted tubule → Most of the water is reabsorbed by osmosis. → All of the glucose and amino acids are reabsorbed by active transport. → Most mineral slats are reabsorbed by diffusion and active transport.

2. Loop of Henlé → Some water are reabsorbed by osmosis → some mineral slats are reabsorbed by active transport.

3. Distal Convoluted tubule → Some water is reabsorbed by osmosis → some mineral salts are reabsorbed by active transport.

4. Collecting duct→ Reabsorbs some water by osmosis

Osmoregulation

Osmoregulation is the control of water potential and solute concentration in the blood to maintain a constant water potential in the body.

How do kidneys help to regulate the water potential of blood? (loss of water)

• When there is loss of water, water potential of blood plasma decreases. It will stimulates Hypothalamus in the brain.

• Pituitary gland releases more ADH into the bloodstream.

• Cells in the walls of the collecting duct become more permeable to water

• More water reabsorbed from the collecting duct into the capillaries.

• Smaller volume of urine produced

• Urine produced is more concentrated

• Water potential returns back to normal

How do kidneys help to regulate the water potential of blood? (large intake of water)

• When there is large intake of water, water potential of blood plasma increases It will stimulates Hypothalamus in the brain.

• Pituitary gland releases less ADH into the bloodstream.

• Cells in the walls of the collecting duct become less permeable to water

• Less water reabsorbed from the collecting duct into the capillaries.

• Larger volume of urine produced

• Urine produced is more diluted

• Water potential returns back to normal

How does a Dialysis Machine work?

1. Blood is drawn from the vein in the patient’s arm and is allowed to be pumped through the tubing in the dialysis machine.

2. The tubing is bathed in a specially controlled dialysis guild. The walls of the tubing in the dialysis machine are partially permeable.

3. Such small molecules such as urea and other metabolic waste products diffuse out of the tubing into the dialysis fluid. Blood cells, platelets and large molecules such as protein remain in the tubing

4. The filtered blood is then returned to a vein in the patient’s arm

Features of a Dialysis Machine

• The Dialysis fluid contains the same concentration of essential substances as healthy blood. → this to ensure that essential substances do not diffuse out of the blood into the dialysis fluid. If the patients lacks these essential substances, these substances will diffuse from the dialysis fluid into the blood.

• The dialysis fluid do not contain metabolic waste products → sets up a concentration gradient that allows waste products to diffuse out of the tubing into the dialysis fluid. The waste products are removed from the blood

• The tubing in the machine is narrow, long and coiled. → increases surface area-to-volume ratio which help posed up the rate of exchange of substances between three patient’s blood and the dialysis fluid.

• The direction of the blood flow is opposite to the flow of dialysis fluid. → maintains a concentration gradient for removal of waste.