Excretion in Humans

Question 1

How does ultrafiltration work?

Answer 1

It is caused by high hydrostatic blood pressure or high blood pressure.
The afferent arteriole that brings blood into the glomerulus is wider than the efferent arteriole that carries blood away
This creates a high blood pressure in the glomerulus.
The hydrostatic blood pressure provides the main force required for the filtration process.
The partially permeable membrane that wraps around the glomerular blood capillaries is like a very fine filter. It is called the basement membrane. The basement membrane has very small pores that allow only water and very small molecules to pass through.

Question 2

What does ultrafiltration do for the urine?

Answer 2

The high blood pressure in the glomerulus forces water and small molecules (e.g. glucose, amino acids, mineral salts and nitrogenous waste products) out of the glomerulus into the Bowman’s capsule, forming the filtrate. Blood cells, platelets, and large molecules such as proteins and fats, are retained in the glomerular capillaries.

Question 3

How does selective reabsorption take place?

Answer 3

At the proximal convoluted tubule, most of the mineral salts (sodium ions) and, in a healthy person, ALL of the glucose and amino acids are reabsorbed through the walls of the tubule into the surrounding blood capillaries. These solutes are reabsorbed via diffusion and active transport. This reabsorption is highly selective, and only those substances required by the body are reabsorbed readily.
Most of the water in the filtrate is reabsorbed by osmosis here.

Question 4

Where is water reabsorbed?

Answer 4

PCT
Loop of Henle
DCT
Collecting Duct

Question 5

How is the water potential of blood related to blood pressure?

Answer 5

The kidneys control the water and solute levels in the blood by controlling the amount of water that is reabsorbed at the nephrons. The amount of water reabsorbed affects the blood volume (amount of blood in the body). As a result, the kidneys also play a vital role in the control of blood volume and blood pressure. If the blood volume increases, then the blood pressure will rise.

Question 6

How does a dialysis machine work?

Answer 6

Blood is drawn from the vein in the patient’s arm and is allowed to be pumped through the tubing in the dialysis machine.
The tubing is bathed in a specially controlled dialysis fluid. The walls of the tubing in the dialysis machine are partially permeable.
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 remain in the tubing.
The filtered blood is then returned to a vein in the patient’s arm.

Question 7

What are the various features of the dialysis machine?

Answer 7

• The dialysis fluid contains the same concentration of essential substances as healthy blood
• The dialysis fluid does not contain metabolic waste products
• The tubing in the machine is narrow, long and coiled
• The direction of the blood flow is opposite to the flow of the dialysis fluid