Excretion as an example of homeostatic control (5.1.2)

Excretion is the removal of metabolic waste products produced by cells, such as carbon dioxide and urea.

It prevents toxic build-up and helps maintain a stable internal environment (homeostasis) for efficient metabolic activity.

The liver is divided into many cylindrical units called lobules, which provide a large surface area and ensure cells are in close contact with the blood, allowing efficient exchange of substances.

Hepatocytes are the main liver cells. They have a dense cytoplasm and contain numerous organelles, particularly mitochondria and rough endoplasmic reticulum, to support their many metabolic functions.

Kupffer cells are specialised macrophages that line the liver sinusoids and remove pathogens, debris and old red blood cells from the blood.

The liver plays several different roles within the body, including:

• Storage of glycogen: excess glucose is converted to glycogen for storage. Glycogen can later be broken down to release glucose and help regulate blood sugar levels.
• Detoxification: the liver breaks down harmful substances such as alcohol and toxins, making them less harmful to the body.
• Formation of urea: excess amino acids are deaminated in the liver, producing toxic ammonia. Ammonia reacts with carbon dioxide in the ornithine cycle to form urea, which is less toxic and is excreted by the kidneys.

The kidney is made up of many tiny tubular structures called nephrons. These are the functional units of the kidney, responsible for filtering blood and producing urine.

Nephrons begin in the cortex at the Bowman’s capsule before extending into the medulla and back up into the cortex, where they join a collecting duct.

There are several blood vessels that are key to nephron function:

• The renal artery carries oxygenated blood containing urea and other substances into the kidney.
• This branches into the afferent arteriole, which carries blood into the glomerulus at high pressure.
• The glomerulus is a network of capillaries that are surrounded by the Bowman’s capsule, where ultrafiltration occurs.
• Blood leaves the glomerulus through the efferent arteriole and is carried to other capillaries around the tubule.
• Finally, blood drains into the renal vein, which carries filtered blood away from the kidney.

Selective reabsorption occurs in the proximal convoluted tubule.

Sodium ions are actively transported from epithelial cells into the blood, lowering the sodium ion concentration inside the cells. Sodium ions then diffuse from the filtrate into the epithelial cells via co-transporter proteins carrying glucose with them.

Glucose leaves the cells and moves into the blood by facilitated diffusion. Under normal conditions, all glucose is reabsorbed in the proximal convoluted tubule.

Water is reabsorbed by osmosis due to the lower water potential of the blood.

Cells lining the proximal convoluted tubule are specialised and adapted for selective reabsorption:

• Their cytoplasm contains many mitochondria to provide ATP for active transport.
• They possess microvilli to increase the surface area for reabsorption.
• Their membranes contain numerous co-transporter proteins for the uptake of glucose, amino acids and sodium ions from the filtrate.

Following selective reabsorption in the proximal convoluted tubule, the filtrate is mainly water, urea and excess ions.

The loop of Henle establishes a low water potential in the medulla. In the descending limb water moves from the filtrate into the medulla by osmosis.

In the ascending limb, sodium and chloride ions are transported from the filtrate into the medulla, but water cannot leave. This creates a low water potential in the medulla, meaning that water leaves the collecting duct by osmosis.

The loop of Henle creates a concentration gradient in the medulla because its two limbs have different permeabilities. The descending limb is permeable to water but not ions, whereas the ascending limb is impermeable to water and transports sodium and chloride ions into the medulla. This establishes a low water potential in the medulla.

The collecting duct passes through the medulla, allowing water to be reabsorbed by osmosis. Its permeability is controlled by ADH; higher ADH levels increase water reabsorption, producing a smaller volume of more concentrated urine.

In the Bowman’s capsule, ultrafiltration forms the glomerular filtrate.

Small molecules such as water, glucose, ions, and urea are forced out of the blood, while proteins and blood cells remain in the capillaries.

Blood flows through the glomerulus and due to the afferent arteriole being wider than the efferent arteriole, hydrostatic pressure in the glomerulus is higher than in the Bowman’s capsule.

This pressure forces water and small molecules, such as glucose, ions, urea and amino acids, out of the blood and into the Bowman’s capsule. Large proteins and blood cells are too large to pass through the filtration barrier and remain in the blood.

The filtration barrier between capillaries and the Bowman’s capsule are adapted to enable ultrafiltration to occur:

• Capillary endothelium is fenestrated (has pores) for blood plasma and dissolved substances to pass through.
• Basement membrane is a fine mesh, acting like a sieve, to ensure large molecules do not pass through.
• Podocytes (specialised cells) in the Bowman’s capsule epithelium have filtration slits between them that allow small molecules to pass into the Bowman’s capsule lumen and prevent large molecules from passing through.

Osmoregulation is the control of the water potential of the blood.

It maintains a stable balance of water and dissolved substances, preventing cells from gaining or losing excessive water by osmosis. This helps prevent cells from bursting or shrinking and maintains normal blood volume, blood pressure and cell function.

A high water potential (high concentration of water) means water is more likely to move out of the blood down its concentration gradient by osmosis. A low water potential means water is less likely to move into the blood by osmosis.

The hypothalamus contains osmoreceptors that detect changes in the water potential of the blood. It controls the release of ADH (antidiuretic hormone) from the posterior pituitary gland.

If blood water potential is too low, more ADH is released, increasing the permeability of the distal convoluted tubule and collecting duct to water. More water is reabsorbed into the blood, producing a smaller volume of concentrated urine.

If blood water potential is too high, less ADH is released, reducing water reabsorption and producing a larger volume of dilute urine.

Kidney failure occurs when the kidneys can no longer effectively filter blood or maintain homeostasis.

The glomerular filtration Rate (GFR) is the volume of filtrate formed by the kidneys each minute. A low GFR indicates reduced kidney function.

Kidney failure also disrupts the regulation of water and electrolyte levels, leading to the accumulation of waste products and ions in the blood and impairing homeostasis.

A kidney transplant involves surgically replacing a failed kidney with a healthy donor kidney. It can provide a long-term solution to kidney failure and removes the need for regular dialysis, improving quality of life.

A transplant is a major operation and recipients must take immunosuppressant drugs to reduce the risk of organ rejection. In addition, suitable donor kidneys may be in short supply, resulting in long waiting times.

Urine testing for anabolic steroids is used to detect the use of banned performance-enhancing drugs in athletes. Steroid molecules are filtered by the kidneys and can be detected in urine samples.

Techniques such as gas chromatography (often combined with mass spectrometry) separate and identify steroid molecules based on their chemical properties, allowing banned substances to be detected.