Excretion and Urea Formation

• Excretion is how organisms remove metabolic waste and toxic substances.
• All living cells undergo numerous metabolic reactions.
• Respiration yields energy, water, and carbon dioxide.
• Animal cells require energy and may utilize water, but carbon dioxide is a waste product.
• In mammals, carbon dioxide from respiration gets excreted by the lungs.

Urea

• Urea, produced in the liver, is a waste product in humans.
• Too much urea in blood is toxic.
• Kidneys filter urea from the blood, along with excess water and ions, and then produce urine.
• Urine flows from the kidneys through the ureters to the bladder, where it is stored.
• Urine exits the body through the urethra when the sphincter muscle relaxes.

How Urea is Produced (Role of the Liver)

• Urea forms in the liver from excess proteins and amino acids.
• Animals, including humans, cannot store proteins or amino acids.
• Excess amino acids get broken down to form urea.
• Digestive enzymes break down proteins into amino acids in the stomach, duodenum, and ileum.
• These amino acids are absorbed into blood capillaries in the villi of the ileum.
• Blood capillaries join to form the hepatic portal vein, which transports amino acids to the liver.
• The liver allows some amino acids to flow in the blood to other areas.
• Excess amino acids get stored as carbohydrates through the removal of the nitrogen-containing amino group.
• The process of removing the nitrogen-containing part of an amino acid is deamination.
• The amino group from deamination turns into urea, then dissolves in the blood plasma to be excreted by the kidneys.

The Kidneys

• Kidneys are solid, oval, red-brown structures enclosed by a transparent membrane and attached to the abdominal cavity's back.
• The kidneys constantly remove toxic urea from the blood, excreting it in urine.
• The renal artery, branching from the aorta, delivers oxygenated blood.
• The renal vein carries deoxygenated blood from the kidneys to the vena cava.
• The ureter, a tube, runs from each kidney to the bladder in the lower abdomen.

Microscopic Structure of the Kidneys

• Kidney tissue contains capillaries and renal tubules.
• A cut through the kidney reveals a dark outer cortex and a lighter inner medulla.
• The pelvis refers to the space where the ureter connects to the kidney.

Nephrons

• Kidneys consist of thousands of tiny tubules called nephrons.
• Each nephron begins in the cortex, loops into the medulla, back into the cortex, and then descends into the medulla.
• Nephrons join with the ureter.

Urine Formation

• Blood flows into the kidney via the renal artery, which divides to form glomeruli (tiny, coiled capillaries).
• As blood flows through a glomerulus, it gets filtered.
• Small molecules like water, urea, glucose, and ions pass through to form filtrate, while larger molecules and blood cells cannot pass through.
• All filtrate molecules enter the nephron.
• Useful substances, like water and glucose, are reabsorbed back into the blood.
• Reabsorption involves the fluid flowing through the nephron.
• The final liquid, urine, contains urea, salts, and water then flows out of the kidneys, along the ureters, and into the bladder for storage before excretion through the urethra.
• Kidneys efficiently reabsorb water; humans reabsorb over 99% of water entering the tubules and filter 170 dm³ of water, but produce only 1.5 dm³ of urine daily.
• Urine volume depends on excess water; more liquid intake leads to diluted urine excretion.
• Less fluid intake or water loss through sweating results in concentrated urine.
• The colour of urine indicates hydration levels.