Why Birds Get Rid of Waste as a White Paste Instead of Urine

Breaking down proteins and nucleic acids leaves nitrogen-containing wastes such as ammonia, urea and uric acid. If they build up, they poison cells, so animals must remove them — in humans, mainly through a pair of kidneys.

This part covers types of nitrogenous waste, the kidney, the nephron, and how urine is formed.

Ammonotelic animals excrete highly toxic ammonia, which needs plenty of water; ureotelic animals excrete less toxic urea, which needs less water; and uricotelic animals excrete nearly insoluble uric acid as a paste, losing the least water.

Ammonotelism:

- Ammonia is the most toxic form and needs large amounts of water to remove
- Examples: many bony fishes, aquatic amphibians and aquatic insects

Ureotelism:

- Ammonia is converted to urea in the liver and released into blood
- Examples: mammals, many terrestrial amphibians and marine fishes

Uricotelism:

- Uric acid leaves as a pellet or paste with minimum water loss
- Examples: reptiles, birds, land snails and insects

An everyday example. The white patches in pigeon droppings on a balcony railing are mostly uric acid.

The substance. Toxicity and water need go together — the more toxic the waste, the more water it takes to remove it safely.

The human excretory system has a pair of kidneys, a pair of ureters, a urinary bladder and a urethra; each kidney has an outer cortex, an inner medulla divided into renal pyramids, cortical extensions called columns of Bertini between the pyramids, and a funnel-shaped pelvis leading to the ureter.

The organs:

- Kidneys — reddish-brown, bean-shaped, between the last thoracic and third lumbar vertebra
- Ureters — carry urine from the kidneys
- Urinary bladder — stores urine
- Urethra — carries urine out of the body

Inside the kidney:

- Hilum — a notch on the inner concave surface where the ureter, blood vessels and nerves enter
- Renal pelvis — a broad funnel-shaped space inside the hilum, with projections called calyces
- Cortex — the outer zone, under a tough capsule
- Medulla — the inner zone, divided into a few conical medullary pyramids projecting into the calyces
- Columns of Bertini — extensions of the cortex between the pyramids

An everyday example. A rajma bean has the same shape as a kidney, with its notch matching the hilum.

The substance. Nephrons span both zones — they begin in the cortex, and their loops dip into the medulla.

Each nephron has a glomerulus inside Bowman's capsule, together called the Malpighian body, followed by the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule, which opens into a collecting duct; cortical nephrons have short loops, while juxtamedullary nephrons have long loops reaching deep into the medulla.

Parts of a nephron:

- Glomerulus — a tuft of capillaries formed by the afferent arteriole; blood leaves by the efferent arteriole
- Bowman's capsule — a double-walled cup around the glomerulus
- Malpighian body (renal corpuscle) — glomerulus + Bowman's capsule
- PCT — a highly coiled tubule after the capsule
- Loop of Henle — a hairpin with descending and ascending limbs
- DCT — another highly coiled tubule
- Collecting duct — receives many DCTs and runs through the medullary pyramids to the renal pelvis

Vasa recta. A U-shaped vessel from the efferent arteriole runs beside the loop of Henle.

Two types:

- Cortical nephrons — the majority; short loops dipping only a little into the medulla; vasa recta absent or highly reduced
- Juxtamedullary nephrons — very long loops running deep into the medulla; well-developed vasa recta

An everyday example. A home water purifier first strains water through a fine filter and then treats it in later stages, much as a nephron filters blood and then processes the filtrate.

The substance. The Malpighian body, PCT and DCT lie in the cortex, while the loop of Henle dips into the medulla.

Urine forms in three steps: blood pressure pushes fluid from the glomerulus into Bowman's capsule as filtrate, the tubules reabsorb most of the water and useful solutes, and they secrete unwanted ions into the filtrate.

Glomerular filtration:

- Blood pressure in the glomerular capillaries filters blood through three layers — capillary endothelium, the epithelium of Bowman's capsule, and a basement membrane between them
- The capsule's epithelial cells, podocytes, leave minute gaps called filtration slits
- Almost all plasma constituents except proteins pass into the capsule

GFR. The filtrate formed by both kidneys per minute is the glomerular filtration rate, about 125 mL per minute in a healthy person.

Worked example — daily filtrate and reabsorption.

$$125\text{ mL/min}\times 60\times 24=180000\text{ mL}=180\text{ L per day}$$

With about 1.5 L of urine released per day:

$$\frac{180-1.5}{180}\times 100\approx 99\text{ per cent reabsorbed}$$

Reabsorption. Tubular cells reabsorb substances actively, such as glucose, amino acids and Na${}^{+}$, or passively, such as nitrogenous wastes and water in the early segments.

Secretion. Tubular cells secrete **H${}^{+}$, K${}^{+}$ and ammonia into the filtrate, helping maintain ionic and acid-base balance.

An everyday example. Draining boiled rice through a colander lets water through while keeping the grains, much as the glomerulus lets plasma pass but holds back cells and proteins.

The substance. Filtration is not selective**, so the tubules must recover the glucose and water the body cannot afford to lose.