Animal Physiology I

Main notes

Animal physiology explains how organ systems cooperate to keep an animal's cells supplied with materials and free of wastes. This topic covers four of those systems: circulation, gas exchange, digestion, and osmoregulation. These systems set the stage for Topic 11, which covers the neural and endocrine controls that coordinate them, and they explain many patterns in the ecology of Topic 12, such as why large active animals need four-chambered hearts and why desert animals excrete uric acid.

Circulatory Systems and the Cardiac Cycle

Diffusion alone moves materials only over microscopic distances, so large animals need a circulatory system of vessels and pumps to carry oxygen, nutrients, hormones, and wastes between the body surface and deep tissues. There are two broad designs. In an open circulatory system, found in insects and most mollusks, blood called hemolymph washes directly over organs in body spaces called sinuses, pushed along by a simple tubular heart. In a closed circulatory system, found in vertebrates, earthworms, and cephalopods, blood stays inside vessels and exchanges materials only across the thin walls of capillaries. Closed systems sustain higher pressure and faster flow, which supports higher metabolic rates.

Vertebrates also differ in how many times blood crosses the heart per trip around the body. Fish use single circulation: blood travels from the heart to the gills, then to the body, then back to the heart in one loop. Birds and mammals use double circulation with two separate loops: the pulmonary circuit between heart and lungs and the systemic circuit between heart and the rest of the body. The four-chambered heart that powers double circulation has two atria and two ventricles, and it keeps oxygenated and deoxygenated blood completely separate. That separation matters, because the second pump restores blood pressure after the low-resistance lungs, letting blood reach tissues with force and supporting the high metabolic rates of birds and mammals.

The cardiac cycle is the repeating pattern of the heartbeat. During diastole, the heart muscle relaxes and the chambers fill with blood. During systole, the ventricles contract and eject blood into the arteries. The familiar lub-dub sound comes from valves snapping shut: the atrioventricular valves make the lub when they close at the start of ventricular systole, and the semilunar valves make the dub when they close as the ventricles relax. At rest the human heart beats about 70 times per minute and pumps roughly five liters of blood each minute, which matches the total blood volume of an adult.

The vessels complete the system. Arteries carry blood away from the heart under high pressure and have thick, elastic walls. Veins return blood to the heart under low pressure and contain one-way valves that prevent backflow. Capillaries, one cell layer thick, are the only site where materials are exchanged with tissues. The blood itself is the cargo: about half of the five liters is red blood cells packed with hemoglobin, an iron-containing protein that binds oxygen. Each hemoglobin molecule carries up to four oxygen molecules, and about 98.5 percent of the oxygen in blood rides on hemoglobin rather than dissolving in plasma.

1. Deoxygenated blood returns through the venae cavae into the right atrium.
2. The right ventricle pumps it through the pulmonary arteries to the lungs.
3. Oxygenated blood returns through the pulmonary veins into the left atrium.
4. The left ventricle pumps it through the aorta into the systemic circuit.
5. Capillaries exchange gases and nutrients, and the blood returns to the right atrium.

Common Mistake: Arteries do not always carry oxygenated blood. The pulmonary arteries carry deoxygenated blood to the lungs, and the pulmonary veins carry oxygenated blood back to the heart. Judge a vessel by which circuit it serves, not by its name.

ELI-10

Picture a delivery service with two trucks. One truck carries packages to the cleaning station, and the other truck delivers them to every house. After cleaning, packages return to the depot to be sorted again. Every package is cleaned before it reaches a house, so the two routes never mix.

Gas Exchange and Respiratory Mechanics

Gas exchange is the diffusion of oxygen into the body and carbon dioxide out of it. Like all diffusion, it runs down a concentration gradient, and its rate rises with surface area and falls with distance. Every effective respiratory surface is therefore thin, moist, and large. Animals have built many solutions with this same recipe. Fish gills are folded sheets through which blood and water flow in opposite directions, a design called countercurrent exchange that keeps oxygen diffusing into the blood along the entire gill, pulling far more oxygen from the water than a parallel flow could. Insects deliver air directly to each cell through a branching network of tracheae. Mammals exchange gases in millions of tiny air sacs called alveoli, each wrapped in capillaries and together providing a surface of about 70 square meters in an adult human.

Ventilation follows simple pressure logic. The lungs sit inside the airtight chest cavity. Inhalation is active: the diaphragm flattens and the external intercostal muscles lift the ribs, expanding the chest, which lowers the pressure inside the lungs below atmospheric pressure, so air flows in. Exhalation is mostly passive: the muscles relax, the chest recoils, pressure rises, and air flows out. This pattern is negative pressure breathing, meaning the lungs are inflated by a pressure drop and never pushed by muscles from inside. At rest, a single breath moves a tidal volume of about 500 milliliters of air, and the maximum air a person can exhale after a deep breath, the vital capacity, is about 4.5 liters.

1. The diaphragm contracts and flattens as the external intercostals raise the ribs.
2. The chest cavity expands, so pressure inside the lungs falls below atmospheric pressure.
3. Air flows down the pressure gradient through the airways into the alveoli.
4. Oxygen diffuses into the blood and carbon dioxide diffuses out of it.
5. The diaphragm and intercostals relax, the chest recoils, and air is pushed out.

In the blood, oxygen travels almost entirely on hemoglobin, because plasma alone could not carry enough to fuel the body. Carbon dioxide takes the reverse path: most of it diffuses into red blood cells and is converted to bicarbonate ions in plasma, with the remainder bound to hemoglobin or dissolved. Bicarbonate also acts as the blood's main buffer against pH change.

Common Mistake: The lungs do not pull air in like a straw. The diaphragm and ribs change the size of the chest, and air simply flows in to equalize the pressure. This is why a puncture in the chest wall collapses the lung: the chest can no longer hold a pressure difference.

ELI-10

Imagine squeezing a plastic bottle and then letting it pop back into shape. Squeezing pushes air out, and letting go pulls air back in. Your rib cage and diaphragm do the same job for your lungs. Air simply moves from high pressure to low pressure, exactly like the air in the bottle.

Digestion and Absorption

Digestion breaks large food molecules into monomers small enough to cross cell membranes, and absorption moves those monomers into the blood. The tract is one long tube, and every station has a different job. Peristalsis, waves of muscle contraction, pushes food along the way. In the mouth, teeth and tongue physically break food while salivary amylase begins digesting starch into maltose. The stomach adds gastric juice: hydrochloric acid and the enzyme pepsin, secreted as inactive pepsinogen so it cannot digest the stomach itself. The acid unfolds proteins, pepsin splits them into short peptides, and churning mixes the contents into chyme.

The small intestine is the main site of both digestion and absorption. The duodenum, its first segment, receives bile from the liver, which emulsifies fats into tiny droplets, and juice from the pancreas, which supplies enzymes that digest fats, proteins, starches, and nucleic acids plus bicarbonate that neutralizes the acid arriving from the stomach. Brush border enzymes on the intestinal surface finish the job. The lining is folded into fingerlike projections called villi, and each villus cell is covered with microvilli, multiplying the surface area hundreds of times over what a smooth tube would offer. Monosaccharides and amino acids cross into the blood capillaries inside each villus. Fatty acids and monoglycerides are reassembled into chylomicrons and taken up by lacteals, lymphatic vessels that drain into the bloodstream. The large intestine then reabsorbs water and salts, leaving the feces to be eliminated.

Common Mistake: The stomach is not the main site of absorption. Almost all digestion and nearly all nutrient absorption happen in the small intestine, and the stomach mainly breaks down proteins with acid and pepsin. Alcohol and a few drugs are the main substances that pass directly through the stomach wall.

ELI-10

Think of a recycling factory with several workstations. One station crushes big boxes, another shreds them into strips, and a final station turns the strips into clean fibers. Each station can only handle the size its machine is built for. Your digestive tract works the same way, one station after another.

Osmoregulation and Excretion

Osmoregulation keeps the water and salt content of body fluids in balance, and excretion removes nitrogenous wastes. Water follows solutes by osmosis, so an animal controls its water by controlling its salts. Marine bony fish drink seawater and pump excess salt out through their gills. Freshwater fish face the opposite problem, taking in water passively, and they excrete large volumes of very dilute urine. Terrestrial animals fight constant water loss and use every trick to conserve it.

The breakdown of proteins and nucleic acids leaves nitrogen to be disposed of, and animals excrete it in three forms that trade energy for water. Ammonia is cheap to make but very toxic, so fish that live in water release it directly into the surrounding water. Mammals and amphibians convert ammonia to urea, which is much less toxic and can be stored safely. Birds, reptiles, and insects go further and make uric acid, a nearly insoluble semisolid that costs the most energy but loses almost no water. Uric acid is a clear advantage for egg-laying animals, whose embryos cannot wash wastes away.

The nephron is the functional unit of the kidney, and each human kidney holds about a million of them. Filtration begins under blood pressure in the glomerulus, a capillary knot where plasma is forced into Bowman's capsule. The filtrate contains water, salts, glucose, amino acids, and urea, but not blood cells or large proteins. Each day the kidneys produce about 180 liters of filtrate, yet a person urinates only about one to two liters, because the tubules reabsorb roughly 99 percent of the volume. The proximal tubule reclaims most of the glucose, amino acids, salts, and water. The loop of Henle builds a salt gradient in the kidney tissue, and the distal tubule and collecting duct fine-tune the final urine. Two hormones adjust the final step: ADH increases water reabsorption, making urine more concentrated, and aldosterone increases sodium reabsorption, with water following by osmosis. How those hormones are released and controlled belongs to Topic 11.

Common Mistake: Urine is not simply blood squeezed through a sieve. Filtration alone would produce about 180 liters of urine every day, which is impossible. The tubules reabsorb about 99 percent of that filtrate, so the one to two liters that leave are the leftover wastes.

ELI-10

Picture a coffee filter holding back the grounds while water runs through. The kidney filter lets small molecules pass but holds back blood cells and large proteins. Most of what passes through is put right back into the body, like pouring the coffee back into the pot. Only the waste that should leave stays in the cup.

High-Yield:

• Track the blood: right atrium, right ventricle, lungs, left atrium, left ventricle, body.
• Every exchange surface is thin, moist, and large because diffusion drives gas exchange and absorption.
• The small intestine, not the stomach, is the main site of digestion and absorption.
• The kidney filters about 180 liters of filtrate daily and reabsorbs about 99 percent of it.
• ADH concentrates urine by increasing water reabsorption, and aldosterone increases sodium reabsorption.

Quick Review

• Double circulation separates the pulmonary and systemic circuits with a four-chambered heart, and the cardiac cycle alternates filling during diastole with ejection during systole.
• An adult human holds about five liters of blood, and each hemoglobin molecule carries up to four oxygen molecules.
• About 98.5 percent of oxygen is transported bound to hemoglobin, while most carbon dioxide travels as bicarbonate.
• Breathing works by pressure differences: the diaphragm and ribs expand the chest, and tidal volume at rest is about 500 milliliters.
• Fish gills use countercurrent exchange, insects use tracheae, and mammals use alveoli.
• The small intestine is the main site of digestion and absorption, with villi and microvilli multiplying its surface area.
• The nephron filters about 180 liters of filtrate each day and reabsorbs about 99 percent of it, leaving one to two liters of urine.
• Nitrogenous waste shifts from ammonia to urea to uric acid as animals need to conserve more water.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.