Comparative Excretory Systems and Mammalian Heart Cycle Stages

Invertebrate Excretory Organs: Structure and Function

Invertebrates utilize various specialized organs to manage waste and maintain osmotic balance:

• Protonephridia

  Found in small animals like Platyhelminths (flatworms) and Nematodes. These consist of a series of closed, branched tubes. The inner ends terminate in ciliated cells (flame cells or solenocytes) that capture waste products. Excretory fluid circulates inside the tube, and once excess water is reabsorbed, the final product is eliminated through excretory pores.

• Metanephridia

  Found in Annelids and Mollusks. These are tubes open at both ends. The internal opening, called the nephrostome, is funnel-shaped, ciliated, and opens into the coelom (general body cavity). The other end opens to the outside via the nephridiopore. Fluid is collected by the cilia of the nephrostome and passes into the tubules where useful substances are reabsorbed. Waste products are expelled through the nephridiopore.

• Malpighian Tubules

  Typical of Insects. They consist of thin outgrowths of the alimentary canal. The closed end is in contact with the body cavity (hemocoel), and the open end connects to the digestive tract. Hemolymph is filtered here, and waste products are excreted into the gut.

• Green Glands (Antennal Glands)

  Exclusive to Crustaceans. They are located at the base of the antennae or maxillae. They consist of:

  1. A sac for collecting waste products.
  2. A tubule that reabsorbs useful substances.
  3. A bladder that eliminates waste through the nephridiopore.

Mammalian Cardiac Cycle Stages

The cardiac cycle is the period that elapses between one heart contraction and the next. It consists of three main stages:

Atrial Systole

Duration: 0.1 seconds. This is the contraction of the atria. Blood passes through the open atrioventricular valves (mitral and tricuspid) into the ventricles. The semilunar valves remain closed.

Ventricular Systole

Duration: 0.3 seconds. Once the ventricles are filled, they contract. The mitral and tricuspid valves close (preventing backflow into the atria). The semilunar valves open, and blood is forcefully ejected from the ventricles into the arteries.

General Diastole

Duration: 0.4 seconds. Both the atria and ventricles are relaxed and expanded, creating negative pressure. Blood flows passively from the veins into the atria and then into the ventricles. The ventricles continue to expand until they reach about 9/10 of their capacity. The cycle concludes when a new atrial systole forces the remaining blood into the ventricles, completing the filling process.