Circulatory and Excretory Systems in Animals: Function and Diversity

Posted on Dec 14, 2024 in Biology

Nursery: Transport

Liquids possess multicellular intracellular (inside the cells) and extracellular (liquids bathing cells) components. This internal environment is formed by interstitial liquid and blood, and it renews lymph. It leads to nutrients and withdraws waste.

Sponges

They have no internal environment; the aquatic environment in which they live is in charge of nourishing them. This is favored by flagellar, ciliary, or corporal movements, which allow water to flow through the animal’s body.

More Complex Animals

They are composed of cells. Diffusion fails in them, which is why the interstitial liquid renews the liquid in the circulatory, digestive, respiratory, and excretory systems.

Homeostasis

It consists of maintaining a relatively constant internal environment. It is necessary to remove waste and add the substances necessary for cells to remain alive. The systems and apparatuses collaborate in homeostasis.

Components of the Circulatory System

Hydrolymph

(Echinoderms: starfish and sea urchins) Similar in composition to seawater, it contains defensive cells, the amebocytes, which phagocytize foreign substances. It has no respiratory mission.

Hemolymph (Higher Invertebrates)

It contains the respiratory pigment hemocyanin and defensive cells. It is related to the celomatic cavities and the tracheal system.

Blood (Insects, Worms, Vertebrates)

It contains hemoerythrin in annelids and hemoglobin in vertebrates. It is formed by plasma, composed of water, mineral salts, proteins, nutrients, respiratory gases, etc. It contains white blood cells, red blood cells, and platelets.

Lymph (Vertebrates)

It contains lipids and leukocytes, but no erythrocytes or platelets.

Types of Hearts

Partitioned: It has 2 chambers: atria, for blood input, and ventricles, for output.
Tubular: Tube-shaped.
Accessory: Speeds up movement in certain areas.

Blood vessels: Tubes that carry the circulating medium from the heart to all the animal’s tissues.

Types: Arteries, arterioles, capillaries, venules, and veins.

Functions of the Human Circulatory System

Distributes nutrients
Transports oxygen
Carries waste products
Carries hormones
Regulates temperature
Protects the body against pathogens
Prevents blood loss

Open or Lacunar Circulatory System

Blood vessels do not form a complete circuit because there are no capillaries. Therefore, blood leaves the arteries, bathes the organs, and returns to the heart. Hemocoel: Space where the organs are found.

Mollusks: Heart – Artery – Hemocoel – Gills – Hemolymph – Heart (1 or 2 auricles and 1 ventricle).
Arthropods (Insects): Tubular heart – Hemolymph – Hemocoel – Internal organs – Ostioles – Heart.

Closed Circulatory System

It is a complete circuit; blood flows inside the vessels (vertebrates, annelids, and cephalopods). The heart pumps blood at high pressure. The vessels have thick and muscular walls capable of withstanding high pressure. Plasma is filtered through the capillaries and forms interstitial fluid, which then returns by osmosis to the capillaries. The lymphatic system collects the rest.

Annelids: Arteries are more muscular and elastic than veins. Arterioles and venules are very thin. Capillaries have only 1 layer of endothelium. Heart – Contractile ventral vessel – No gas exchange through the skin – Dorsal cup – Contractile heart.
Cephalopods: Closed circulation, accessory hearts in the gills to accelerate circulation in this area.
Vertebrates: The heart is formed by different chambers. Types of circulation:
- Simple: Blood only passes once through the heart (fish).
- Double: Blood passes twice through the heart (birds, mammals, amphibians, and reptiles).
- Incomplete: Oxygenated and deoxygenated blood mix in the heart (amphibians and reptiles).
- Complete: There is no mixing (birds, mammals, and fish).

Fish

Complete and simple circulation. It has 2 chambers. The auricle is the chamber with less muscle mass. The ventricle has more. Disadvantage: The capillaries of the gills offer so much resistance to blood flow that blood pressure drops when entering the central aorta.

Amphibians

Double and incomplete circulation. It has 3 chambers: 2 atria and 1 ventricle.

Reptiles

Double and incomplete circulation. It has 3 chambers: 2 atria and 1 partitioned ventricle, except in crocodiles.

Birds and Mammals

Double and complete circulation. It has 4 chambers: 2 atria and 2 ventricles.

Heart

It is located in the chest cavity between the lungs and is enveloped by a membrane (pericardium). It is a hollow organ whose walls are formed by the heart muscle, which makes rhythmic contraction and dilation possible.

Atrioventricular and Semilunar Valves

Atrioventricular valves separate the atria from the ventricles. Semilunar valves allow blood to pass between the pulmonary artery and the aorta when the ventricles contract, but they prevent it from returning when blood passes to the arteries. The tricuspid valve connects the right atrium with the right ventricle, and the bicuspid valve connects the left atrium with the left ventricle.

Cardiac Cycle

Alternation of systole and diastole of the heart cavities.

General diastole: Atria and ventricles are relaxed, and semilunar valves are closed.
Atrial systole: Atria contract, pushing blood into the ventricles.
Ventricular systole: Ventricles contract, semilunar valves open, and the valves that separate the ventricles from the atria close.

Double Circulation

Pulmonary circulation: Blood poor in O₂ passes from the right atrium to the right ventricle. From there, it goes to the pulmonary artery, which divides into 2 branches, one for each lung. It then goes to smaller arteries, then arterioles, and then capillaries, where gas exchange takes place. It then passes from the capillaries to the venules and then to the major veins, which converge into the 4 pulmonary veins that flow into the left atrium.
Systemic circulation: Oxygenated blood passes to the left ventricle. When it contracts, it leads to the aorta. The arteries divide into arterioles, which branch into capillaries, where the exchange of substances occurs. They then congregate into venules, which lead to the inferior and superior vena cava, which flow into the right atrium.

Lymphatic System

Functions: Returns fluid to the blood, transports fat from the small intestine to the blood, and helps defend the body against pathogenic microbes.

Difference Between Excretion and Defecation

Excretion removes substances produced during metabolism, while defecation removes substances that have not been absorbed and are not normally digested in the feces.

Excretory System

Functions: Removes waste, filters the blood, maintains the body’s water balance, and regulates ion concentration.

Homeostasis: It is in charge of maintaining the constancy of the internal environment in animals.

Processes by which the excretory system aids in homeostasis: Excretion of waste products, regulation of ion concentration and other substances, and maintenance of water balance.

Organs involved in homeostasis: Liver, kidney, digestive system, skin, and lungs.

Excretion products: Water, CO₂, nitrogenous products, and uric acid.

Excretion Processes

Sponges and cnidarians lack specialized structures for excretion. It passes by diffusion from the intracellular to the external environment. They expel their waste directly by diffusion through the body surface.

Excretion involves 4 processes:

Filtration: Occurs due to the difference in pressure between the fluid of the internal environment and the inside of the excretory organs.
Reabsorption: Certain substances return to the internal environment through the cells of the excretory organ.
Secretion: Elimination of molecules that were not filtered from the internal environment to the inside of the excretory organs.
Expulsion of urine: Discharge of urine to the outside.

Excretory Systems in Invertebrates

Protonephridia (Flatworms)

They are formed by tubules whose end is a flame cell. Water and some dissolved waste products enter the tubules by filtration, where the flagella generate a current that drives the fluid throughout the network. The resulting fluid exits through pores.

Metanephridia (Annelids)

They are tubules, 2 per ring, open at both ends. The internal end opens into the coelom through a nephrostome, which continues into a long, unbranched tube that opens to the outside through the nephridiopore of the body.

Antennal or Green Glands (Crustaceans)

They are tubular structures located in the anterior part of the cephalothorax. They produce filtration in a blind sac that continues into a glandular chamber. In the tubular portion, reabsorption takes place. The fluid is removed and accumulates in a bladder, and is expelled through a small hole.

Malpighian Tubules (Insects)

Exclusive to dry environments. They are closed tubes at one end in contact with the hemocoel, while the other end opens into the intestine. When the fluid reaches the posterior part of the intestine, useful substances and water are reabsorbed.

Kidneys

Renal cortex: Outermost area formed by tissue.
Medulla: Composed of pyramidal structures. At the tip of each one is a renal papilla containing pores, which are the openings of the collecting tubules.
Renal pelvis: A funnel-shaped cavity that collects the filtrate and carries it to the ureters.

Nephron Structure

Bowman’s capsule: Where blood filtration occurs.
Proximal tubule: Located in the cortical area of the kidney.
Loop of Henle: In the shape of a loop, with a descending branch that enters the medulla and an ascending branch.
Distal tubule: Located in the cortical area of the kidney and flows into the collecting tubules along with the tubules of other nephrons.
Renal artery: Supplies blood loaded with waste products to the kidney.

Urinary Tract

Ureters: Carry urine to the bladder.
Urinary bladder: Where urine collects.
Urethra: Connects the bladder to the outside of the body.

Urine Formation

Glomerular filtration
Tubular reabsorption
Tubular secretion
Urine concentration
Expulsion of urine