The Heart and Blood Circulation: A Detailed Explanation

Posted on Dec 3, 2024 in Biology

The Circulatory System

Transporting Substances

The circulatory system, comprised of blood vessels and the heart, transports vital substances throughout the body. Numerous channels reach all body cells, delivering nutrients and removing waste products. This exchange occurs within the internal environment, specifically the interstitial plasma.

Interstitial Plasma Renewal

Continuous renewal of interstitial plasma is essential. Blood circulates throughout the body, providing fresh nutrients and removing waste products. Blood, approximately 5.5 liters in a human, performs several key functions:

  • Transports nutrients and oxygen
  • Collects waste products
  • Plays a crucial role in defense mechanisms
  • Carries hormones
  • Regulates body temperature

Blood Composition

Blood plasma, yellowish in color, consists mainly of water, which dissolves various molecules, including mineral salts, nutrients, waste substances, proteins, and hormones.

Blood Cells

  • Red Blood Cells (Erythrocytes): Disc-shaped, lacking a nucleus, and containing hemoglobin, a red, iron-rich pigment that transports oxygen from the lungs’ alveoli to all body cells.
  • White Blood Cells (Leukocytes): Involved in defending the body against microbes. Several types exist:
    • Granulocytes: Have granular cytoplasm and a multi-lobed nucleus.
    • Agranulocytes: Lack cytoplasmic granules and include lymphocytes and monocytes. Granulocytes and monocytes destroy microbes through phagocytosis, while lymphocytes produce antibodies.
  • Thrombocytes (Platelets): Cell fragments containing substances that enable blood clotting, preventing rapid blood loss and potentially fatal bleeding.

Blood cells are formed in the red bone marrow, located within certain bones. Some leukocytes mature and acquire their function in other organs, such as lymph nodes.

Blood Vessels

  • Arteries: Carry blood away from the heart, progressively branching into smaller vessels called arterioles.
  • Capillaries: Microscopic blood vessels branching from arterioles, reaching all body tissues. Their thin walls, composed of a single layer of flat cells, facilitate the exchange of substances between blood and surrounding cells.
  • Veins: Capillaries merge to form venules, which then combine into larger veins, returning blood to the heart. Veins have thinner walls than arteries and contain valves that ensure one-way blood flow toward the heart.

The Heart

The heart pumps blood throughout the circulatory system. This muscular organ, with a thick wall called the myocardium, is lined by the endocardium and enclosed by the pericardium. It receives blood from veins and pumps it into arteries.

  • Heart Rate: The number of times the heart beats per minute.
  • Cardiac Output: The volume of blood pumped by the heart per minute.
  • Atrial Systole: The atria contract, pumping blood into the ventricles through the atrioventricular valves.
  • Ventricular Systole: The ventricles contract, pumping blood into the pulmonary artery and aorta.
  • Diastole: Blood from the vena cava and pulmonary veins enters the atria.
  • Blood Pressure: The force exerted by blood against the vessel walls.

Blood circulation is driven by the heart’s suction during diastole and the contraction of muscles surrounding the veins.

Circulatory Circuits

  • Systemic Circulation: Blood flows from the left ventricle through the aorta to all body tissues (except the lungs) and returns to the right atrium.
  • Pulmonary Circulation: Blood flows from the right ventricle to the lungs and back to the left atrium.

The Lymphatic System

Lymph capillaries in tissues collect lymph and merge into lymphatic vessels, which empty into blood vessels. Lymph nodes filter lymph, containing lymphocytes and plasma.

Other Excretory Organs

  • Liver: Removes byproducts from the breakdown of hemoglobin in old red blood cells, as well as small amounts of cholesterol and certain toxins.
  • Urinary System: Removes waste products through urine.
  • Respiratory System: Provides oxygen to the blood and removes carbon dioxide.
  • Sweat Glands: Produce sweat, which contains excretory substances.

Waste Products

  • Carbon Dioxide: Produced during cellular respiration.
  • Urea and Uric Acid: Result from the breakdown of proteins and nucleic acids.
  • Ingested Toxic Substances: Transported to the excretory system.

The excretory system also regulates water balance, mineral salt concentration, and blood acidity.

Kidneys

  • Renal Cortex: The outer layer of the kidney.
  • Renal Medulla: The inner layer, where urine is collected.
  • Ureters: Two tubes that carry urine from the kidneys to the bladder.
  • Urination Reflex: Triggered when the bladder is full and contracts.
  • Urethra: The tube that carries urine from the bladder to the outside of the body.

Kidney Function

  • Glomerular Filtration: Substances from the blood pass into Bowman’s capsule.
  • Tubular Reabsorption: Nutrients are reabsorbed from the filtrate.