Human Blood, Lymphatic, and Urinary Systems: Composition and Functions

Posted on Dec 7, 2024 in Biology

Blood Composition and Functions

Blood constitutes 8% of our body weight. It consists of blood plasma and cellular components suspended in plasma.

Plasma

Plasma is a liquid composed mostly of water, minerals, glucose, proteins (fibrinogen, albumin, and globulin), and other substances such as vitamins and hormones.

Cellular Components

The cellular component constitutes about 40% of the blood. This volume remains constant and is called hematocrit. Its decrease indicates anemia. There are three types:

  • Red Blood Cells (Erythrocytes): These are small, biconcave cells without a nucleus. They are the most abundant (4.5 million per mm3), with men having slightly more than women. They contain hemoglobin, a protein that contains iron, gives blood its red color, and transports gases (oxygen and CO2).
  • White Blood Cells (Leukocytes): These defend the body against microbes and tumor cells. They are larger and less numerous (8,000 per mm3). An increase in their number indicates an infection. There are three types: lymphocytes, monocytes, and granulocytes.
    • Lymphocytes: These are small cells with a large nucleus, representing 32% of white blood cells.
    • Monocytes (Macrophages): These are large cells with a kidney-shaped nucleus, representing 4% of white blood cells.
    • Granulocytes: There are three types:
      • Neutrophils: These contain a segmented nucleus and granules loaded with enzymes, representing 60% of white blood cells.
      • Eosinophils: These have a bilobed nucleus and granules filled with enzymes, representing 3% of white blood cells.
      • Basophils: These contain granules laden with histamine and heparin, representing 0-1% of white blood cells.
  • Platelets: These are cellular fragments derived from large blood cells. There are about 300,000 per mm3. A decrease in platelets can cause unreported hemorrhages because they collaborate in blood clotting.

Functions of Blood

  • Transport and Exchange of Substances: Blood provides nutrients and oxygen to cells, and these give waste products (urea, CO2) to the interstitial fluid. Some of these are dissolved in plasma, while others are linked to proteins (cholesterol and vitamin A). Oxygen and CO2 are linked to the hemoglobin of red blood cells.
  • Regulation of Body Temperature: As it circulates throughout the body, blood functions as a cooling system, transporting heat between the internal organs and the outside.
  • Defense: Blood defends the organism when it suffers trauma or when pathogens attempt to penetrate it. For example, if an injury occurs, it triggers coagulation, which prevents blood loss through damaged vessels.

Lymphatic System

The lymphatic system is composed of lymph vessels, lymph nodes, and terminal collectors.

  • Lymph Vessels: These are small-diameter collecting tubes with many valves that prevent lymph from retreating. Their terminal ends are in the form of a sac.
  • Lymph Nodes: These alternate with lymphatic vessels. They participate in defending the organism, as they produce and store lymphocytes and macrophages. They are distributed throughout the body and are common in the armpits, groin, and neck.
  • Terminal Collectors: These are the thoracic duct, which receives lymph from three-quarters of the body and originates from a reservoir called the cisterna chyli (Pecquet’s lymphatic dilation), and the right lymphatic duct, which collects lymph from the right upper limb. Lymph returns to the venous system from the thoracic duct and right lymphatic duct into the left and right subclavian veins, respectively.

Functions of the Lymphatic System

  • Maintaining the balance of the internal environment’s fluid.
  • Serving as a circulatory circuit.
  • Participating in the body’s defense.

Urinary System

  • Ureters: These are two tubes that come from each kidney and carry urine to the bladder.
  • Bladder: This is a muscular organ that fills with urine until it is expelled.
  • Urethra: This is a common conduit from the bladder to the outside of the body. Urine is expelled through it during urination. The output of urine is produced by the opening of a voluntary muscle sphincter.
  • Kidneys: Within the kidneys, a liquid similar to blood plasma is filtered, changing its composition as it is processed to become urine. Kidneys are bean-shaped and coated with a thin, tough membrane called the capsule, which is dark brown and shiny. The renal artery and vein enter, and the ureter exits through the anterior median area. In a longitudinal section, a clear external zone, the renal cortex, and a darker internal zone, the renal medulla, are visible. The medulla extends into the cortex (medullary rays). In the medulla, there are a series of renal pyramids; the vertices of these pyramids (papillae) are areas through which urine is discharged into the renal pelvis, a funnel-shaped cavity that narrows to become a tube (ureter).

Sweat Glands

Sweat glands are found inside the skin, surrounded by capillaries. These glands filter blood along with the waste substances it transports, forming sweat, which is a liquid similar to urine but less concentrated in salts and urea. Sweat is excreted through the skin’s pores and acts as a temperature regulator, cooling the body’s surface as it evaporates.

Renal Function

The kidney maintains the blood’s constant chemical composition and regulates water loss in urine. To do this, it has approximately two million microscopic structures called nephrons. The main component is a long, winding tubule surrounded by capillaries called the renal tubule. This duct communicates with the Malpighian corpuscle, a spherical structure formed by a capsule (Bowman’s capsule) and a tangle of capillaries (glomerulus) inside. The nephron performs its functions in three stages: filtration of blood, reabsorption (return of useful substances to the blood), and secretion of harmful substances into the urine.

  • Filtration of Blood: This occurs when blood passes through the glomerulus. Small substances (urea, uric acid, glucose, water) pass through to the renal tubule, but not large molecules (proteins or cells). This forms the primary urine.
  • Reabsorption of Useful Substances: Glucose, amino acids, and 99% of water are reabsorbed from the renal tubule back into the blood.
  • Secretion of Harmful Substances: Substances such as urea or some mineral salts are secreted from the capillaries into the renal tubule throughout its length, forming concentrated urine that is discharged into a collecting duct, common to several nephrons, which flows into the renal pelvis. As a consequence of this process, the blood that leaves the kidneys through the renal veins is cleansed of waste products.